Collins New Naturalist 50

Pesticides and Pollution
Kenneth Mellanby

Table of Contents

Cover Page

Title Page

Editors’ Preface

Author’s Preface

CHAPTER ONE

INTRODUCTION

CHAPTER TWO

AIR POLLUTION

CHAPTER THREE

WATER POLLUTION

CHAPTER FOUR

RADIATION

CHAPTER FIVE

POLLUTION OF THE OCEAN AND THE SHORE

CHAPTER SIX

HERBICIDES AND WEED CONTROL

CHAPTER SEVEN

FUNGICIDES

CHAPTER EIGHT

INSECTICIDES AND INSECT CONTROL

CHAPTER NINE

SUBSTANCES USED TO CONTROL OTHER INVERTEBRATE PESTS

CHAPTER TEN

THE CONTROL OF VERTEBRATE PESTS

CHAPTER ELEVEN

THE FUTURE – IMPROVEMENT OR DISASTER?

Bibliography

Appendix

Index

Plates

Copyright

About the Publisher

EDITORS’ PREFACE

The New Naturalist series, now some way beyond its half century of volumes and its quarter century of years, has had much cause for gratitude to the senior officers of our kingdom’s Nature Conservancy who have so valuably contributed to its books and its task.

It could be expected that a servant of a statutory body, when discussing problems of conservation and ecology of political, economic and social moment (as nearly all such problems are) might adopt a somewhat statutory tone of voice. None of our Natural Conservancy authors has yet done so; nor has Dr. Mellanby, who handles in this book what can be vulgarly described (if it is not mixing a biological metaphor) as the hottest potato in the nature business. The impact of modern industry’s chemical products (themselves the products of vastly expensive and brilliant research) upon our environment—the air we breathe, the water we drink, the food we eat, and the animals and plants with which we try peacefully to coexist—is a subject so vast, so emotion-rousing, so socially provoking that almost anybody could be forgiven for approaching it with fearful frenzy on the one hand, or with careful dissemblance or even dissimulation on the other. Not so Mellanby: this is a calm book, and a deeply thought-out book, and patently a balanced book: the kind of book we expected from the leader of one of the finest teams of ecological analysts in the country. Of course we knew it would be so, when we persuaded the Director of the Monks Wood Experimenal Station to write it.

Kenneth Mellanby’s approach is magnificently lucid, the more so because of his deft use of illuminative detail, alternating with wise generalisations that show the deepest understanding of the history of pollution and the eternal struggle of man against predators and pests. He has had to specialise in being general: be not only historian but geographer, physiographer, chemist and physicist as well as biologist to arrive at a sense of proportion and balance, and a true evaluation of the present tides and streams of wastes and poisons, their natural history, control, cause, cure and care.

This book has been written without fear or favour, and with the fairest analyses of mistakes and successes. It persuades us of the need for everybody’s deeper understanding of the problems involved, and that our human stock, with the increase of its population and its civilised wants, has courted risks, certainly invited disasters and suffered a few—and yet may have succeeded in arriving at a point of common-sense confidence in a clean (or cleanish) planet in the predictable future. The planet is presently a pretty dirty one: but some clever ecologists and conservationists have voices that are now being heard, and may be heeded before it is too late.

All books on pesticides and pollution must be compared with the late Rachel Carson’s classic Silent Spring of 1963, which started the general public of all the educated world wondering. Kenneth Mellanby’s book is not of the same genre, and it would have been most inappropriate if it had been. Miss Carson’s book was a chamber of horrors, and, as regards the insecticidal events of its time, as every responsible naturalist (including Dr. Mellanby) would agree, accurate. It did a power of good. This book, we predict, will do further good; for it does the next thing. It does not say how awful! for this has already been said, and in Rachel Carson’s context justifiably. It says how does the business really work, and what next? and proceeds to spell it out, in a masterly style and depth that we are proud to be associated with.

THE EDITORS

AUTHOR’S PREFACE

In this book I have tried to deal with the subject of environmental pollution in Britain in an objective way. The public, and particularly those members of the public who are interested in the conservation of wild life, are very familiar with many types of pollution, but they cannot always judge their importance. Sometimes atomic radiation seems to be all-important, particularly when questions of military strategy are discussed. Our many fishless rivers are clear evidence of the serious effects of the pollution of fresh water, and every year our newspapers have pictures of dead seabirds covered with oil on our beaches. Great publicity has been given to agricultural chemicals. The remarkable impact of Rachel Carson’s Silent Spring has suggested that insecticides are the greatest danger. I have deliberately avoided dealing with this book in the text, not because I underrate its contribution to the subject, but because I think that the time has come to try to look at all sides of the problem. Rachel Carson, when dealing with insecticides and herbicides, was careful to give us the facts as they applied to the United States, but she selected her facts, and gave us an advocate’s case. At the time, this was a useful service to science, and equally selective rejoinders from the chemical industry have done little to reassure the public. Other and more objective books on the effects of pesticides are listed in the bibliography. One of my main tasks has been to try to fit pesticides into the general picture of pollution from all sources.

The writer of a book like this needs to call on many others for help. Over a good many years I have discussed these problems with scientists from many countries, and I have tried to digest their views and the contents of their publications. The question of pesticides must have special mention. I have been fortunate in being able to discuss these problems with Dr. N. W. Moore, head of the Nature Conservancy’s Toxic Chemicals and Wild Life Section at Monks Wood Experimental Station, and with the members of his team. Dr. Moore was the first scientist in Britain to organise research work on this subject, and he and his colleagues have made major contributions towards the understanding of their problem. I have received immense help from them at all stages and have taken up a great deal of their time in detailed discussion. I hasten to add, however, that they are in no way to blame for any faults in my presentation of the subject.

The editors of this series have given valuable help. Sir Julian Huxley originally suggested that I should write this book, and made useful proposals as to its contents. Sir Dudley Stamp also gave me much encouragement, and the most courteous application of the spur whenever I fell behind my schedule. Without this I would never have finished the book. He read the manuscript and I shall treasure the appreciative letter he wrote me about it not long before his untimely death.

As I have already said, I have tried to make this book an objective account of pollution. I fear that I shall be attacked from all sides. In discussions I have been accused of exaggerating the dangerous effects of industrial processes and of beneficial agricultural chemicals. I have also been told that I play down the dangers of these substances. So long as the attacks do indeed come from these different quarters I shall not feel that I have entirely failed in my objective.

CHAPTER ONE INTRODUCTION

This book is essentially an account of the way in which man is unintentionally contaminating his environment. This is a world-wide problem, but for the most part I am restricting my scope to consider the situation in Britain. I do this for two reasons, first because this treatment seems appropriate to a volume in the New Naturalist series, and secondly because although the problem is more acute in these small, developed and densely populated islands than in many other parts of the world, it is possible that what we do can help and guide others, and serve as a warning to prevent damage in the countries at present undeveloped which might otherwise accompany their economic development.

I think it is quite logical to bring together in one book the effects on our environments and on our wild life of pollutions which arise from urban conditions and from industry, and the rather special case of pesticides, which have recently been shown to constitute such an important contribution to environmental pollution. In all cases we are dealing with effects which were not deliberately planned by those who produce them. Pollution, including radiation, has grown as a side effect of the increase in human population, and of increased material productivity. Pest control began in quite a different way, but it is the side effects of modern pesticides, substances which could only have been produced and developed in an industrial system, and which may be effective weapons when properly used, which are causing so much concern.

In this book I have attempted to review the present situation, show how it has arisen, and discuss how man may either continue to pollute the earth and to impoverish it by destroying and restricting its flora and fauna, or alternatively may control his own activities for the benefit of posterity. The situation is serious. Much irremediable damage has already been done. On the other hand, we have learned how to avoid the worst types of pollution. Public opinion is strongly in favour of controlling pollution, and we are not so dogmatically in favour of eradicating any forms of life we consider to be “pests” as perhaps we used to be. If mankind is prepared to make a determined effort, to support much more research to make that effort effective, our descendants may not be condemned to live on an impoverished planet devoid of so much of the varied life which has made it so interesting and so beautiful. There is one other point which should be noted. Some pollutions, particularly from insecticides, seem to be harmful to wild life but not, as yet, to man. Air pollution is more often seen to damage plants than to harm man. Perhaps we should look upon these as useful “early warning” systems, and use the opportunity to reduce the chances of future damage to our own species.

Man deliberately alters the face of the earth, to a degree to which no other species of animal has aspired. He builds cities, factories and roads which wipe out wild life over large areas. He does this quite intentionally, assuming that the gain far outweighs the loss, though his pollution of the surrounding countryside with domestic and industrial effluents is not intentional. However, important as are the effects of urban and industrial development, it is as a farmer that man has the most profound effects on the landscape, and on the plants and animals which live there. An excellent account of these changes is given in Man and the Land by Sir Dudley Stamp. Farmers destroy the natural vegetation, and substitute wide areas of alien plants grown in monoculture. They deliberately try to get rid of plants which compete with their crops – that is, weeds. They try to destroy animals which eat their crops and so reduce the amount harvested – that is, pests. As agriculture has developed, new methods of dealing with weeds and pests have been discovered and put into use. To-day we are all conscious of the fact that chemical controls have been developed, and that poisonous substances are widely used to achieve control. We know that these poisons may kill birds and other forms of wild life, and we fear that they may endanger human health. However, we tend to forget that these possible toxic side effects may be less catastrophic to wild life than are the ecological effects of “traditional” agriculture.

