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.