exceed the federal standard on a typical day and so are almost constantly exposed to very poor air. On stagnant days this area is much larger.

It is not possible to make as direct a comparison of the HC isopleths with federal standards because the federal standard uses different time periods and a slightly different chemical compound for measurement. However, since hydrocarbons and carbon monoxide have essentially the same sources and enter the atmosphere in much the same way, the areas of high HC pollution are likely to be areas of low ambient air quality. Precise comparison with the federal standard is less important in the case of HC for another reason. We are examining people and pollution. The important task is to show variations in air quality, and HC isopleths do this.

Unlike the HC isopleths, the isopleths for particulates and sulfur dioxide can be compared with the federal standards, since they are all based on an annual average. Even though Washington lacks heavy industry, the city ranks about average in SO₂ level and only a little below average in particulate levels. The federal particulate standard is 75 μg/m³ and the SO₂ is 80 μg/m³. Extensive areas in Washington are above these standards, so that we may safely conclude that people in these areas breathe chronically polluted air.

The last isopleth map, particulates for 1973, was developed from data supplied by the Metropolitan Washington Council of Governments. This map used data from 40 locations within the area of the study and was produced with the help of experts from the Health and Environmental Protection Department of the Council, who provided annual mean readings at each point. They show very marked reductions in pollution levels since 1969, indicating the effectiveness of air pollution control measures. The reason for this air quality improvement is discussed below.