rapid photochemical conversion made possible by the presence of exhaust hydrocarbons will be retarded in some inverse relationship to our success in eliminating vehicular pollutants.

The California Air Quality Standards, insofar as they relate to engine exhausts at all, are based on an assumption that the objectionable properties of Los Angeles smog would be effectively eliminated by keeping the levels of exhaust emission at about 30 percent of the 1960 values. In order to achieve this 1940 state of relative freedom, rules were adopted requiring afterburners to perform in such a manner as to destroy 80 percent of the hydrocarbons, and 60 percent of the carbon monoxide contained in an average exhaust. Unfortunately, it is now quite certain that the "average exhaust" on which the absolute emission standard of 275 parts per million HC was based, is not at all representative of the output performance of numerous fleets of cars tested more recently. It seems reasonably certain that the present goal of 275 parts per million of hydrocarbon per vehicle would represent a reduction of only 50 to 60 percent of the actual emission.

Even this degree of abatement would, if realized immediately, have a marked minimizing effect on formation of photochemical smog. However, we are dealing with a real situation in which perfection in application of the rule appears to me to be unattainable for many years. First there are the technical facts of life. All of the proposed devices with which I am familiar require a "warmup" period before becoming functional at all. Since this time is normally several minutes, it follows that for a substantial fraction of the average triptime the control device will be inoperative. Thus the overall effect of installation on all vehicles would be curtailed to something like 40 percent reduction of current output.

But the current level is not the level of output with which we have to be concerned. Nothing appears likely to change the rate of increase in number of operating automobiles during the next decade. Nor is economic or political perspicacity of a very high order necessary to foresee that about 10 years will have to be used in the process of getting exhaust burners on all vehicles, new and already in use, unless they can be made at a cost not now predicted. All of which means that the current program will have achieved its goal just about in time to maintain the status quo for a brief period.

This is not an optimistic prospect; in fact, as a prediction, it will be quite unpalatable to many, and quite discouraging to all. In the context of my present discussion, it has very definite implications. It means that automobile exhaust not only is, but for a long time will continue to be, the major pollutant involved in Los Angeles smog. It means that the emission standards for automotive vehicles will have to be made more stringent. It means that every other contributory source of primary pollutants will have to be more and more rigorously restricted, especially those producing substances such as NO or NO2, capable of reacting photochemically in combination with exhaust products. And it means that the battle against air pollution cannot finally be won so long as numbers of sources are increasing, just as the last schoolroom cannot be built so long as the number of students increases.

Perhaps it would be better for all of us now, or it will be a necessity in the future, to consider air pollution as a relatively minor but annoying symptom of the inherent malignancy of urbanizationoverpopulation of limited areas. Polluted water, devastated natural resources, death by automobile, juvenile delinquency, mental illness, physical and moral decay are other symptoms. No one of them can be dealt with or even considered intelligently out of context with the others and with the monstrous growths from which they emanate. Even less useful is it to attempt to alleviate the one symptom-air pollution by excluding all but one of its components from consideration. In some places, and at some times, one source may be of prime importance, but our cities grow and our problems change with time and with different points in space. It is much too easy to base immediate actions on the exigencies of next year's budget justification, or the necessities of biennial reelection. One can dream that some mechanism be established, well removed from the immediacy of political and economic myopia, to consider atmospheric pollution as a changing manifestation of a dynamic metropolitan complex, and to plan actions designed to cope with future as well as present circumstances.

As a biologist I am inclined to view manmade air pollution as a facet of nature's inexorable system of checks and balances. We have been endowed with no well-developed group capability for present action in the future interest of our species. When we foul our environment, three alternatives

are possible: we temporarily clean up so the next and larger generation can foul it even more copiously, or we move to a new nest and start the process over again, or we remain in filth until it overwhelms us and there is no new generation. In the first case, with time, the cost of maintaining a tolerable site eventually becomes so burdensome as to be itself intolerable and a population limit is thus imposed. In the second case the population limit is by choice; while in the third case the population limit is the product of inaction.

Of course you have sensed that I am about to conclude these loosely related remarks by appending a thesis I have never failed to set forth under some pretext since I first became interested in public health problems. So long as we have uncon

trolled and exponential increases in populations, we will have proportionate increases in the complexity and cost of maintaining a satisfactory environment. It is easy to show that present levels of air pollution are generally proportional to population density even though dominant sources differ from one metropolitan area to another. Within rather broad limits the basic unit source of air pollution is one man.

This thesis is often countered with the view that any expectation of effective control at the real source is beyond hope. I suggest that any expectation of ultimate solution of our growing public health problems at any but this basic level is beyond possibility, even though palliative measures may carry us uneasily through a few more generations.

NORTON NELSON

Director, Institute of Industrial Medicine New York University Medical Center New York, N.Y.