Man has existed for hundreds of thousands of years, but it is only within the last ten thousand that he has had any considerable ecological effect, and only within the last millenium that he has really begun to change the whole appearance of the earth’s surface. Until about 8,000 B.C. man was a hunter and a gatherer of wild plants, with no domestic animals and no crops. Life, as Hobbes has said, was “nasty, brutish and short.” Man had little effect on his environment. He ate fruit and berries, and helped to disperse the seeds as do birds and animals to-day. His waste-products were probably dispersed over the land and helped to fertilise it. When he died his remains decayed and the nutrients were returned to the system. The nastiness of his life was probably made worse by pollution, even at that stage of his development, and pests contributed to its shortness. Some animals are naturally clean. They excrete in recognised latrines some distance from their usual resting places, and they do not leave the decaying remains of food in inconvenient places. Other animals, and primitive man was probably one of them, are not so hygienic, and if he lived in a cave it probably stank disgustingly. Even when he buried his dead he sometimes did so beneath his dwelling, just where decay would be most objectionable. Under these conditions many diseases must have been rife.

Early man probably thought of the larger carnivores as the most serious pests. He competed with them for animal food when he became a hunter, and he himself formed a part of their prey. At first he did little positive to control these pests; his main object was to avoid them. Later, but long before he became a farmer, man in some parts of the world learned to trap and hunt even the most ferocious wild animals, and he probably accelerated the extinction of several species. Recent work on the so-called “Pleistocene overkill” goes so far as to suggest that man exterminated nearly half the larger mammals in Africa some 50,000 years ago, and that in North Africa (where man arrived much later) he similarly killed off at least sixty per cent of the species of large mammals around 10,000 B.C. If these conclusions are finally substantiated, they will have a profound effect on ecological thinking. Early man probably paid little attention to the smaller insect pests, lice, bugs and fleas, from which he no doubt suffered. He did not realise that these were not only a nuisance, but were also the carriers of diseases which were far more deadly than all the lions and tigers and snakes which he so greatly feared. At this stage man was simply an animal, competing with other animals, and doing little to upset the uneasy balance of nature.

There were cases where hunters profoundly changed the landscape. It is likely that North American Indians deliberately burned the forest, and so encouraged grassland which could maintain larger herds of buffalo. This could be considered an early example of wild life conservation! It must also have had profound effects on all the other animals and plants in the region. Incidentally, at a much later date, when the prairies were cultivated to grow cereal crops, the buffaloes became “pests” and were almost completely exterminated.

Primitive man suffered from pests and pollutions, even if he was not always aware of this. When he became an agriculturalist, he recognised the fact. Settled agriculture, with the growing of some crops, has gone on in parts of the world for perhaps ten thousand years, but extensive farming has only existed for about five thousand years. Arable farming is essentially a process where certain plants are encouraged, and others, which would compete with them, are discouraged. The unwanted plants are pests or weeds.

It is difficult to find a satisfactory definition of a pest, other than to describe it as a plant or animal living where man does not want it to live. The same animal may sometimes be treated as economically valuable, at other times as a dangerous competitor. Thus mink, escaped from fur farms in England, where they are prized, are considered as dangerous pests in other parts of the country. The same plant may be a valued crop at one time and a weed at another; an obvious example is the potato, for a few tubers, accidentally left in the soil when this crop is harvested, are troublesome weeds in cereals grown in succeeding years.

A great part of the energy expended in arable farming goes in weed control. Ploughing, harrowing and cultivation are all means of reducing weeds and their growth, as well as of making conditions suitable for planting crops. It is therefore somewhat ironic that weeds are important largely because man has produced conditions in which they flourish. Most weeds were rare plants before man became a farmer, and some are now uncommon or even extinct except on farm land. The history of weeds and their development has been fully described by Sir Edward Salisbury in his masterly book Weeds and Aliens, to which any interested reader must refer.

At different times farmers have used different methods of weed control, and different species of plant have been economically important. When wheat and other corn crops were sown broadcast, hand weeding was the only practicable method. When drilling in rows was introduced in the eighteenth century, it became much easier to keep crops clean. At the same time improved methods of separating crop from weed seeds were devised, so that sowing did not itself greatly contaminate the ground. Thus with clean seed and properly planted crops, cheap labour and comparatively simple horse-drawn machines kept the fields clean. At the end of the nineteenth century there was no serious weed problem for the good farmer in most parts of Britain. As agricultural wages rose, mechanisation was introduced, and some farming processes were improved, but many crops became weedier and weedier, so that different rotations had to be developed, not always with success. In recent years the situation, for the farmer, has been saved by the introduction of selective weedkillers. These have revolutionised agriculture, but have produced their own problems, as will be seen in chapter 6.

Parasitic fungi cause a great deal of crop damage. Early man was aware of some of these diseases, and the danger of eating corn made poisonous, for instance, by the fungus causing ergot, particularly in rye. However, many fungus diseases were not recognised as such, and their damage was accepted as a normal risk of farming, until the latter part of the nineteenth century. As shown in chapter 7, many fungus diseases are now controlled with little risk to other forms of life.

Farmers, from neolithic times onwards, were aware of mammalian pests, ranging from deer which damaged their crops to wolves which preyed on their herds. Rats and other rodents were known to consume much of the stored grain, and many ingenious methods of excluding them were devised. Early pest control was in effect hunting; the results were sometimes successful, as in the case of large and slow-breeding animals, and quite ineffective against small mammals which bred rapidly. Brown bears were exterminated in England in Roman times, and in Scotland before the Norman invasion. Wolves continued much longer. They were quite common, particularly in Wales, into the medieval period, and the last survivor is believed to have been killed in 1740; a few lingered on in Ireland for another thirty years.

Pest control in the English countryside has usually been related to game preservation and sport as well as to agriculture. The wolf was clearly too large and voracious an animal to be tolerated, and so it was eliminated. No one to-day seriously suggests its reintroduction, though conservationists (if not the local farmers) are concerned about its future in Spain. The fox, however, is still quite abundant, although it undoubtedly kills poultry. It would be difficult, though not impossible, to exterminate all the hill foxes in the wilder parts of Britain, but a determined effort could get rid of this animal in areas of intensive farming within a couple of years. Foxes may not be deliberately preserved, but they have been tolerated for many years because of fox-hunting, and those naturalists who are opposed to blood sports should realise their debt in this connection. Recent research has shown that foxes live as much on carrion and on small animals and insects as upon poultry and game, so they are likely to continue to survive unless accidentally wiped out by chemicals (see here). Recently foxes have been reported in increasing numbers in suburbia, raiding dustbins and feeding on garbage. These habits will not endear them to the more sentimental members of the public, who may equate garbage-eating foxes with rats. More of the animals are likely to survive, and they may eventually become pests in a new role if they become too common.

Gamekeepers were for a long time the main enemies of carnivorous animals and birds, which they spoke of as “vermin” on the assumption that they lived mainly on game. Most keepers until recently had their “gallows” on which the rotting corpses of stoats, weasels, hawks and owls were hung, presumably pour encourager les autres. Systematic shooting of slow-breeding predatory birds effectively controlled their numbers. In the nineteenth century kites, formerly distributed throughout the country, were eliminated except from a few mountainous areas. A careful investigation by Dr. N. W. Moore shows how the buzzard has fared in the last 150 years. At the beginning of the nineteenth century buzzards were quite common breeding birds over most of Britain. By 1865 they had been exterminated in most of East Anglia and the Midlands, and by 1900 they were only to be found in Cornwall, Wales, the Lake District and Western Scotland. By 1954 the situation had somewhat improved, and the birds had recolonised many of the areas occupied in 1865. This spread was clearly due to the decrease in game preservation during and after the 1914-18 war. If the data for buzzards and the numbers of gamekeepers are mapped side by side, this shows a good negative correlation.

To-day many gamekeepers are more enlightened. Although hawks and other carnivorous birds do eat some game birds, they prey much more on small mammals (mice, voles, etc.) which compete with game for food. Most predators are now themselves legally “preserved,” though they can be shot if caught in the act of taking poultry or game. We may see an improvement in numbers, similar to that manifested by the buzzard between 1900 and 1954, for many other species, if the new danger from agricultural chemicals can be overcome.

Smaller, non-carnivorous mammals and birds do a lot of damage, the amount of which is not always recognised. Rabbits were probably introduced into England by the Normans in the eleventh or twelfth century and were comparatively uncommon, prized as game animals until the nineteenth century. Then, for some reason which we do not yet understand, they suddenly increased in numbers so that a density of twenty animals to an acre was not uncommon, and as many as 100,000,000 carcasses were sold in a year, without noticeably depleting the numbers still at large. It was not until rabbits were wiped out in many areas by myxomatosis in 1954 that the extent of the damage they had done was recognised. Pre-myxomatosis control, by shooting and trapping, was rarely effective, and merely served to “crop” the population. The importance of myxomatosis is discussed in chapter 10.

Fig. 1 Changes in the distribution of the buzzard in the British Isles. (from Dr. N. W. Moore with acknowledgement to British Birds).

KEY: Black: Breeding proved, or good circumstantial evidence of breeding.

? on black: Circumstantial evidence suggests that breeding probably took place.

? on white: Inadequate evidence of breeding.

White: No evidence of breeding.

Fig. 2 a. The breeding population of the buzzard in 1954.

KEY: Black: 1 or more pairs per 10 square miles.

Cross-hatch: More than 1 pair per 100 square miles, but less than 1 pair per 10 square miles.

Diagonal hatch: Less than 1 pair per 100 square miles.

White: No breeding buzzards.

+ means that breeding density may belong to the category higher than that indicated.

—means that the breeding density may belong to the category lower than that indicated.

b. Game preservation in 1955.

KEY: Black: 3 to 6 gamekeepers per 100 square miles.

Cross-hatch: 1 to 2 gamekeepers per 100 square miles.

Diagonal hatch: Less than 1 gamekeeper per 100 square miles but more than 1 per 200 square miles.

White: Less than 1 gamekeeper per 200 square miles.

G. Principal grouse-preserving areas. On these, and also on some very large estates, the numbers of keepers may be higher than shown on this map.

