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of the British Medical Research Council Group on air pollution) had in mind, several years ago, when he said regarding British urban air pollution: "We have no more right to expect an easy solution than to overlook a simple answer. The field is bedevilled by paradox, not the least striking of which is the persistence with which we apply exquisitely fine techniques to crude problems and at the same time expect delicate mechanisms to yield to the hammer blows of our clumsier methods."

The British are plagued by pollution problems characterized more by SO.and its involved atmospheric permutations to SO, and sulfuric acid than by the automotive kind of smog. But this is changing, of course, as autos and traffic congestion increase, just as the SO.:-complex never was exclusively a British problem.

Sulfur dioxide in the air comes mostly from combustion of sulfur-containing fuels-coal and low-grade or residual fuel oils are the chief offenders. Natural gas and light petroleum fractions like kerosenes and gasolines are relatively low in sulfur or can be made so with little difficulty. Some SO, also comes from the smelting of sulfide ores. The ash constituents invariably present in the air help to catalytically oxidize SO., to SO,,, and hydration of SO, yields the sulfuric acid which is responsible for the blue color typical of So,-laden exhaust plumes.

The more difficult and critical transformation of So, to SO, is probably accomplished photochemically by near ultraviolet radiation; this mechanism may be most effective in the presence of particles of manganese and iron salts or oxides, under the moisture-rich conditions available in most stack gases and during humid weather conditions more generally. No doubt the strongly oxidizing atmosphere created in typical photochemical (Los Angeles) smog also contributes significantly to this otherwise slow oxidation step. Both kinds of smog are invariably present to greater or lesser degree in urban atmospheres and their deleterious effects, as to some extent their photochemical histories, are intricately entangled, but distinguishable.

The U.S. Clean Air Act specifically recognized the sulfur problem too, and it directed Health, Education and Welfare to conduct a major R&D effort aimed at developing cheaper and better techniques for removing sulfur from the offending fuels. Much additional work is being done on an alternative potential solution-removing the sulfur compounds from exhaust gases before emitting them to the atmosphere. Several lines of development look promising here. Some involve adsorption of So, on activated carbon char or reacting it with alkalized alumina. Others approach the problem as the atmosphere itself does, by catalytically oxidizing the So, to SO, and con

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ppm; this limit is for 5-day-a-week, 8-hr-aday exposures of healthy adult workers. A value of twice this amount or 10 ppm observed in a single measurement in ambient air in Los Angeles would result in a so-called "third alert” in which all-out community protection measures would be authorized. Most of the time the LA concentration of NO and NO:which are usually measured together since there appears to be no control purpose to be served in distinguishing between them in view of the rapid conversion of NO into NO.:--stays below about 1 ppm. One thing that may help keep NO and NO., well below more potentially dangerous levels most of the time is that during the daylight hours, they are continually used up in maintaining the series of equilibrium reactions with ultraviolet light and hydrocarbons. But some knowledgeable scientists, such as Philip Leighton of Stanford University and Albert Bush of UCLA, have warned that reducing hydrocarbons alone especially to drastically low levels such as 100 ppm or less—may in fact raise the total of nitrogen oxides in the air over the long term, eventually perhaps to more dangerous levels. Hamming and most others discount this possibility, however.

Such fundamental uncertainties as these must have been what Dr. P. J. Lawther (then

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verting the latter to sulfuric acid, which is apparently recoverable in amounts sufficient to at least partly defray the costs. It's an ill wind that blows pollution your way

When you think about the huge dimensions of the ocean of air that lies above us, it's hard to believe that the activities of urban man, which are carried on over just about 1% of the total land surface, can create vast, slowly drifting, Sargasso-like seas of pollution. In fact, the major portion of man's airborne effluvia is carried away by turbulent winds and vertical updrafts and diluted to undetectable concentrations throughout the entire 10mile thickness of the lower atmosphere. But a considerable proportion often cannot be dispersed this way.

With surprising frequency-an average of perhaps one-third of the time over much of the U.S. for example—there is an effective limit to the upward dispersion of contaminants, at altitudes of 500 feet or less.