I have been asked to cover a very large topic in a limited period of time and, accordingly, I can. only highlight the major features of the extensive information that has been developed on these manyfaceted problems. Publications, both original and review, have been increasing in number in recent years as interest and research have increased. Thus, there are available a number of excellent and upto-date works outlining the present state of knowledge, and these should be consulted for further details and for full documentation.

I have taken the liberty of deviating somewhat from the area of discussion assigned to me by including in my comments the effects of air pollutants from sources other than motor vehicles. I have been obliged to do this for two reasons. First, an understanding of the effect of air pollutants from motor vehicles is very much dependent on the information available to us on the effects of contaminants from other emissions. Second, in most real situations air pollution comes from a number of mixed sources and it is quite impossible to allocate rigorously the observed effects to a specific origin. As noted by other speakers, motor vehicle emissions are normally major contributors to the socalled photochemical smog, and use of this term will generally imply such origin.

In dealing with the topic assigned to me, I will give most attention to the two areas of adverse effects of air pollution that have been examined the most extensively, namely, the effects on vegetation and on health. A related area, the general economic effects of air pollution, is certainly very important but the dearth of information permits only the briefest comment.

In respect to economic factors, it is apparent to the most casual observer that the general burden of

air pollution on the economy is very substantial and unquestionably of sufficient moment in itself to provide adequate grounds for correction of the nuisance. That the costs are very great is quite clear; one very rough estimate for 1950-51 for air pollution from all sources was that the losses amounted to $1,500 million per year in the United States; that is, about $10 per inhabitant per year (1). A more recent statement notes that $65 per inhabitant per year would be more reasonable (2).

It is, however, not possible at this time to make any really accurate appraisal of the cost to society of such factors as soiling and damage to clothing, household furnishings, stocks in warehouses, paint damage, and corrosion of stone and metals, which are produced by air pollutants from many sources. Obstruction of visibility and rubber deterioration are well known and costly consequences of photochemical smog, to which auto emissions are major contributors. Admittedly, the measurement of most of these factors will be difficult, in some cases impossible. This need was noted in a recent document prepared at the request of the U.S. Public Health Service, "National Goals in Air Pollution Research" (3), which recommended air pollution research needs in the next decade. This gap should be recognized and studies should be put under way aimed at developing efficient and effective means for assessing the economic costs of air pollution. Once such procedures are available they should be applied in a systematic way so that a running ledger on costs can be maintained. It seems quite probable that well based data of this sort can provide a most effective basis for the early institution of appropriate control measures.

By contrast with the uncertainties in this area there is a body of well established knowledge on the

effects of air pollution on vegetation. The damaging consequences of air pollution to crops, forests, and ornamental plants has been known for a long time. The magnitude of the problem was dramatically signalled on this continent by several extremely vivid examples (4) of flagrant violations of the countryside by uncontrolled industrial discharges. Although there seems little likelihood that instances of equal intensity will be allowed to occur again, it has become incontestably clear that very significant damage to vegetation is occurring in many areas in this country by more typical and routine levels of air pollutants. "Smog" damage to crops is estimated to currently amout to some $8 million annually in California (5); motor vehicle emissions are unquestionably prominent contributors to these losses. Similarly, the cost of damage to agricultural crops in the eastern part of the United States has been estimated at $18 million; it is believed that this is mostly from ozone (6); again, auto exhaust should be suspect.

Research into the nature of the factors involved in the action of air pollutants on plants has been extensive and fruitful. In fact, the relationship between the concentration of air pollutants and the resulting plant injury has been sufficiently well defined so that plants show considerable promise as a means of measurement of air pollution.

Injury from air contaminants can involve nearly every kind of plants, whether these are wild or domesticated, ornamental or utilitarian, grown for pleasure or as a means of livelihood.1 Although a large number of substances can damage plants, at the present time focus is on a relatively small group of substances. These include sulfur dioxide, fluorine, and, associated with motor vehicle emissions, "oxidants" and ozone.

The term "oxidant" covers a variety of unspecified air contaminants having in common the ability to reduce potassium iodide in neutral solution. Although ozone is a part of this complex, it is not predominant. The analytical procedure and the term have provided a very useful index for measurement of photochemical smog, characteristic of, and first described for, the Los Angeles air. It is, however, by no means restricted to either Los Angeles or to the west coast. "Oxidant" injury to plants is characteristically seen as a silvering, or glazing, of the lower leaf surface. This is typical of the plant

damage widely seen in the Los Angeles area. It has not been possible to identify the specific constituents in smog responsible for this damage, although one strong candidate proposed is peroxyacyl nitrate (PAN).

Ozone, which is catalytically generated in photochemical smog, differs from "oxidant" in the nature of the injury produced on exposed plants; with ozone the upper surface of the leaf is attacked rather than the under surface. Exposures of a few hours at 0.2 part per million result in injury. As noted above, extensive damage to tobacco crops attributed to ozone has been reported in the eastern part of this country (6).

