Most reports on air quality trends typically begin with 1970, with the passage of the first Clean Air Act and the beginning of systematic monitoring of emissions and ambient levels of air pollution. Data from early monitors and evidence from air quality models, however, show that many forms of air pollution started to decline—in some cases rapidly—before the Clean Air Act of 1970.
For sulfur dioxide, data from 21 urban monitors show that the average ambient level fell approximately 40 percent between 1962 and 1964, as shown in Figure 1. This set the stage for progress after the Clean Air Act; the national average dropped 66 percent since then. Local data for New York City show that ambient sulfur dioxide fell by almost 50 percent between 1964 and 1970, as shown in Figure 2. Fragmentary data also show that carbon dioxide levels were falling in the mid-1960s.
Figure 1: Ambient SO2 Level, Mean Annual Average, 1962-1997
Source: Indur Goklany, Clearing the Air: The Real Story of the War on Air Pollution, Washington DC, Cato Institute, 1999
Figure 2: Ambient SO2 Concentrations in New York City, 1963-1972
What accounts for the decline in air pollution before national regulations were enacted? There are several factors, including state and local efforts to reduce pollution before the federal government acted. By the time of the Clean Air Act in 1970, about half the states and more than 100 metropolitan areas operated air pollution control agencies. In some cases, such as smoke in Pittsburgh, local private sector and government efforts to reduce pollution went back decades.
The role of economic growth and technological progress is perhaps more significant, however. Figure 3 shows the trend for “settleable dust” (which would be very large particulates) in Pittsburgh between 1925 and 1965. The rapid decline in the early years between 1925 and 1940 is attributable to the simple efficiency gains from industry upgrading its technology. The industrial drive for cost-saving efficiency also typically leads to cleaner technology.
Figure 3: Settleable Dust in Pittsburgh
London offers a more dramatic long-term example. British environmental scientist Peter Brimblecombe developed a model to estimate air-pollution levels in London as far back as the 16th century, which shows that levels of both sulfur dioxide and smoke peaked more than 100 years ago and declined steadily throughout the 20th century, with London air today cleaner than it was 300 years ago. (See Figure 4.)
Figure 4: Long-Term Air Pollution Trends in London, 1580-1990