Mice and rats invaded man’s home as soon as there was a home to invade. They also lived in his grain stores and farm buildings. Early man tried to make his granaries rodent-proof, sometimes with remarkable success. Control by trapping and poisoning was generally inefficient, and the rodent population in contact with man was roughly a measure of the amount of food he made available. A mouse-proof larder is more effective than an apparently efficient trap. The domestic cat was probably the most useful method of local control, though cats, like other “pesticides,” have had their side effects. Those that have escaped and become feral have important effects on other wild life. Modern methods of rodent control are much improved, but these animals still do much economic damage in our cities and on our farms to-day. It is perhaps surprising that the black rat (Rattus rattus), which is found mainly in towns, where it is particularly at home in hot-water ducts in tall buildings, was our “original rat,” though some think even it only arrived about A.D. 1200, and the brown rat (Rattus norvegicus), which is the species commonly found in the country, only arrived in Britain in the eighteenth century. The black rat was apparently driven from the rural haunts by the brown invader.

Pigeons and sparrows are serious agricultural pests, against which no really satisfactory control measures have so far been devised. Pigeons become more numerous each year. This increase is probably due to the increased amount of winter food, particularly clover leys on farms, that is available. The recent fall in the number of hawks may also have had some effect. Organised shoots give some sport to the participants, but have negligible effects on the pigeon population. Work on poisoning or narcotising pigeons is progressing, but the danger to other and more desirable species of birds is difficult to prevent. Sparrows probably increase because suburban householders feed them in winter. This enables them to survive in cold weather, and the increased population does more harm on the nearby farms to which it migrates in summer and autumn. Again shooting and trapping has little effect.

Farmers and others were then soon aware that wolves and other large mammals and birds might be pests, competing with them in various ways, even if they often overestimated the damage done by the carnivores and sometimes underestimated the amount of food taken by rabbits and other herbivores. They took active, if sometimes misdirected, steps to control these animals. On the other hand, they almost always underestimated the harm done to their crops and to their health by insect pests, and it is only in recent years that serious attempts have been made at control in this direction.

In Britain we do not have plagues of locusts, which in many countries can consume the whole of a crop, but it is estimated that to-day some £300,000,000’s worth of food is lost each year because of pests of crops and insect damage to farm stock. This sum is almost exactly the same as is spent annually on the support of agricultural prices (“farm subsidies”). Some insects, like the caterpillars of Cabbage White butterflies, eat crop plants, reduce the yield and make many plants unsaleable. Other insects do little damage themselves but carry organisms which cause diseases. Thus aphids carry the virus causing virus yellows in sugar beet; this can seriously reduce the value of the crop. For many years farmers and gardeners accepted insect damage as something they could not prevent. They learned by experience that it could sometimes be avoided or reduced to a minimum by timing their operations carefully. Thus if broad beans in the garden are sown early, seed is set before aphids (“black fly”) are numerous and a good crop is obtained, but in two years out of three a late sown crop will be smothered by insects and prove a failure. Cultural devices such as this are valuable and important, but will seldom allow a late crop of broad beans to be produced. Field beans, which flower and set seed over a long period during the summer, cannot be successfully grown in a way to avoid attack in a bad aphid year. Resistant strains of crop plants have been recognised and used for many years, and in future are likely to prove very important, where resistance is linked with high yield and quality. Farmers generally prefer to be able to grow the most profitable crop at the most convenient time and wish to attack pests by any possible means. Before 1939 most effective insecticides (except general poisons very dangerous to man) could only be produced in relatively small quantities and at a price which made their use on many crops uneconomic. The farmers therefore have welcomed the synthetic insecticides which can be produced in unlimited quantities and which, at first at any rate, seemed the perfect answer. The dangers from their use are described in later chapters.

Insect pests of crops were soon recognised as such, even if little was done about the problem until very recently. The importance of insects as vectors of disease has only been understood for about seventy years, though man and his habitations have provided niches for troublesome parasitic species in the same way that wild animals have supported their own parasites. Man has sometimes controlled insects of medical importance effectively without understanding the problem. Malaria was formerly widespread in Britain, but it was almost eliminated long before man knew the parasitic organism concerned or that it was carried only by the Anopheles mosquito. This was because man avoided marshy areas, thinking that malaria was caught from the “bad air,” and so he kept away from the breeding places of the mosquitoes. He also drained the swamps, usually to produce better agricultural land, but in so doing he got rid of the insects which carried the disease. Incidentally naturalists are now very concerned at the continued draining of marshes and swamps, which are now the last refuges of many species of wild life. This is just one of the ways in which non-chemical pest control can have effects which have end results which may be as disastrous to wild life as the most indiscriminate use of chemicals.

In the Middle Ages most people of all ranks of life harboured body lice on their persons, and as recently as 1940 the majority of the girls in our industrial cities had lousy heads. Personal cleanliness could eliminate these pests, except where conditions were grossly overcrowded, but in war and after any disaster infestation, and the risk from louse-borne typhus fever, grew. Persistent insecticides now control these insects, and properly applied to man and his clothing they present little danger to any other organisms.

Pest insects which attack man, but depend on his having a permanent home, include fleas and bedbugs. Fleas are not only a nuisance, but also carry plague, a disease which died out in Britain many years before effective insecticides were discovered to control the vectors. Improved hygienic conditions rather than chemicals have made fleas uncommon insects.

Human bedbugs are very similar to the species which attack bats and swallows, and primitive man may have become infested first when he also lived in caves. Bugs were common from the earliest times in the warmer parts of the world. However, Britain had no bedbugs before the sixteenth century, possibly because the houses were too cold. Bugs certainly existed in Italy in classical times. When bugs arrived in Britain, they soon spread through overcrowded slums, and before the 1939 war most houses in our cities, except detached surburban villas, harboured at least a few. However, they were only common in unhygienic and overcrowded dwellings, and improved conditions soon reduced their numbers. Modern insecticides, particularly those which put a persistent film in the cracks which the bugs haunt, control the insects effectively without seriously contaminating the environment.

When his numbers were few, pollution was not a serious problem to man. Many pests, both plant and animal, have become common only because man has produced suitable conditions. In some cases pests have been controlled with little harm to the environment, in others pest control has become a new and potent form of pollution. The great difficulty is to assess accurately just how much pollution affects the environment and the plants and animals it contains. We are seldom able to give simple answers. Sometimes an animal has obviously been killed, perhaps from the effluent from a factory, perhaps by accidental contamination with a pesticide. Generally, however, we have to depend on circumstantial evidence of damage, and this is the reason for the controversy which so often surrounds our subject.

Toxicology is difficult and complicated. The results of analyses of animals’ bodies, where traces of poisonous substances are found, are not easily interpreted. In the case of well-known poisons like arsenic, strychnine or cyanide the situation may be less mysterious if large amounts are found. We know, for instance, that if a man eats five grams of lead arsenate he is likely to be fatally poisoned. If the pathologist finds ten grams of lead arsenate in the stomach of a corpse, he will be almost certain that this poison caused death. If he finds only a few milligrams, he will be almost certain that death was due to some other cause. The finding of intermediate amounts makes diagnosis difficult. Consideration must be given to the site where the poison is found in the body, and to losses due to vomiting or excretion. The situation is even more complicated where poisons are broken down in the body, either as part of the process of damaging the victim, or due to post mortem changes. If we do not know accurately how toxic a chemical is to a particular animal, and if we are not fully familiar with these chemical changes, we cannot usually say for certain whether a small residue of poison in a live or dead specimen has any significance.

It is generally fairly easy to establish the acute toxicity of a substance, that is the amount which, in a single dose, is lethal. Experiments with rats, chicks or fish are commonly made. Groups of animals are given different doses, and the least amount of poison which kills is found. Usually different individuals of a species show a somewhat varied susceptibility, and instead of determining the amount which kills them all, the so-called LD₅₀, that is the amount which kills half of a batch, is determined. In most instances few animals die from a single dose of half the LD₅₀, and twice the LD₅₀ is likely to kill almost every individual. However, this is not always the case. Sometimes a population contains a few individuals which can survive relatively large doses of certain poisons; under certain circumstances these may be selected out and may breed a strain which is more resistant than the normal to a toxic substance. Resistance or susceptibility to poisons is not necessarily correlated with unusual or subnormal “vigour,” and this type of chemical selection may leave a species less well adapted to normal environmental conditions.

Although acute toxicity is not difficult to determine in the laboratory, it can only be done with a limited number of species, and values for others (including man) can usually only be inferred. Also the effects of a specific poison may differ even with the same batch of the same species depending on how it is administered, e.g. neat, in suspension, in oily solution, on an empty stomach, through the skin, by inhalation and so forth. These difficulties have usually meant that, at least where man is exposed, a fairly large “safety factor” has been applied. Thus if work with rats suggests that the LD₅₀ for substance “X” is 50 milligrams per kilogram, it could be assumed that half of a group of 50 kilogram men would probably die if they ate one gram each of “X.” It would generally be found that a single dose of one hundredth of this amount, i.e. of 10 milligrams, would be unlikely to be harmful. In many cases this assumption is quite justified but contamination of food to this extent would not normally be tolerated.

While there are sometimes difficulties in establishing the effects of single, large doses of poisonous substances, the study of the effects of repeated small doses, each of which would probably be harmless, spread over long periods, presents even more serious problems. Poisons which are unstable are unlikely to be very dangerous under these circumstances. Those which are stable, particularly if they are stored in the body, may present great risks even if they are not acutely poisonous in single doses. All these factors are borne in mind when, for instance, new insecticides are tested. Their action on a number of insects, particularly pests, is determined. Then long-term experiments, lasting over severa, years and a number of generations, are then made with rats, chickens and other animals. It is obviously impossible to include more than a few species in such trials, so it is not surprising that sometimes a desirable species of bird, or mammal, is found (too late) to be unexpectedly susceptible. The effects of chronic exposure to low-level industrial and urban pollution is even harder to study. Some, impressed by the complexity of the situation, fear that the ecological effects of pesticides may bear little relation to their gross toxicity.