Detailed studies into the mechanisms by which plant injuries are produced by these various toxins are presently under way and should yield fruitful information. However, a more systematic evaluation of plant injury in this country than has been undertaken to date will be required if a valid assessment of the overall impact of plant damage on agriculture and the general economy of this country is to be achieved.

Turning now to the effects of air pollution on man, one first must call attention to the important contribution of air pollution to odors and to eye irritation; although these do not result in actual disease, they are nevertheless of widespread concern. Eye irritation occurs from a variety of irritant materials and is of particularly frequent occurrence in photochemical smog. Eye irritation in American cities is by no means limited to the west coast but is significant in major cities throughout the country. The specific materials in smog responsible for eye irritation are not definitely identified but probably include aldehydes, other oxidized hydrocarbons, and such compounds as PAN and nitro-olefins. Motor vehicles are major contributors to the production of these substances. The close correlation of annoyance with smog indices has been confirmed repeatedly (9).

As was the case with respect to damage to plants, public concern for the impact of air pollution on health is to a considerable extent traceable to major and dramatic episodes. In fact, had they not occurred it is conceivable that the major effort now under way to define the impact of air pollution on health would never have been undertaken. Motor vehicles were not of critical importance in these earlier incidents.

Two episodes in particular have become embedded in historical discussions of air pollution, one

occurring in Donora in 1948 leading to some 20 deaths (10) and another in London in 1952 (11), leading to an estimated 4,000 excess deaths. Periods of unusual atmospheric stagnation were linked with high, although not extreme, levels of contamination in both episodes.

Controls since instituted should lessen the likelihood of similar episodes in the future; nevertheless, where such controls have not been installed, or have been inadequate, the risk of major difficulties does remain, and another episode awaits only the recurrence of the required meteorological conditions to allow accumulation of lethal levels of toxicants. Fortunately, the particular set of meteorological conditions sufficient to produce the required circumstances is quite rare.

Of greater present interest than these major disasters is the extent of health impairment resulting from the more typical levels of air pollution found frequently in our major cities. At the outset it can be stated that this question is quite a difficult one, which for precise answers will require methods of a higher degree of refinement than have been generally undertaken in the past. One source of the difficulty in defining the health effects of air pollution results from the fact that there is no specific air pollution disease; that is to say, there is no particular pattern of symptoms that defines itself as having arisen from exposure to air pollutants. On the contrary, it is apparent that air pollution in its effects on health-and there is good reason to believe that there are such-acts primarily through the exacerbation of existing disease. Nevertheless, if air pollution introduced a major perturbation in the disease pattern it should be relatively easy to distinguish. Such a major perturbation has not been discernible to date, and perhaps we can conclude that air pollution is not a predominant source of disease in comparison to all other factors involved in disease induction, whether these be degenerative diseases, infectious diseases, or malignancy. One may further ask, since it takes such refined techniques to detect an effect, can such factors be important? The answer is clearly, yes; that is to say, a definitely significant, although not predominant, role of air pollution in disease causation may be present and still be difficult to measure. Included in the term "significant" here is the implication of such disease for the individual and for society. It will be useful to examine the evidence that is accumulating on this issue, as well as

to consider current research, some still incomplete, which may give more decisive answers than are now available.

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Two materials, carbon monoxide and lead, particularly associated with automobile exhaust, have been steadily increasing in concentration as community air pollutants in most cities in this country. Whereas only a few years ago, reported concentrations of these substances were far below levels regarded as significant for health, more recently each of these materials has been found in concentrations not too remote from the levels considered acceptable in industrial work places. Thus, carbon monoxide concentrations of 30 to 40 parts per lion and lead concentrations of 20 to 30 micrograms per cubic meter are to be found in a number of regions of heavy traffic density (5). The industrial MAC's (maximum allowable concentrations) for these materials are 100 parts per million and 200 micrograms per cubic meter, respectively. Although the margin is still large, obviously, for a variety of reasons, more rigorous standards must be used with community populations than with industrial workers. Thus, community exposures are often superimposed on occupational exposures, individuals exposed to community air pollution may include individuals of all ages and of varying states of health and disease, and finally, exposures may be of longer duration.

With reference to these particular materials, there is no evidence at this time that health impairment does arise from such exposure; however, it will be important to establish more firmly the health implications of currently prevalent levels, and to guard against further increase in their concentrations.

Much attention has been given to the role of irritant chemicals in respiratory disease. Many materials appear to have in common the ability to produce irritation of the respiratory tract, individually and jointly. Such irritants have many sources. The combustion of sulfur-containing coal and oil produces sulfur dioxide and sulfuric acid. Motor vehicle emissions and the consequence, photochemical smog, bring their own constellation of irritants, including ozone, nitrogen oxides, and a group of oxidized organic compounds such as aldehydes, acids, peroxy acids, and so forth.

These materials may all produce irritation, which in turn may be the basis of interference with lung performance and the enhancement of disease.