The science of particulate air pollution and health has a long, rich, and complex history. More detailed versions of various aspects of this history can be found elsewhere.1-5 A simple stylized version of this history may suggest that, until the early to mid-1900s, the health effects of air pollution were largely a matter of instinctive observations, suspicion, and superstition. In fact, one of the earliest and ultimately most acclaimed successes of environmental epidemiology was John Snow’s hard-earned evidence, in the mid-1800s, that cholera was transmitted by a waterborne pathogen—not “bad air.” Fascinating, well-written, historical accounts of this remarkable story are readily available.6,7
As late as the 1980s, evidence of adverse health effects of ambient air pollution was based largely on a few well-documented but extreme air pollution episodes of the 1930s to 1950s, several poorly esteemed ecological cross-sectional mortality studies, and various other loosely connected studies. Contemporaneous and highly comprehensive discussions of the evidence at that period of time and the conflicting interpretations of the evidence were published.8-10
In the late 1980s through the mid-1990s, the evidence that air pollution adversely affects human health was substantially strengthened by several controversial single-city daily time-series mortality studies,11-17 studies of a unique but somewhat isolated natural experiment,18 and panel studies that found that exposures to air pollution were associated with reduced lung function and increased respiratory symptoms.19 Also, two prospective cohort studies, commonly referred to as the Harvard Six Cities study20 and the ACS cohort study21 reported that long-term exposures to fine and sulfate particulate pollution were associated with surprisingly large increases in risk of mortality—especially cardiovascular or cardiopulmonary mortality.
Furthermore, there was a rapid growth in the reporting of daily time-series mortality studies. Similar studies of hospitalizations for respiratory and cardiovascular disease, and studies of various other designs began to be reported. These studies suggested that air pollution, even at moderate levels common to U.S. cities, remained a public health concern. They were met with much skepticism and were highly contested. The early to mid-1990s, therefore, was a galvanizing period in the history of air pollution science, and resulted in a substantial increase in research efforts to understand particulate matter health effects, which has continued until today. Reviews of the state of the science during the 1990s and discussions of the accompanying controversy are provided elsewhere.22-25