Everyone wishes to abolish the damage which may be caused to man and to wild life by pollution from every source. As, however, we are not always agreed as to when damage is being caused, or how exactly some obvious damage arose, an easy solution will not be found. Man has always polluted his environment; he has always suffered from pests, but because of the “population explosion,” these problems have become more serious in recent years. The need for more research in these subjects is obvious, if irreparable damage to wild life, and to man, is to be avoided. Equally important, we must make sure that the results of such research are quickly and efficiently applied.

CHAPTER TWO AIR POLLUTION

Perhaps the most obvious way in which man has contaminated his environment is by polluting the air with smoke and with the waste products from industry. Everyone has seen the pall of smoke hanging over a city. He knows that many plants and animals are not found in the middle of a city. It is, however, difficult to find exactly how this pollution has affected wild life, notwithstanding much intensive study of the subject. Although some lichens and other plants seem to be particularly susceptible to the effects of atmospheric pollution, and their distribution may be correlated with it, nevertheless the position is far from simple. This is perhaps not surprising, as we seldom have a constant amount of any noxious substance in the air at any place over any long period of time. The smoke emitted from a domestic fire or from a factory is in bursts followed by periods of comparative inactivity; in some towns factories are only allowed to give out black smoke for five minutes in an hour. The weather has a profound effect; calm clear periods, particularly when temperature conditions prevent upward circulation, allow the pollution to concentrate, while strong winds ventilate the area though they carry the substances in detectable amounts to distant parts of the country.

As soon as man discovered fire, he made smoke and so polluted the atmosphere. The effects were local and slight until about the thirteenth century, when coal fires in cities were found to produce winter fog and punitive laws were introduced, apparently with little permanent effect. As cities were small, and little coal was burned, probably no great damage was done except perhaps to men themselves living in unventilated houses. When cities grew, smoke became, and still is, a major problem.

Industrial development in the nineteenth century was accompanied by new types of pollution. Hydrochloric acid gas from alkali works caused a public outcry, with resulting legislation. Attempts have since been made to restrict all the emissions from factories to a “safe” level. This happened none too soon. Much of the gross pollution accompanying the dereliction in areas like the lower Swansea Valley was airborne from factories in the area.

The results of atmospheric pollution differ in an interesting way from those of insecticides which are discussed in later chapters. Man himself has been the major victim of polluted air; insecticides have had serious effects on wild life, but man has seldom been injured by the direct effect of these substances. The ecological significance of this difference is discussed in later chapters.

Every urban housewife is only too well aware of the reality of atmospheric pollution. Curtains and furnishings remain clean for months or years in the country; in the towns they are grimy in a matter of days. Students of pathology who have only seen inside the corpses of city-dwellers are amazed, and think they have found some new disease, when they see for the first time the healthy red lungs of a farm worker who has never lived in or near a town. Walkers on the moors of the Peak District know that their clothes will be blackened if they sit on the heather, and most flocks of sheep there, except immediately after shearing, seem to consist only of black sheep. The Peak District sheep on moors surrounded by industrial towns contrast with the much whiter animals found in the remoter highlands of Scotland, and this colour difference has been suggested as a rough and ready means of estimating pollution.

Air pollution in Britain to-day is mainly due to burning coal and oil. Local effects from many chemical processes, and petrol and from diesel engines also make their contribution. Perhaps the most serious chemical problem is due to fluorine, mainly from brick works, and this is specially mentioned below. Legislation and regulations have reduced the amount of many pollutions to such an extent that wild life is usually not seriously harmed, except in particular danger areas, but the amounts of dust, smoke and sulphur dioxide produced from fuel are so enormous and so unaesthetic that they cannot be ignored.

Britain consumes annually about 200,000,000 tons of coal and 25,000,000 tons of fuel oil. The output of noxious products is estimated at 1,000,000 tons of dust, 2,000,000 tons of smoke and over 5,000,000 tons of sulphur dioxide. Coal produces relatively more smoke and dust, and oil more sulphur dioxide. This pollution is obviously very unevenly spread over the country. The Ministry of Technology, formerly the Department of Scientific and Industrial Research, compiles reports from some 2,111 recording instruments spread all over Britain. These show that in heavily industrialised areas over 1,000 tons of grit and dust must fall on each square mile in a year; this corresponds to about two pounds on each square yard. In cities generally the figure is in the region of a quarter of a pound, and in rural districts it may be less than a tenth of an ounce. Sulphur dioxide, being a gas, is dispersed more readily, and the rural concentration is probably about a tenth of the urban or industrial figure, though under unfavourable conditions much higher values may be obtained adjacent to some factories. The housewife knows that polished silver or copper tarnishes more quickly in the town than in the country; this is correlated with the SO₂ in the air.

The effects of industrial pollution on man have been studied intensively, but with somewhat confusing results. It is believed that the four-day “smog” in December, 1952, killed some 4,000 Londoners. Exactly how smog, which is looked on as a brand of fog containing more contaminants and smaller and more penetrating particles, kills is not understood. It may act as a general irritant which acts as the “last straw” in the weak and those with respiratory trouble. It has been suggested that the excess of free sulphuric acid is the lethal factor, but total amounts are small (only 0·05 parts per million as a maximum) and this view is not generally accepted. There is no doubt that smog is a killer, and it kills other animals than man if they are exposed (many cattle died at the 1957 Smithfield Show), but fortunately it does not often spread outside our largest cities. Mist, which consists of relatively clean water particles, is of course widespread. Fog, which is essentially mist containing amounts of smoke, penetrates some distance from industrial areas, but seems to have comparatively little acute effect on man or animals.

Acute effects on man and animals of smog, and possibly of fog, can be shown to occur even if they cannot be fully explained. Chronic effects of the usual urban levels of pollution no doubt occur, but are not so easily demonstrated. Lung cancer is higher in cities than in the country, but we do not know the precise cause. Respiratory diseases are similarly commonest in industrial areas. Although we ourselves filter the air we breathe and reject much of the dirt, city dwellers’ lungs are impregnated with dirt particles, and it is difficult to feel sure that this is not harmful. For these reasons considerable efforts are being made to reduce atmospheric pollution. “Smoke-free” zones have been scheduled in most cities, and some progress is being slowly achieved to reduce the smoke and dust. Fogs and smogs are less serious than they were, though the amount of sulphur dioxide in the air is less easily controlled and tends to increase even in smoke-free zones.

Farmers near to cities suffer from the effects of smoke and grime. It has been estimated that pollution, by damaging pastures in particular, costs the East Lancashire farmers over two and a half million pounds a year. Horticulturalists find that smoke reduces light intensity indoors and out, and obscures the glass of greenhouses, covering them with deposits which are difficult and costly to remove.

Smoke, by reducing light intensity, will obviously retard plant growth, and may encourage some species at the expense of others, though there seems remarkably little evidence of this happening except in industrial areas. Many city gardens do indeed suffer from the lack of light, but this is not due to pollution so much as to shading from buildings, and, more particularly, from trees. The luxuriant growth on bomb sites was a revelation to many. Here shading from buildings and trees was reduced to a minimum. Often one finds that spring flowers do quite well, before the trees are in leaf. In the confined space of a small city garden we may prefer trees to flowers, but we can seldom have both.

The effects of heavy deposits on leaves may be even more important. Evergreen species in heavily polluted areas have been shown to have a rate of transpiration of only about one tenth of normal and the leaves last a much shorter time than they do in pure air. Thus in some conifers the leaves normally live for up to eight years, and contribute to photosynthesis and growth for the whole of their lives. With moderate pollution the leaves may die and fall off in three or four years; heavy pollution may cause annual leaf fall and such trees hardly grow perceptibly and usually die. Some workers have suggested that particles of grime act by bunging up the stomata, but usually it seems that these are left patent and the effects are due to reduced transpiration, and, in some cases, to poisoning from sulphur or other substances. Deciduous trees which lose their leaves each year are often less susceptible to damage from pollution, as the leaves can complete their normal work before they are put, partly or entirely, out of action. Those responsible for planting in public parks in cities and industrial areas are well aware that spruce and firs are less likely to succeed than larch or oak. The exact way in which pollution harms trees is not fully understood.

I have already mentioned that atmospheric pollution by sulphur dioxide is becoming worse rather than better. The air in cities commonly contains 0·1 parts per million, that in rural areas 0·1 parts, but sometimes concentrations as high as 1 part per million may occur locally, under particular weather conditions, at distances from the source. Experiments have shown that most flowering plants show no damage to 0·1 parts per million even with long exposures, but higher concentrations usually cause damage such as leaf blotching and loss of yield. Some of the crop reductions on farms near towns are probably due to this cause, but it seems unlikely that there is much damage to wild life in rural areas. Sometimes this type of pollution may be economically advantageous; the absence from industrial areas of the fungus causing rose mildew is almost certainly due to sulphur in the atmosphere. This suggests that other species of fungi, which are in general much more susceptible to sulphur poisoning than are flowering plants, may be similarly affected. This could be of considerable ecological importance, but there seems little information on the subject. However, as rose mildew soon manifests itself in the suburbs, it would seem likely that other fungi, and susceptible plants of other groups, suffer little damage outside very polluted areas. Nevertheless it would be wrong to be dogmatic about this. Small quantities of sulphur or of other gaseous and solid pollutants which are dispersing through our environment may be more harmful than is generally realised.

It should be noted that although trees may suffer from the effects of pollution, at the same time they do something to alleviate this condition. It has been shown that trees growing in industrial areas may do this in several ways. They filter the air, so the deposits on their leaves are removed from general circulation. They cause turbulence and deviation of the air flow, which may help to ventilate (with less contaminated air) an area of otherwise stagnant pollution. They also remove carbon dioxide and liberate oxygen, an important function on a global scale, but, as mentioned below (see here) even seriously polluted air is almost never deficient in oxygen and dangerous concentrations of carbon dioxide are uncommon. Incidentally, in a highly polluted area where trees are likely to improve conditions, it may be best to plant broad-leaved deciduous species, which are harmed less than evergreen conifers, even though they have less effect in winter when the branches are bare. Conifers will be more efficient, and in some circumstances may be used and considered as “expendable.”