This upper limit to dispersion is created either locally or over large regions by a thermal inversion, a condition you're probably familiar with, in which the normal decrease of air temperature with height above the ground which heats it is reversed. At some elevation above the ground-known as the inversion base-air temperatures begin to rise instead of continuing to drop (see margin). This anomalous temperature gradient persists upward throughout the inversion layer to an altitude which is determined by large-scale weather patterns that create the inversion in the first place. The base of the inversion layer

acts as the effective lid.

Imagine a bold parcel of polluted air-such as a hot, high-velocity jet of stack gases-one that has the temerity to try to rise into the inversion layer itself. Although it cools markedly on the way up, on penetrating the inversion layer it finds itself much cooler and more dense than the surrounding air, in which the temperature is going up not down. Consequently, it quickly sinks back toward the inversion base and has little if any time in which to disperse its pollutants to higher altitudes. It and its burden of pollution are confined to the appropriately named "mixing layer" that lies below the inversion base and extends to the ground.

The average prevailing thickness or depth of this mixing layer varies with time and place—it reaches a mile or two at times—but it is always far less than the full thickness of the lower atmosphere. Yet, in general these mixing depths would suffice to dilute pollutant concentrations, if the winds that handle horizontal circulation blew hard enough and with enough turbulence for enough of the time. At some seasons of the year and at many places they don't. Still worse, winds that are too weak can compound pollution troubles.

Helmut Landsberg, head of the climatology section of the U.S. Weather Bureau, has shown this for the northeastern chain of cities, extending from Richmond, Virginia, to Portland, Maine. When weak winds involving 100 miles or less of net air transport a day blow the right way–in this case mostly from the south or southwest (see margin)—the pollutants emitted in any one city either stay in the local area or are wafted gently toward the next city in the chain, perhaps adding to its pollution burden. Such weather conditions are far from rare for at least parts of the chain.

This doesn't mean that recent comments by New York City's Mayor Wagner, in which he described the city as lying at the end of a "3000-mile long sewer” of air pollution, are technically correct (as pollution control people in California are at some pains to point out). It does mean however that regional airsheds exist. These, at some seasons and some places, are in many ways analogous to watersheds. In both cases pollution can increase in the downstream direction. But air, unlike water, cannot be cleaned up for general use. Pollution in it can only be controlled at the source. In order to do this we're going to need more and better ways to monitor and trace the movements of pollution clouds that migrate downwind, from the central cities into suburbs and the surrounding countryside. Characteristic patterns of pollution-caused damage to plants offers some grim help here.

In the U. S. some data vital for these and other purposes are starting to come in from a PHS National Air Sampling Network of more than 200 stations-urban and rural

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is no safe exposure other than zero to chemicals such as these.

What should the attitude of an agency like the Public Health Service be in setting allowable limits for substances such as these, charged as it is with a vested interest and proper bias on the side of public health and safety? Should PHS do the job of setting criteria, or should it be delegated instead to a cabinet-level body like the Federal Radiation Council, or to some other august body like the National Academy of Sciences, which can juggle benefits versus risks through less safety-tinted glasses?

As one who has breathed for some time and hopes to continue doing so for a long time to come, I hope that someone with a more health-biased viewpoint will do the job, as it is now in the process of doing it. Soon.

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which have been operated since 1953 on an intermittently scheduled basis that has yielded some 4500 samples a year. A little closer approach to the kind of assault that's needed has been provided by a recently activated PHS system of highly automated, computer integrated monitoring stations which provide continuous 24-hr-a-day, every day, measurements of CO, SO2, NO, NO2, O3, total oxidants, and total hydrocarbons—at one location in each of nine cities. Society, or somebody, must call the shots

Technical needs in pollution research and control activities are in many ways obvious. They can and will be met as soon as sufficient resources are devoted to them. The problems of setting quality criteria for air, and seeing to it that they are enforced, are much more difficult. Solving them requires not only scientific, technical, and medical data and decisions but social and moral ones as well. There's nothing new about this. Society requires many such decisions.

