A study released earlier this week indicates that airborne pollution in China may have shortened the lives of 500 million Chinese by 2.5 billion years. The paper, published in PNAS on Monday, examined pollution data and death records to see whether coal burning, long a source of air pollution, could have damaged public health across northern China in the 1990s. The findings raise concern for developing countries across the globe.

Michael Greenstone, an economist at Massachusetts Institute of Technology and a primary author of the study, says he is driven by the question, "Over the course of a lifetime, what are the costs of exposure to high levels of air pollution?" Yet, he notes, finding answers has been extremely difficult, because simply comparing pollution levels and health in different locales can be misleading. For one thing, people often move from place to place and experience varying levels of pollution, so it is not safe to assume that all have had the same exposure. And access to health care differs across population groups around the world. "I've been searching for the right setting to answer this question for more than a decade," Greenstone says.

A few years ago one of Greenstone's colleagues saw a possible solution in a decades-old Chinese policy. From 1950 to 1980 the Chinese government was in a period of socialist transformation. During this time the “Huai River policy” provided free coal for heating homes and offices north of the Huai River, which runs west to east across eastern China. Meanwhile budget constraints kept free coal from being provided south of the river. At the same time, government rules restricted a family's ability to move, so that many lived in one location for decades.

By examining rates of mortality and respiratory-related illnesses on both sides of the river, Greenstone’s team identified a difference: life expectancies are lower and pollution concentrations are higher north of the Huai, where coal burning was widespread. "We've imposed a really demanding test on the data: we want to know whether or not there's a jump in air pollution right at the Huai river border," Greenstone says. "And we found that. All other factors are identical."

To make these connections, Greenstone and his team examined pollution data from sites across the country for the years 1981 to 2000. They then collected mortality data from China's Disease Surveillance Points, 145 sites chosen by the government to accurately represent the wealth and geographic dispersion of the populace.

After collecting data for five years, Greenstone's team plugged the information into equations that measured the sensitivity of the mortality rate to pollution levels on either side of the river. The results estimate that lifelong exposure to 100 micrograms of “total suspended particulates,” or TSPs, (minuscule solid particles floating in the air, such as pollutants) per meter of air cubed will shorten a person’s life by three years, on average. For the years between 1981 and 2000 northern China’s air contained a daily average of 551.6 micrograms of TSPs, whereas the south held an average of 354 micrograms. In the U.S. the legally required standard for air quality from 1971 to 1987 was 75 micrograms.

Most TSPs are visible, and not all pose health risks. “Typically, our respiratory system is designed to handle whatever is in the environment,” says Kent Pinkerton, director of the University of California, Davis, Center for Health and the Environment. The nose, mucus in the esophagus and cells in the lungs all filter foreign substances to facilitate clean breathing. Some pollutant particles can overwhelm the body’s natural defense systems, however, causing inflammation in the lungs. This inflammation could result in breathing difficulties, exacerbate preexisting conditions and, in extreme cases, cause death.

As more countries in Asia and Africa power toward industrialization, air pollution becomes an increasing concern. In China between 1981 and 2001 concentrations of TSPs were more than double the country’s earlier average of 200 micrograms per meter cubed and more than five times the average amount in the U.S., even before the passage of the latter nation's Clean Air Act in 1970. The pollution problem is not restricted to outdoor air, as families in developing countries also encounter pollution inside the home; according to Pinkerton, almost half the world prepares food with wood, charcoal or coal, all common sources of air pollutants.

“Developing countries are really trying to strike the right balance between economic growth to confront poverty and environmental quality and public health,” Greenstone says. “I think this study will help them—it shows a relationship between pollution and health.”