Two large studies in the 1990s further cemented particulate pollution as a danger. Both studies compiled immense data sets on ordinary Americans and their environments. The so-called Six Cities study, begun at Harvard in 1974, found that in the study area with the fewest particles, Portage, Wisconsin, there were 26 percent fewer deaths from lung and heart diseases than in the city with the dirtiest air, Steubenville, Ohio. The heart- and lung-damaging effects of particles of 10 microns in diameter and smaller have since been reproduced by many other studies, including the American Cancer Society’s survey of air quality in 150 American cities. In 1997, these studies prompted the EPA to tighten its regulations on particulate pollution, and the agency began regulating even smaller particles, those just 2.5 microns across.
These tiny particles penetrate the lungs deeply, where they can trigger asthma attacks and cause scarring like that from cigarette smoking, says air quality researcher Francesca Dominici of Harvard University. People with asthma and other lung diseases are at risk for lung damage from particulate pollution, but the large studies demonstrate risks to people with heart disease and diabetes, too. An ongoing review of Medicare hospitalizations, first published in 2006, indicates that particle pollution accounts for “hundreds of thousands of deaths each year” from strokes, heart failure and lung diseases, says environmental epidemiologist Joel Schwartz of Harvard.
“In the research community, no one has any question any more that even low levels of particulate matter and ozone are associated with adverse health effects,” says Dominici. In addition, the large studies show that pollution disproportionately impacts the poor, who tend to live near industrial areas and highways.
The EPA is now reviewing these studies during its years-long process of updating its regulations on particles. Dominici says the challenge lies with identifying sources of particles that can be controlled, as power plants, factories, vehicles and wind-blown dust all contribute to the problem. “Lowering the levels of these pollutants is not so easy,” she says.
Lowering the concentrations of ozone, another major pollutant from industry and vehicles and the main component of smog, presents another big challenge. Ozone forms when sunlight reacts with various pollutants, so concentrations can soar on hot, sunny days. The EPA tightened its ozone limit in 2008 and proposed an even tighter standard in January 2010. But while setting a standard is one matter, achieving it is another. About 174 million people live in counties that don't meet the 2008 ozone requirements, according to the American Lung Association.
In 2009, the EPA looked to the future and declared six greenhouse gases, including carbon dioxide and methane, dangers to human health. The agency said that it expected climate change to increase ground-level ozone and further endanger vulnerable populations from heat waves, other extreme weather and transmissible diseases that thrive in warm climates. The implications of the EPA's declaration, which followed on a 2007 Supreme Court ruling that greenhouse gases fall under the Clean Air Act, are unclear. The EPA has not regulated emissions of the gases; instead it urged Congress to pass comprehensive climate change legislation.
Lowering air pollution to zero – the only known safe level – is probably impractical. But researchers say opportunities abound to continue improving air quality – and human health. "There are still a lot of benefits to be had from reducing pollution," says Samet.