Just as it is possible to define relatively accurately the water catchment area which feeds a given stream or a whole river system, so it now appears that "air sheds" can be identified with physical features on the earth which may have the effect of gathering, containing, and channeling air currents. This effect could in some cases assure on the average a rapid flow over an area whereas in other instances it could tend to inhibit flow, with obvious impact upon the degree of pollution experienced by the inhabitants. Such was the case, for example, when in 1948 fog and a low-level temperature inversion covered the horseshoe-shaped valley of the Monongahela River in Pennsylvania, and nearly half the people around the town of Donora became ill and twenty died.
On the west coast of California, where there is a prevailing westerly breeze, there exist several well-defined urban air sheds, one in the San Francisco Bay area and another in the Los Angeles area. Each is bounded by mountains or hills, but is more or less open to the west.
The notion of an airshed may be a useful concept for analysis and control of air pollution, but the problems in its use should not be minimized. In the case of water pollution, the variable character of streamflow presents difficulties in measuring associated costs and in designing and operating optimum systems for water quality control. The airshed adds more dimensions to this problem. It may be likened to a stream which varies its course rapidly (within defined boundaries), changes specific gravity, and from time to time decides to flow uphill.
To the extent that air sheds are definable, air shed authorities or compacts or districts are conceivable and may be useful administrative devices. There is some precedent here in the formation of the San Francisco Bay area air pollution control district. In Germany an important part of the work of the Siedlungsverband (Land Planning Authority) is to deal with problems of air pollution. The primary means used in Germany is the control of location of industrial activities. Efforts of this kind present useful laboratories for studying the costs and effectiveness of various approaches. For example, in the Ruhr Basin of West Germany, where the air is heavily polluted by the dust and smoke derived from intensive industrialization, over 15 percent of the children were reported to show symptoms of rickets in 1961, as against half this percentage in a control city situated in the Rhine Valley.
As in the case of water pollution, there may be some administrative and regulatory alternatives to standards which demand serious consideration, for example some variety of air pollution charge or tax. This would be levied on the theory that use of a congested facility, air, should be reduced by putting a price on its use. Such a tax would be based on some measure of pollutants discharged at the source and could be weighted according to such items as location of the source, the external costs of specific pollutants, timing of releases in relation to peak loads of air congestion, and wind direction.
Based on the air pollution chapter of Quality of the Environment, by Orris C. Herfindahl and Allen V. Kneese (an RFF Research Report now in press).
A version of this article appeared in print in the May 1965 issue of Resources magazine.
