Читать книгу Pesticides and Pollution - Kenneth Mellanby - Страница 7

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.