Life expectancy in northern China was 5.5 years shorter than in southern China in the 1990s, and a health risk disparity lingers today, a difference almost entirely due to heart and lung disease related to air pollution from the burning of coal, a new study shows.

The research, published Monday in the Proceedings of the National Academy of Sciences (PNAS), not only adds to the large body of evidence on the risks of small particle pollution, it provides the most precise measure yet of the health impact of smoke from combustion.

Drawing on what they said was the most comprehensive data set ever compiled in the developing world, the researchers aimed to provide a yardstick that public policymakers can use as they consider the implications of decisions now being made on energy. The findings come at a time when coal is on track to surpass oil as the world's top energy source and 2.8 billion people rely on wood, crop waste, dung, and other biomass to cook and heat their homes. (See related: "Five Surprising Facts About Energy Poverty.")

"We can now say with more confidence that long-run exposure to pollution, especially particulates, has dramatic consequences for life expectancy," said Michael Greenstone, economics professor at the Massachusetts Institute of Technology, who collaborated with researchers in China and Israel. "To be able to say with some precision what the health costs are, and what the loss of life expectancy is, puts a finer point on the importance of finding policies that balance growth with environmental quality."

An Accidental Experiment

The stark new evidence of the impact of particulate pollution emerged due to an unintended social experiment that heightened health risks for 500 million people.

For 30 years, from 1950 to 1980, the Chinese government provided free coal for home and office winter heating systems for anyone living north of the Huai River and Qinling Mountain range. Central planners chose this demarcation because the Huai River follows the January 0°C (32°F) average temperature line. The policy has had a long-lasting impact, because so many of the long-lived heating systems remain in place. Coal is no longer free in the north, but it is still subsidized, while there are still very few cities in the south with heating systems like those in the north.

Though the policy's purpose was to provide warmth in winter to those who needed it most, the result—the new research shows—was a dramatic rise in mortality due to cardiorespiratory illness.

The air concentration of total suspended particulates (TSP) in the north was 55 percent higher than in the south, and life expectancies were 5.52 years lower, between 1981 and 2000, the researchers found.

"It's not that the Chinese government set out to cause this," Greenstone says. "This was the unintended consequence of a policy that must have appeared quite sensible."

Since there are 500 million residents of Northern China, that means air pollution is associated with the loss of more than 2.5 billion life years of life expectancy, the study says. Northern China includes numerous cities that are perennially on the list of the world's most polluted metropolitan areas, including the capital city of Beijing; neighboring Tianjin; Lanzhou, in the Gansu province; and Harbin in Heilongjiang province. (See related: "Quiz: What You Don't Know About Cities And Energy," and "In China's Icy North, Outfitting Buildings to Save Energy.")

Dan Greenbaum, president of Boston-based Health Effects Institute, who was not involved in the new study, said its findings are important because it underscores that although life expectancy has grown significantly overall in China, air pollution is in fact eroding health. "The beauty of this study is they were able to look at different parts of China with different levels of pollution in a way that really helps you to understand the impact on life expectancy that much better," he said.

Greenbaum added, however, that the study, for all its strengths, assumes that home heating systems are the source of the particulate pollution. Northern China also is home to most of the nation's coal-burning power plants, and could also be contributing to the health burden, he said. "I'd say the regional analysis and differences in air pollution is right on the money, but I'd be hesitant to say that any one coal-fired source is better than another," he said. (See related: "Pictures: A Rare Look Inside China's Energy Machine.")

The Health Effects Institute was involved in the landmark Global Burden of Disease study published last year in The Lancet, which tallied 3.5 million annual deaths from respiratory illness due to burning of wood, brush, dung, and other biomass for fuel—making it one of the largest environmental threats to health. (See related, "Cookstove Smoke is 'Largest Environmental Threat,' Global Health Study Finds.")

A Measure of Health Impact

The results suggest that long-term exposure to each additional 100 micrograms per cubic meter of TSP is associated with reduction in life expectancy at birth of about three years. Particulate-matter levels were more than 400 micrograms per cubic meter in China during the years studied. (Beijing made headlines in January when its air pollution levels reached a staggering 700 micrograms per cubic meter, but that was based on a number of pollutants, not just TSP particulates-those measured in this study.)

In contrast, the U.S. Environmental Protection Agency's health-based national air quality standard is 50 micrograms per cubic meter for PM-10. (That stands for particulate matter of 10 micrometers or less, a standard that has come to replace monitoring of "total suspended particulates," which includes anything less than 100 micrometers.)

Although the researchers did not analyze mortality data from more recent years, they did compile air pollution data from 2003 to 2008 indicating that TSP concentrations remain 26 percent higher north of Huai River, "suggesting that residents of the north continue to have shortened lifespans," the study said. Greenstone said the researchers are continuing to work on an effort to gain access to more recent mortality data.

In an email, he said that even if all differences in policy were removed tomorrow, "It will presumably continue to affect health for decades, as the North China population is likely to have been weakened by the many years of exposure to high-pollution concentrations."

The real-life experiment provided an extraordinary opportunity to track air pollution health effects. During the time period studied, China's hukou, or household registration system, restricted mobility so individuals could be assumed to have died where they spent most of their lives. In previous studies, researchers have had to assume that was the case, even though migration is high in the United States, where much of the research has been done.

The researchers collected information on annual daily average concentrations of TSP for 90 cities from 1981 to 2000, hand-entering some of the figures from Chinese language publications, and supplementing this with World Bank data. The life expectancies were calculated from 1991 to 2000, using mortality data derived from China's Disease Surveillance Points (DSP) system, a set of 145 sites chosen to be nationally representative so that it captures the nation's variation in wealth, urbanism, and geographic distribution. The study also included analysis on a wide range of other factors, including temperature data, education, manufacturing's share of employment, and the percent of residents with access to tap water, to rule out causes other than air pollution as the reason for the north-south disparity.

Greenbaum, of the Health Effects Institute, said the new study is likely to be given weight in policymaking in China because it was conducted in collaboration with Chinese researchers.

"There are still challenges, though," Greenbaum said. "The process of implementing rules that deal with air pollution are significant-having the rules in place, having staff to implement and enforce the rules, and making sure enforcement systems are not subject to potential corruption."