In the field of nuclear energy for example, the Federal Radiation Council was established to assess social benefits versus risks in face of the current overwhelming scientific judgement that there is no threshold or limiting value below which adverse biological effects do not occur—there is no "safe" level of exposure to radioactivity other than zero. There are comparable problems in the air-pollution field, especially in the case of polycyclic hydrocarbons like 3-4 benz-pyrene which are potent carcinogenic agents in experimental animals. Vernon MacKenzie of PHS notes that polycyclic hydrocarbons in air appear to come mostly from coal combustion, whether in furnaces or engines, from burning waste materials, and from some industrial processes. They cannot be practically eliminated from the air unless the total economic and technical fabric of society is altered. Yet there

A cosmic joker in the deck?

Even complete success in controlling pollution of the kinds we have been discussing may prove to be a Pyrrhic victory in the not very long distant end. There is inconclusive evidence that the atmosphere's total content of carbon dioxide has increased by some 13% due to man's increasingly industrial way of life since the 19th century. Co, is not usually thought of as a pollutant since it is not harmful. Indeed it and water are the ideal non-toxic end products of all fuel combustion and metabolic processes. The observed increase agrees strikingly well with estimates of the Co., increase that could be expected since the 19th century on the basis of sharp rises in fossil fuel use. Projecting such estimates into the future, it appears that CO, in the atmosphere may be 50% higher by the year 2000 than in pre-industrial days, assuming that atomic power doesn't replace power from fossil fuels to any significant degree.

This increase in itself shouldn't bother anybody's breathing or other activities, but it might have larger-scale effects on the climate of the entire earth. CO, is an important absorber of the longer wave infrared energy that the earth's surface reradiates as it cools through the nights and the seasons. If all of the extra CO., remains in the atmosphere, instead of being taken up by plants or dissolved in sea water, and nobody knows exactly how much is removable in these ways, it seems likely that the earth's average temperature could go up several degrees. Some provocative though largely speculative estimates suggest that this increase might be enough to melt all or most of the glacial ice on earth. In turn this would raise sea level everywhere by a few hundred feet-enough to put most of smoggy Manhattan and the Los Angeles Basin under water, for example. Which is indeed one long-range solution to the problem of polluted urban air.

THE INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS, INC. 4 OCTOBER 1965

IEEE Spectrum

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Airborne asphyxia-an international problem

Air pollution is no longer a minor inconvenience; it is a tangible, frightening, and intolerable situation that affects virtually every major industrial city in the United States and Europe

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Until quite recently, the smog in Los Angeles was the butt of every radio and television comedian. It was subjected to ridicule as the world's worst example of an outof-control condition that was apparently without remedy.

But today the joke has gone sour. A recent—and disturbing-survey made by a special committee under the aegis of the New York City Council reveals that the type of air pollution-a dangerously high level of sulfur dioxide-in this huge metropolis, and particularly in the Borough of Manhattan, may be more hazardous to human life and health than that of Los Angeles.

It is estimated that the air is so foul over the island of Manhattan (see Fig. 1) that a monthly sootfall of more than 60 tons per square mile is commonplace during the winter. The City Council report alleges that most of the pollutants are belched out of the chimneys of industrial plants, electric utilities (see Figs. 2 and 3), and some 14 000 apartment house incinerators. Additional noxious fumes are spewed into the air from the tailpipes of thousands of motor vehicles that clog almost every square inch of city streets.

The commission's report asserts that breathing city air for one day would have the same effect on human lungs as inhaling two packs of cigarettes within the same time interval. So, as one wag put it: if one happens to be a habitual smoker in the two-pack-a-day category, and cannot shake the habit, it may be more healthful for one to give up breathing!

The sad story of air pollution, however, does not end with New York; Houston, Atlanta, Louisville, Chicago, St. Louis, Birmingham, and a host of other American and foreign cities are suffering the same atmospheric miseries (Fig. 4) in varying degrees and from various sources. In Germany, England, Belgium, France, and Luxembourg, where coal is the principal industrial fuel, the air pollution and smog problem is equally intense. Cities such as Essen, Dusseldorf, and other industrial centers in the Ruhr Valley are plagued by air

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Fig. 1. "Darkness at high noon"-a view of 42nd Street in New York City under a blanket of heavy smog caused by a temperature inversion, on October 20, 1963..

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