In December 1952, a deadly smog settled over London. Trapped by cooler air above, the dirty cloud enveloped the city for four days. Rich with soot from factories and low-quality home-burned coal, the Great Smog, as it came to be known, caused some 12,000 deaths that winter.
Similar, though smaller, lethal clouds choked Liege, Belgium, in 1930, killing at least 60 people, and Donora, Pennsylvania, in 1948, accounting for a score of deaths.
These disasters forced the world to face the dangers of air pollution and inspired an ongoing movement for cleaner air. The United Kingdom adopted broad air pollution regulations in 1956, the first country to do so. In 1970, the United States created the Environmental Protection Agency and passed the Clean Air Act. The act originally empowered the EPA to determine safe limits and regulate six major air pollutants, now expanded to include 189 potential threats.
“It’s a huge act,” says Jonathan Samet, an air pollution researcher and professor of public health at the University of Southern California. “We’ve had tremendous declines in major air pollutants as a consequence.”
Thanks to these tighter regulations and improved technology, the United States has enjoyed huge reductions in airborne lead, sulfur compounds and chlorofluorocarbons. Concentrations of carbon monoxide – once spewed by every car and truck but now removed by catalytic converters – have dropped by about 90 percent in large American cities, says Samet. Concentrations of particulate matter, a classification covering a wide range of pollutants in the microscopic to near-microscopic range, have dropped by about 80 percent from their peak, and ozone has plummeted as well. The result: from 1980 to 2000, according to a 2009 study in the New England Journal of Medicine, average life expectancy increased five months because of the nationwide drop in air pollution.
But even these dramatically clearer skies can be dangerous: Decades of increasingly sophisticated research suggest that no amount of air pollution is safe. Air pollution still accounts for tens of thousands of deaths each year in the United States. The Clean Air Act requires that the EPA base its pollution limits on the latest available science, so as research has pointed out more and more health risks, the EPA has steadily tightened its standards.
The EPA set some of its early pollution standards based on “challenge studies“ in human volunteers. In the 1970s, people with asthma, coronary artery disease and other conditions were exposed to ozone and carbon monoxide in a lab. Their lung capacity dropped and their blood vessels constricted “at lower levels of exposure than had been expected,” says Daniel Greenbaum, president of the Health Effects Institute, which is funded by the EPA and car makers. As a result, the EPA pushed for stricter standards, and by 1975 catalytic converters were mandatory on new vehicles.
Indoor air quality first became a focus of attention in the 1980s. As knowledge of smoking’s dangers increased, flight attendants became the first group to press for cigarette smoking bans. Smoking was more and more restricted on airplanes until it was finally banned altogether in 2000, when exceptions for flights to and from the United States were abolished. Indoor smoking bans are now de rigueur across the nation.
A radon scare in the mid-1980s swept through the country after a Pennsylvania home was found to harbor enough of the airborne radioactive element to close a uranium mine. Soon thereafter, the EPA began recommending radon testing for all homes. Despite the increased attention and widespread home testing, radon remains the second leading cause of lung cancer in the United States; the National Academy of Sciences reports that some 20,000 people die from radon-induced lung cancer each year.
A months-long strike at a steel mill near Provo, Utah, in the late 1980s provided a vivid demonstration of the health risks of airborne particles, especially those made of metal, which were prevalent in the area. Researcher Arden Pope of Brigham Young University seized on the closure as a natural experiment, an opportunity to correlate airborne particle readings with hospital records. His findings, published in 1989, were striking. When the mill was closed, local hospital admissions for children experiencing respiratory problems dropped to a third of what they were when the mill was open. Admissions for adults with respiratory problems dropped by about half. “It’s a classic cause-and-effect study with a powerful conclusion,” says Greenbaum.
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.