(Outdoor) Air Quality in the United
After indoor air concentrations (or appropriate proxies), the next best indicators for the public health impacts of air pollution are outdoor (or ambient) air concentrations at approximately ground level.
Systematic national efforts to monitor the air began in 1953, when the U.S. Public Health Service's Division of Sanitary Engineering began sampling suspended particulate matter using "high-volume samplers" in 17 cities in cooperation with local (and a few state) agencies. By 1956, particulate sampling had expanded to 66 communities nationwide. In 1957, the National Air Sampling Network, which planned to operate about 100 sampling stations each year in urban and nonurban areas, was established, followed in 1959-60 by the Gas Sampling Network, which was to collect 24-hour samples of SO2 and nitrogen dioxide. Then in 1962, the six-city Continuous Air Monitoring Project (CAMP) was begun to continuously measure CO, NOx, SO2, total hydrocarbons, and total oxidants.1 The order in which these monitoring programs came into being also reflects the order in which the various pollutants intruded into the consciousness of national policymakers and the public as having significant real or potential effects on public health (see Table 1-1).
Data from these sampling networks—which, at any one time, used more or less consistent protocols and procedures for gathering, handling, analyzing, and reporting data for each pollutant—allow us to construct national trends for the various pollutants. Using these data and data from successor networks managed by—or reported by states and local agencies to—EPA, it is possible to construct "national" trends provided measurements are appropriately adjusted to account for any changes in protocols, procedures, instruments, and instrument placement that may have occurred from time