Motor vehicles are responsible for widespread pollution in town and country. The exhaust gases contain a high concentration of carbon monoxide, which is very poisonous to mammals and birds. This gas may reach dangerous levels, particularly to car drivers in traffic blocks in towns, but it is probably dispersed too rapidly in the country to have an appreciable effect. Some three thousand tons of lead are emitted with the exhaust gases of cars in Britain each year. This has been found to accumulate in the vegetation and soil along roadside verges, and although serious damage has been seldom reported up to now, a dangerous concentration could build up locally over a period of years. Lead could possibly enter food chains and have damaging effects far from the source of pollution.

Carbon dioxide is another common constituent of the exhaust from fires, factories and vehicles. It has seldom been found in the high concentrations which are harmful to life, and its presence may even promote plant growth in the way it has been shown to do when CO₂-enriched air is pumped into a glasshouse. Thus if the CO₂, which is normally only some 0·03-0·04 per cent of the total air, is increased to 0·15 per cent, the rate of photosynthesis in a glasshouse may be more than doubled, and crop yields can be substantially increased. The effects of CO₂ from industrial pollution on outdoor crops and on natural vegetation have not yet been thoroughly investigated. It is possible that quite small differences in CO₂ may affect the whole pattern of vegetation by stimulating one species of plant more than another. More work on this problem is clearly required.

Recently it has been suggested that CO₂ may eventually have a drastic effect on world climate. Coal and other “fossil fuels” are being burned at such a rate that the CO₂ content of the whole atmosphere may be raised as much as 25 per cent by the year A.D. 2000, and the level will probably continue to rise. The effects of this are not fully understood but some scientists think the temperature and other properties of the stratosphere may be affected. This could alter the world’s radiation balance, possibly melting the polar ice cap. So far little or nothing has been done to reduce the output of CO₂, though some research on ameliorating its possible effects has been suggested. So far most scientists have thought that CO₂ pollution was of little importance; it now seems possible that it may cause greater changes to the world than any other man-made factor in our environment. On the other hand, this may be a completely false alarm.

Ozone, the form of oxygen with three atoms in the molecule (O₃) instead of the normal two (O₂), occurs naturally in tiny quantities, and pollution, particularly from motor vehicles, may increase the amount. As little as one part of ozone in 10,000,000 parts of air has been found, in the U.S.A., to harm many plants and trees, and such ozone poisoning is said to be important in both California and Connecticut, in which state an annual loss of $1,000,000 to vegetable crops is reported. So far, I know of no cases of ozone damage to vegetation in Britain, but with the increasing number of motor vehicles it seems likely to occur either now or in the near future.

Air pollution also affects the soil. Near cities the soil is often considered to be “sour,” because of the sulphur dioxide and other acid-forming substances washed in by the rain. This effect probably does not extend very widely, but many of the chemicals found in rain-water may come from industrial pollution. In the moorland areas of the Pennines we know that the rain brings in substantial quantities of minerals, which contribute to the fertility of the soil. Much of this comes from the ocean, but some from pollution, which here may be having an advantageous effect. The quantities of nutrients are significant, but probably not sufficient to have detrimental effects such as those produced by similar nutrients in much larger amounts in purified sewage, which upset the balance in many rivers (see more).

Botanists have studied the effects of pollution on a wide range of plants, mostly with inconclusive results. They have attempted to find “indicator species” which may be used to measure pollution. Such a species would only grow where pollution was below a certain level. The most successful work has been with lichens. Several species of lichen are absent entirely from the industrial areas of high pollution, and reappear on the outskirts. This problem has been studied in Northern Ireland, near Belfast, and around Newcastle upon Tyne. Fig. 3 shows how the lichen cover of tree trunks increases from the city centre of Belfast to its outskirts. It has been reported that the habit of growth of individual species was affected, so that some seemed barely able to exist where others grew normally. The subject is, however, not an easy one. It is necessary to be competent to recognise individual lichen species accurately, and to distinguish these in their sterile sorediate forms which often occur under unfavourable conditions. I must confess that I personally have been disappointed by the potentialities of this group. After reading in the literature that “a salient feature of lichen’s ecology is that these plants are very scarce in the neighbourhood of towns” I visited the Lower Swansea Valley (Plate 1), perhaps the most polluted area in Britain. My first impression of the soil was an almost pure culture of lichen, and a wooden railway bridge was equally encrusted. These were of course resistant species easily recognised by a specialist, but showing that the method used in Belfast and illustrated in Fig. 3 cannot be generally used except by experts. In time botanists may find other plants which are better indicators; in fact there has been some progress in this field, but the confusion to-day relating to the effects of pollution suggests that unless it is very severe it may not be a factor of major importance in the ecology of most regions.

Fig. 3. Increase of lichen cover outside the city of Belfast. (After A. F. Fenton.)

One particular element – fluorine – requires special attention. Fluorine occurs in minute quantities in all plants and animals, and it is one of the essential elements of protoplasm. If the natural level falls below a minimum, and this occurs in nature, harmful effects may be seen. One (but only one) of the reasons for the poor teeth found in many parts of Britain and North America is that the natural water may have a very low fluorine content, less than one tenth part per million, and combined with a “sophisticated” diet this may cause fluorine deficiency. A tiny additional amount, up to 1 part per million, may then be added to the water, and this has been found to improve tooth structure in children and reduce dental decay. This is one instance of a general principle, that a substance essential in small amounts may be toxic when the proper level is exceeded. The toxicity of fluorine in larger doses has made some people oppose the addition of this element to water, though there is no evidence that drinking water with 1 part per million ever does harm.

Fluorine occurs particularly in the smoke from brickworks, which are often surrounded by agricultural land. Other industries, including iron and aluminium production, are also important in this connection. Unlike active organic poisons, which may break down quickly to harmless substances, once fluorine has contaminated an area it remains a danger until it is physically removed. Fluorine only damages plants at relatively high concentrations, though it is at least ten times as toxic as sulphur dioxide. However, phytotoxic concentrations are rare, even near to industrial sites. The main danger from fluorine is that after deposition it is concentrated by growing plants. For example, grass has been found with as much as 2,000 parts per million. If this grass is eaten by stock, or by wild animals, they will certainly be seriously affected and will probably be killed. Lower concentrations have less drastic effects. The first symptoms of fluorosis are dental; the teeth are rough and mottled. Bigger doses cause bone abnormalities, lameness and general loss of condition. I know of no reports of fluorosis affecting wild life, but small mammals in affected areas are certain to suffer. Its stability, and the way it is concentrated by many food plants, makes fluorine a potential danger anywhere near a source, and abnormal weather conditions and air currents could affect vegetation, and thus animals, over a wider area. Fluorine seems a rather special, and dangerous, case of a poisonous substance entering the atmosphere, but it should make us more careful about accepting pollutions which may contain other, as yet undetected, dangers.

Air pollutions can thus have acute effects, when intense in industrial regions. They can have chronic effects, which may extend further from the source. In these cases emissions are acting as poisons, and the effects depend on the susceptibility of different plants and animals. In general, wild life, being remote from industry, would seem to be little harmed. However, there is one other way in which air pollution affects wild life, indirectly, by altering the physical environment.

We have noted that as much as two pounds of dirt may be deposited in a year on a square yard of ground near a factory. On the outskirts of our towns, the amount is perhaps an ounce. An ounce is quite a large quantity, more than the weight of pigment necessary to turn a blank paper into a valuable painting. Sheep within a considerable distance of industrial towns are black, and so are tree trunks and most animate and inanimate surfaces after a few months’ exposure. We find that a number of different species of moths, which are normally pale coloured in unpolluted districts, are usually represented by melanic forms which are black or at least much darker than the “normal.” This phenomenon of industrial melanism has been fully reviewed by Dr. E. B. Ford in his book Ecological Genetics, so there is no need to go into details here. It has been established that various moths, and the Peppered Moth (Biston betularia) has been most fully studied, have evolved melanic races which are adapted to their new surroundings. In clean areas, where tree trunks are covered with pale lichens, the typical form of the Peppered Moth is difficult to see. The melanic form is very prominent. This difference is not only apparent to man, but to birds which prey on the insects, and readily take them when resting on trees. In industrial areas, where the trunks are blackened and lichens are comparatively scarce, the melanic form is inconspicuous and is preyed upon least. This phenomenon has demonstrated that evolutionary changes may be more rapid than had previously been imagined. Not all evolutionary changes have such obvious morphological differences as we find in the Peppered Moth, and differences in physiology or behaviour may be selected and perpetuated by pollution, with important effects on wild populations which may spread outside the area in which they first become apparent. Thus many types of organism may be changing to-day, as a result of industrial pollution, with far-reaching effects which we do not yet suspect.

Man-made air pollution occurs where man is most numerous, so we are the species most affected. For this reason we take many steps in the attempt to safeguard our own species. Nevertheless it is man who normally is subject to the highest concentration of pollutants, so that he can be said to be acting as a “guinea-pig” for wild life. This is the reason why the countryside is not more seriously damaged though there is no excuse for complacency, or for underestimating the damage in urban and industrial areas. Suspicions that sulphur dioxide and other substances may be more harmful than is at present accepted may make us even stricter in our controls. Pure air in an industrial civilisation is expensive, but it is possible. Already our larger chemical manufacturers have spent millions of pounds on reducing air pollution. There are even vested interests at work. I saw recently a paper entitled “Long-range economic effects of the 1964 Clean Air Act”; I expected it to deal with improved agriculture and health. In fact it foretold up to fifty per cent increases in sales for equipment to control air pollution! Let us hope this target is reached.

