Once prosperity and technology were responsible for air pollution. Today they are necessary for its cleanup. Their transformation— from problems to solutions—began toward the latter part of the last century. The advent of new, clean energy sources and more efficient combustion technologies made it possible to reduce pollution and to prosper in the bargain. Those technological changes propelled by—and in turn propelling—prosperity gathered steam through the past century. And through the decades, one by one, the various pollutants were brought under control, by force of an affluence‐ and technology-driven environmental transition. As if in accordance with a grand design, the problems that were the most obvious and the easiest to control were addressed before others, and each pollutant's transition was determined by factors dependent ultimately on prosperity and technology—namely, knowledge regarding its causes and effects, its risks relative to other societal risks, the availability and affordability of measures to reduce those risks, the mix of energy sources used by society, the structure of the economy, and, of course, the general level of affluence. Improvements were achieved indoors before outdoors; in primary pollutants, in air quality before total emissions; and for primary pollutants before secondary pollutants.
Declines in emissions per GNP—measures of technological change which also serve as leading environmental indicators in a growing economy—indicate that cleanup had begun no later than the 1920s for SO2, the '30s for VOC and NOx, and the '40s for PM and CO. The first improvements came from voluntary, market-driven measures. People of their own accord cleaned up their households of the most obvious problem, smoke. That also improved air quality in their neighborhoods. Then the focus turned to outdoor air. Outdoor smoke went through a transition in urban areas shortly after World War II, if not earlier. With greater prosperity, better health, and reduced mortality, the risks of PM and SO2 became more evident.