Nevertheless we find it difficult to deal with one form of atmospheric pollution, that is with unpleasant smells. Man is not considered to have his olfactory senses particularly well developed as compared with some other mammals, yet he can detect the presence of many odours at a concentration which cannot easily be confirmed by methods of chemical analysis. Anyone who has suffered from smells from farmyards, manure spreading, piggeries or even from chemical factories knows how difficult it is to have such a nuisance abated. He will probably be told that he will soon “get used to it,” and is only certain of more serious consideration when poisonous substances can be detected in amounts which can be shown to be dangerous. The difficulties of stopping intermittent smells being given out from farms or factories are such, and the legal costs which may be incurred without the certainty of success (and then with the prospect of paying the legal expenses of the persons causing the smell) are so great, that many people sell their houses at a loss (hoping that prospective buyers will call when the wind is in the right direction) and move away to another district.

If other mammals have a so much keener sense of smell, they must be even more distressed, perhaps by odours to which we do not object or which we cannot detect. I know of no proof of animals leaving an area because of a smell which is also not toxic, but it seems probable that this sometimes happens. On the other hand the stench in the dens of some carnivores suggests that they are even more tolerant than man of some types of smell.

There is one important point about air pollution which is not always remembered. People complain, usually quite wrongly, that polluted air is short of oxygen, and they believe that they inhale more of this vital gas in the country or on the top of a mountain than when in a town. In fact there is little change in the amount of oxygen in the air even in the stuffiest room; there is certainly more in a crowded lecture room in London than in the rarer, though purer, air at the top of Ben Nevis. Industrial pollution, except for the undiluted exhaust gases from chimneys and engines, hardly reduces the amount of available oxygen. Carbon dioxide, present in pure air in very small quantities (approx. 0·03 per cent) is indeed increased by pollution, but seldom if ever to a concentration which is harmful to animals, and it may even stimulate plant growth. Man’s breathing is upset by air containing 7 per cent of CO₂, and 14 per cent breathed for some minutes can be lethal; such levels of pollution have never been recorded except in such enclcsed spaces as fermentation chambers in breweries. “Stuffiness” is experienced in crowded rooms, but this is not due to the shortage of oxygen or the amount of carbon dioxide. It is due to very small amounts of organic substances given off by the other occupants of the room (“B.O.”), and to shortwave radiation from the walls and people themselves. Many Englishmen – and even more Englishwomen – think a room is stuffy and “polluted” simply because, for once, it is comfortably warm! Polluted air is usually “normal” air, in so far as its content of oxygen, nitrogen and carbon dioxide is concerned, plus the addition of small quantities of added materials. Polluted water, as will be seen in the next chapter, may provide quite different problems.

CHAPTER THREE WATER POLLUTION

As was seen in the last chapter, air pollution has proved difficult to study, and many conflicting results have been obtained. Water pollution seems to have provided a more satisfactory topic for investigation. This is not because the subject is simpler; in fact in some ways it is even more complicated. Air almost always contains sufficient oxygen to sustain life, and “pollution” only means adding a lesser or greater amount of some foreign substance to an otherwise wholesome atmosphere. Water, on the other hand, may be greatly depleted of oxygen so that it cannot sustain most kinds of life, or it may have various substances added, so that animals and plants are poisoned. Some waters have both reduced amounts of oxygen and appreciable amounts of poisons in solution; the inter-relations between these factors, and their effects on aquatic animals and plants, may be very complicated. The advantage of water over air, from the point of view of the research worker, is probably that it forms a definite and restricted environment, from which animals cannot easily move. It is therefore possible to study the long-term effects of pollution under fairly constant conditions, and there is no difficulty in demonstrating the serious effects which pollution can produce. It may also be suggested that water is a marketable commodity of considerable economic value, while air is, theoretically at least, “free.” So although fresh water covers under one per cent of the surface area of Britain, there are probably more scientists studying its pollution than there are investigating air which covers one hundred pet cent of the globe, land and water alike.

Several excellent books on water pollution, and books on freshwater dealing authoritatively on aspects of the subject, have been published in recent years. These include The Biology of Polluted Water by H. B. N. Hynes, Fish and River Pollution by J. R. Erichsen Jones, the New Naturalist Life in Lakes and Rivers by T. T. Macan and E. B. Worthington, and Freshwater Ecology by T. T. Macan. The existence of this extensive and easily obtainable literature has enabled me to make this chapter much shorter than would otherwise be desirable, and to deal with the effects of pollution in a rather different way than would have otherwise been possible.

Man’s requirements regarding water are different from those of “wild life” generally. Man demands what he describes as “pure” water; what he really means is “safe” water. This must contain only a minimum amount of salts, and must be free from those bacteria, protozoa and arthropods which might develop in his body and cause disease. Man thus deliberately interrupts the life-cycle of many other forms of life by the various methods of purification which he uses. He is less concerned with the oxygen content of the water than are the fish and insects which live in it. These efforts to produce a pure water supply for city dwellers can even be thought of as a form of “pollution,” in that water catchments alter, and sometimes sterilise, large areas of the countryside. The conflict between Manchester Corporation and many naturalists and others over the fate of much of the English Lake District illustrates this point.

On the other hand, water which, for public health reasons, is considered to be “grossly contaminated” by sewage, may still be, from the biological point of view, a healthy and desirable environment for many animals. But by the deliberate discharge of his domestic and industrial wastes man most greatly affects streams and lakes, and so alters the whole composition of their flora and fauna.

Natural waters may not only be “impure” from man’s point of view because of the parasites they harbour; they may contain many substances, even poisons, without any human intervention. Quite high concentrations, sufficient to poison some fish and many insects, of lead and copper are found in waters which percolate through strata rich in these metals. Streams running through forests, particularly pine forests, may be contaminated with large amounts of organic matter, and the results may be quite similar to those arising from domestic pollution. As a rule a special flora and fauna is found, consisting of plants and animals adapted to such conditions, in these impure waters. Human pollution usually happens so quickly that impoverishment occurs, often without time to allow the introduction of many of these special types of organism.

Primitive man did not seriously harm the aquatic environment. He often lived beside rivers and lakes, and his waste products must have entered the water, but in insufficient quantities to have adverse effects on the flora and fauna. In fact excrement entering the water in this way no doubt contributed to its nutritive value, and the substances it contained entered into the normal cycles. In some of the less developed and less densely populated areas of tropical Africa we can see a similar situation to-day. The streams and ponds are full of healthy fish; the human beings have a rich internal fauna of parasite worms which pass part of their lives in the water, inside small crustaceans or fish. Man in this way contributes to the richness of wild life in his environment.

When man came to live in towns and cities, however, his increasing numbers had a very different effect. Sewage continued to be poured into the rivers, but the quantities were so great that most unpleasant results were obtained. By the middle of the nineteenth century the Thames, and many other major British rivers, had become open sewers. There are many accounts in the literature. I myself like the account of the Reverend Benjamin Armstrong, from his diary:

“July 10th, 1855. Took the children by boat from Vauxhall Bridge to show them the great buildings. Fortunately the Queen and Royal Princes drove by. The ride on the water was refreshing except for the stench. What a pity that this noble river should be made a common sewer.”

Practically every other river was treated similarly. Even the Cam flowing through the Backs at Cambridge was in this way abused, as is illustrated by the (perhaps apocryphal) story of Queen Victoria’s conversation with the Master of Trinity when she looked over the bridge. “What,” she asked, “are all those pieces of paper in the water?” The Master promptly replied, “Those, Your Majesty, are notices saying that bathing is forbidden.”

The results of all this untreated, or “raw,” sewage, vary greatly, depending on the volume of water and the amount of organic matter. As indicated above, small amounts of raw sewage may be actually beneficial to most forms of aquatic life. To-day in some rivers, including the Bedfordshire Ouse, the comparatively small number of boats present discharge the contents of their water closets straight into the water. This does not cause noticeable offence. In some parts of the Norfolk Broads it does, for there are many boats producing much more sewage and this is dangerous. In really crowded rivers, such as the Thames, such disposal methods are not allowed.

Sewage, in quantities which are large enough to have a biological effect, acts in different ways depending on the temperature, the nature of the water and various other factors. The most important biological effect arises from its breakdown by bacteria; this requires oxygen, and as a result the water tends to become deoxygenated, and so less suitable to support most other forms of life. Almost all pollution of water with organic matter, be it sewage, effluents from factories (particularly food factories and dairies) or sawdust and similar wood waste, has this sort of effect. Organic pollution is usually measured by the “biochemical oxygen demand test” (B.O.D.). Experience has confirmed the value of this test, in which a sample of contaminated water is incubated, in the dark, at 20°C. for five days in a closed container containing a known amount of oxygen in solution; the amount of oxygen taken up by the sample is a measure of its B.O.D. Where this is high, and where the diluting water is not present in large amounts, trouble is likely to occur.

It is not generally realised how little oxygen is present, dissolved, in any sample even of “pure” water. A litre of water, at 5°C., in free contact with the atmosphere, only contains about 9 cc. of oxygen, weighing 13 mgs. As the temperature rises the oxygen content falls, so that at 20°C. it is only about two-thirds the level at 5°C. As the rate of metabolism of cold-blooded animals may treble with such a rise in temperature, an oxygen shortage is easily produced. Air, even polluted air, is a much richer source of oxygen. A litre of air contains about 210 cc. of oxygen, weighing approximately 300 mgs., i.e. over twenty times as much as is found in the same volume of well-oxygenated water. This may help to explain why some chemicals are toxic in very low doses when dissolved in water; an aquatic animal to breathe must make intimate contact with an immensely large volume of water in order to obtain enough oxygen.

Oxygen reaches the water in two main ways. First, it dissolves at the surface from the atmosphere. Still water takes up oxygen slowly, turbulent water rushing over falls takes it up much more rapidly, for this often submerges bubbles which act as does bubbling air through a domestic aquarium. This type of solution will rarely raise the oxygen level above saturation. The second source of oxygen in water is from photosynthesis. Where there are many green plants present, during the hours of daylight the water may often become supersaturated with oxygen. Unfortunately after dark photosynthesis stops and the plants continue to respire and so actually reduce the amount of oxygen in solution. Therefore during a twenty-four-hour period some waters have a range of oxygen levels which varies enormously, from practically nil around dawn to a very high volume in the early afternoon. Many animals are adapted to life under these conditions. Some biologists have not realised that they exist, and have given too much importance to single measurements of oxygen level in samples of water, not realising that in a few hours far more or far less of the gas may be available.

The capacity of organic pollution to deoxygenate water is enormous. The sewage produced by a single human being gives rise to a daily oxygen demand of 115 gms. (¹/₄ lb.). This represents the total amount of oxygen dissolved in 10,000 litres (over 2,000 gallons) if the water is saturated. In most rivers where sewage is discharged the water, before contamination, is usually far from saturation, so an even greater volume may be affected. Some industrial wastes have much greater effects. For instance it has been calculated that the oxygen demand created by the manufacture of a ton of strawboard corresponds to the sewage output of 1,690 persons, so it could deoxygenate some 17,000,000 litres (nearly 4,000,000 gallons) of oxygen-saturated water daily. These figures are somewhat academic, as they do not allow for the considerable amount of oxygen which dissolves into moving water from the atmosphere. Were it not for this important factor almost any river contaminated with any appreciable amount of organic matter would remain completely deoxygenated; deep lakes, with little water movement, become “purified” much more slowly, and severe pollution can have permanent effects.

There is little doubt that the Thames, formerly an excellent salmon river, reached a peak of pollution, and complete deoxygenation, during the nineteenth century. It was almost entirely due to untreated sewage produced by the human population that this disgusting condition was produced. This is not surprising. The flow of the river may be as low as 200,000,000 gallons a day. The water entering the London area is already depleted of oxygen, and as it is slow-moving only relatively small amounts of further oxygen go into solution. The sewage from a population of 100,000 people would, if the water were originally saturated and if no oxygen were added (and these two factors tend to cancel out), produce complete deoxygenation. It is no wonder that much of the sewage remained undecomposed for days, carried backwards and forwards through the city by the ebbing and flowing tide. Notwithstanding the increased population of to-day the situation, through improved methods of sewage treatment, is in fact considerably improved, at least from the aesthetic, and hygienic, point of view, but the water is still frequently completely or almost completely devoid of oxygen and the fauna and flora are of the kind resistant to such conditions. Pollution is now due not only to (treated) sewage effluent, but also to a great deal of industrial waste, which presents many problems mentioned below.

Many methods have been suggested for dealing with sewage. Ideally it should be returned to the land as fertiliser; if all the salts which we pour down the drains and, eventually, into the sea could be recovered, they would replace the greater part of our imports of chemical fertilisers and might replace them in a more desirable form. Various methods of composting sewage have been devised, and successfully adopted in a few places. In China agriculture in many areas depends on the use of human excreta as manure. The main difficulty is that unless carefully done, the composting process may not kill parasitic worms and other pathogenic organisms, and the compost may be a danger to health. Nevertheless I think that eventually these problems may be solved to the benefit of our rivers and our agriculture.

At one time “sewage farms” were commonly developed. The raw effluent was run into channels and allowed to percolate into the ground. Excellent vegetables were grown on ridges between the channels. Where large areas of well drained soil were available, with no rapid percolation into the water supplies, this was a reasonably safe method, and the material was broken down by the soil bacteria in a fairly short time. An optimum addition of sewage gave maximum fertility and no serious pollution, though parasitic worms and pathogenic bacteria often fouled the vegetables which therefore needed careful cooking. However, there is an upper limit to the amount of material which can be treated in any area as over-treatment overwhelms the bacterial fauna and disgusting conditions result. As suitable ground is becoming less easily available, this method has been largely abandoned.

To-day most urban waste is dealt with before being discharged into rivers, though quite a lot of raw sewage is still run directly into the sea and into tidal estuaries. This latter procedure has in recent years been the subject of much justifiable criticism, as it has been a cause of severe health hazards as well as aesthetic unpleasantness; nevertheless it has probably contributed to the richness of the flora and fauna on the shore near to several popular seaside resorts. The usual methods of sewage treatment depend essentially on oxidation by aerobic organisms. The most widely used system includes filtration through trickling filters, which are the circular structures seen in most sewage works. They are made of clinker or broken stones, and the fluid trickles slowly through them, leaving the interstices full of air. It takes some months for a filter bed to reach its maximum efficiency. It becomes covered with many different micro-organisms which feed on, and so remove, most of the organic matter. The filter is prevented from quickly becoming clogged by their growth because insect larvae and worms also develop in large numbers and feed on the micro-organisms. Another system of sewage treatment is the active sludge process. In this the sewage is run into tanks. These are inoculated with the sludge from a previous batch (to make sure the correct micro-organisms are present) and the whole is kept stirred to ensure aeration. The organic matter is broken down as in the filters. A clear effluent, and “sewage sludge” which is dried and may be sold as a fertiliser, is produced.

These methods of sewage treatment, supplemented by filtration through sand in some cases, are remarkably successful. The greater part of the flow of some of our rivers is in fact treated sewage effluent. It is sometimes said that the water of the Thames when it reaches London has been drunk and passed through different sewage works at least five times. The result is, from the point of view of man, very much an improvement on the conditions which obtained a hundred years ago. There are, however, disadvantages in treated sewage effluent as compared with moderate doses of raw sewage from the point of view of some forms of life. Raw sewage is oxidised rapidly, but its breakdown products become gradually available over a period of several days or even longer. In even the most slowly moving river this means that they are diluted and spread over a considerable area. In treated effluent many salts, not themselves poisonous, are present, and are immediately available as sources of nutrition for plants including algae. Thus a “clean” sewage effluent can have a more rapid, and in some ways more undesirable, effect on the vegetation than the same amount of untreated sewage. This emphasises the conflict that may arise between the needs of human hygiene and the preservation of natural conditions in streams.

Severe organic pollution with complete deoxygenation of the water is an obvious and undesirable condition. Life is not completely absent. Many bacteria, some producing poisonous or unpleasant gases like hydrogen sulphide, abound. Some of the insects which actually breathe air at the water surface, such as the rat-tailed maggot Eristalis, are quite common, but insects which remain totally submerged and all fish are absent. This condition obtains in much of the Thames estuary, notwithstanding the marked improvement that has taken place in recent years.

In many rivers and streams, organic pollution is intermittent. At times it is severe, and complete or almost complete deoxygenation occurs. At other times the water is comparatively pure and oxygen is present. Such severe pollution will kill all the fish, many of the plants and most of the insects and other invertebrates. Recolonisation when it ceases occurs, but which animals and plants reappear depends on many factors. After severe pollution of a stretch of a river, the remainder of which is unaffected, recolonisation is rapid. Careful sampling has shown that some species of “coarse” fish come back even when the oxygen tension is still quite low. Fish have the obvious advantage of being quick moving and able to progress against all but the fastest currents. Many species of invertebrates cannot move so fast, and plants are dependent on water and air currents, animals and other factors for their distribution. Ecologists can quickly recognise a river which is recovering from a period of pollution.

The usual effect of organic pollution is partial, rather than complete, deoxygenation. This is a very complicated subject and for details readers should refer to the books already mentioned by Hynes and Erichsen Jones, and to the excellent work which is constantly coming from the Water Pollution Research Laboratory. It is important to remember that unless organic pollution is very severe, most bodies of water exhibit self-purification to a greater or lesser extent. Fig. 4 shows the changes in a river below an organic effluent outfall. This illustrates a case of severe pollution, but insufficient to cause complete deoxygenation. A shows how the oxygen level drops and the B.O.D. rises just below the outfall; farther down this process is reversed until the oxygen level is fully restored. B shows the parallel changes in the chemical constitution of the water. C and D show how the micro-organisms and the larger animals fare. The recovery of the “clean water fauna” will depend on recolonisation, if, as appears in this figure, it is totally eliminated just below the outfall. In some cases the purified river will still be richer in nutrients than above the point where the effluent entered, and the level of the clean water fauna may be actually enhanced. This indicates how moderate pollution, from a small village, for instance, may have little permanent harmful effect. With the growing population of Britain, however, it seems that other methods of disposal than the rivers will always have to be used if the water is to be kept safe for wild life; if it is only to be drunk by man such high standards are not required!

So far we have mainly considered pollution due to organic waste, and consisting of substances which in themselves, and in small quantities, are harmless or even beneficial to life. Many effluents particularly from industry contain toxic substances. Some of these, including phenols and thiocyanates, are usually broken down by bacteria, particularly if they are mixed and diluted with ordinary sewage which, of course, promotes rich bacterial growth. Many, but by no means all, toxic organic substances are affected in this way, but metallic poisons generally pass through filter beds without loss of toxicity. Some metals are remarkably toxic to certain forms of life. Thus copper is used to keep ponds free from algae, when 0·5 parts per million is often effective. Fish survive just over 1 p.p.m., and 2 p.p.m. is tolerated in human drinking water. Zinc affects certain invertebrates at widely different concentrations, some snails are killed by 0·3 p.p.m., some insects survive 500 p.p.m. Much more work is necessary on the long-term effects of metals at low concentrations, not only on fishes and invertebrates, but on man. Recent findings on the effects of low doses of lead on man are disquieting and more harm may be being done to other forms of life than is usually recognised.

One group of organic pollutants which has received special study is the synthetic detergents. They illustrate how serious damage can be done to amenities and wild life by the unexpected persistence of substances not originally expected to be harmful. The oldest known detergents, the soaps, are made from alkaline salts and certain (weak) fatty acids. The soap which went down the drain was broken down or precipitated in the sewage works and was never thought of again; it did little or no harm. Soaps have the disadvantage that they are relatively insoluble in hard water. Since about 1918 a series of synthetic chemicals has been developed which does not have this disadvantage. Various different chemicals have been successfully used as detergents. The housewife, in her home, has no complaint, and has even come to accept the illusion that they make her washing “whiter than white” when optical whiteners (which have not been shown to have any biological disadvantage nor to make the removal of dirt from clothes more efficient) are included. The trouble has arisen in recent years from the strikingly obvious effect of running the sewage effluent into rivers. As soon as a river has run over even the lowest weir, causing a small amount of turbulence, the detergent has produced enormous quantities of persistent foam which has sometimes caused trouble by blowing in large lumps the size of footballs into crowded streets. The apt name of “detergent swans” has been applied to these aggregations. One interesting point about their occurrence should be noted. The foaming is often worst a long way below the point where the sewage works discharges its effluent into a river. This is because foaming is least in dirty water. It is not until some degree of self-purification of the river has taken place that the maximum foam-production is possible.

Fig. 4 The effects of an organic effluent on a river below the outfall. A and B physical and chemical changes, C changes in micro-organisms, D changes in larger animals. (From H. B. Hynes.)

The aesthetic damage by detergent foam is obvious. Its biological effects are less easy to determine. Foam blowing from sewage works has been shown to carry pathogenic bacteria and worm eggs, and so is a hazard to human health. Some rivers contain intermittently as much as ten parts per million of detergent without apparently doing a great deal of harm to the flora and fauna, or to the humans who use the river as their water supply. However, these are usually rivers which have to start with a fair degree of pollution and a sparse fauna and flora. It is known that as little as 0·1 p.p.m. of detergent almost halves the rate at which a river takes up oxygen, and so small residues greatly slow down self-purification. Sensitive fish, like trout, are affected by concentrations as low as one part per million, and show symptoms similar to asphyxia. It seems likely that even a very small amount of detergent in a clean upland stream would have a severe biological effect on the sensitive plant and animal life; contamination of such streams from upland farms and cottages must occur.

The economic effect of detergents in sewage works is serious. These substances reduce the efficiency of the filter beds, which must be extended considerably if the effluent is to be maintained at a given standard of purity. Where this fact has not been realised, strongly polluted effluents have sometimes been accidentally discharged into rivers. The reason detergents are persistent, and the foam such a nuisance, is that the molecules are very stable, and that they are not broken down quickly in sewage filters or in the rivers into which the sewage effluent is run. The most troublesome detergents include substances like sodium tetrapropylene benzene sulphonate (TBS); it has a molecule with many branches in its carbon chain (Fig. 5) and this is associated with its resistance to bacteria. Other substances which act as anti-foaming agents, including kerosene, have been added to effluents to prevent foaming; unfortunately they do not remove the detergent, only mask its presence, and may actually aggravate the pollution. Recently entirely new chemicals, as efficient in washing clothes, but much more easily broken down by bacteria, have been introduced. These have straight carbon chains and include Dobane JN sulphonate, and finally sodium alkane sulphonate (SAS) which is at least 99 per cent broken down as it passes through the sewage works. The breakdown products are, as far as is known, non-poisonous. Already some countries, including Western Germany, have made the sale of the so-called “hard” detergents, i.e. those with branched-chain molecules which are so stable, illegal, and some progress with their replacement by the “soft,” straight-chain substances has been made in Britain. It therefore seems possible that in a few years this form of pollution may have disappeared. However, there are many useful lessons to be learned from this subject. Detergent pollution might not have been noticed but for the appearance of “swans.” Other cases of pollution may go undetected when new substances are used, for instance in industry, so constant monitoring of effluents and the rivers into which they run is obviously necessary.

Other very stable substances are found polluting water, but with more serious results than those resulting from the presence of the relatively non-poisonous detergents. The most serious effects have resulted from the presence of the chlorinated hydrocarbon insecticides, which are both persistent and poisonous to most forms of life, but particularly to fish and aquatic insects. This subject is discussed in detail below (see here), when consideration is given to the whole question of environmental pollution by pesticides.

Many rivers to-day are altered by industry, particularly by electric power stations, by having the temperature of the water raised. Where this warming accompanies organic pollution, the effects are greatly increased, as the oxygen level is lowered while the rate of metabolism of the bacteria and other forms of life is increased. In some cases different animals and plants, more adapted to warm conditions, have colonised regions where heated effluents are discharged. This heating of rivers and lakes is something which is likely to increase with growing industrialisation, and it needs to be watched from many points of view, including that of the preservation of wild life.

Rivers may also be cooled, if heat pumps are introduced to warm our cities. These installations remove the heat energy from the water, and pilot plants have reduced the temperature by two or three degrees. This will probably slow down many biological processes, but it is unlikely to have a very great effect on the composition of the flora and fauna.

It is difficult to foresee just what will happen to the lakes, streams and rivers of Britain in the future. So long as sewage and industrial effluents are discharged into our rivers, these will cease to have their “natural” fauna and flora, even if the more unpleasant symptoms of pollution are no longer tolerated. If no effluent were to be discharged, under present conditions, many of our river beds would be dry for most of the year. The policy of drawing water supplies from the lowest reaches of the rivers instead of from the cleaner streams above the towns is one which appeals to the conservationist more than it did to most water engineers in the past, though changes in policy now bring these different organisations closer together. Certain river authorities now wish to use their upland reservoirs for storage only, discharging them into the rivers which act as channels to bring the water to the lowland towns. This means that the catchment areas can be used for recreation and agriculture, as pollution is less important when the water, abstracted further down, must be purified anyhow.

Fig. 5 Chemical structure of “hard” and “soft” detergents.

A: TBS (hard)

B: Dobane PT (hard)

C: Dobane JN sulphonate (soft)

D: Sodium alkane sulphonate (soft)

If sea-water can be economically desalinated, the pressure on our fresh-water supplies will be eased, and further improvement of their purity will be possible. It may be difficult fully to restore conditions in lakes which have become polluted, and every effort should be made to prevent the discharge of even the “cleanest” effluent into such places. So far comparatively few species of animals or plants have been totally exterminated from fresh water in Britain, and streams which have been cleaned up have usually been recolonised with the appropriate forms. This may not happen in the future unless every effort is made to render our fresh waters not only safe but also pure.

CHAPTER FOUR RADIATION

Since the first atomic bombs were dropped on Japan in 1945, we have been aware of the dangers of “atomic radiation.” Radiation, in sufficient quantities, is clearly dangerous to human and to other forms of life. Man has polluted the earth by releasing radiation and radio-active materials as the result of testing nuclear weapons, by accidental discharges from nuclear power stations, by the waste products from these power stations, by the use of radio-active substances in industry, in research and in medicine and by the use of X-rays in clinical diagnosis and in the treatment of disease. My task here is to assess the danger to man, animals and plants from the pollution that has so far occurred, and to discuss possible future risks from this source.

This is not the place for a detailed discussion of the nature of radiation, but some account is necessary for readers with little background knowledge of the subject. Atomic radiations, which are physically of several different kinds, some consisting of electromagnetic waves and some of bits of atoms moving at high speeds, all have similar chemical and biological effects. These radiations are invisible, and they penetrate living tissues to a greater or lesser extent; some are stopped in the first fraction of an inch of skin, others go deep into the body. All the radiations we are considering here are called “ionising radiations”; this means that they have the property of knocking out electrons from the atoms in the substances they pass through, and producing “ionised atoms” which have great chemical activity. When this occurs in a living cell, the usual effect is for the cell to be damaged. A large dose of radiation, producing many active ions, can cause the cell to die almost immediately. A very small dose may have no noticeable effect, though even the most minute amount of radiation causes some change in some part of the cell.

Ionising radiations arise from various sources. Before man made his contribution natural background radiation existed, and is still the most important source in most areas of the world, and it has the greatest effect on its human and other inhabitants. Three-quarters of the background radiation affecting man comes from outside his body; a third of this fraction is due to cosmic rays reaching the earth from outer space, and two-thirds is due to the local radioactivity of many of the rocks. Cosmic ray radiation is partially absorbed by the atmosphere, and is therefore much greater at the top of mountains; at higher altitudes it may increase enormously, and could be a serious hazard to astronauts. Even high-flying birds will receive more cosmic rays than those which remain near the ground. The radio-activity of rocks varies in different parts of the world. Near uranium deposits it may be high, but the differences between different parts of Britain are probably under 50 per cent.

The rest of the natural radiation affecting man comes from radio-active substances within his body. The most important of these is potassium. Potassium is an essential constituent of all living tissues. A tiny fraction, about one part in ten thousand, of this naturally occurring potassium is radio-active, and this produces radiation within the body. Radiation generated within the body in the vicinity of a vital tissue may be very dangerous, compared with similar radiation coming from outside and perhaps being absorbed so as to cause only superficial damage.

Man has contributed to radiation exposure in various ways. First he has concentrated naturally occurring radio-active substances, particularly radium, and used their radiations for medical diagnosis (X-ray examinations and photographs) and for certain types of treatment. More recently he has learned how to produce radiations for scientific research, and to use them for industrial purposes. Finally he has learned to “split the atom” and from this knowledge have stemmed both nuclear weapons and nuclear power stations. A nuclear explosion is accompanied by intense radiation, the effects of which may go farther than those of the blast and heat. Secondly the explosion liberates a quantity of radio-active dust, which goes high into the sky and is slowly deposited as “fall out.” Atomic power stations produce radiations which would be dangerous to life nearby if precautions were not successful. The greatest worry relating to these stations is, however, that they produce large amounts of radio-active waste products which could cause great danger if not disposed of properly.

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