Headlines were full of grim news this week about a new study that projects pollution from China's coal-burning heating boilers north of the Huaihe River will take an aggregate 2.5 billion years off the life expectancy of the 500 million people who live there.

What that means is about 5.5 years off each person's life expectancy starting at birth where the smog is 400 micrograms of particulate matter per cubic meter of air and for every 100 micrograms per cubic meter of increase, life expectancy drops another three years.

The dismal outlook aside, what's fascinating about the study is the science behind it. It comes from an analysis of data never before compiled, and the story really goes back to a Chinese government policy implemented in the pre-1980s era of central planning, when the government gave free coal for fuel boilers to everyone living north of the Huaihe River, China's age-old de-facto north-south border. The free heating fuel meant that people in the north stayed warmer but at what's now known to be a higher cost to the environment.

The policy also set up - inadvertently, of course - a large-scale human experiment, where a vast population in the north was exposed to pollutants, and an equally vast and comparable population to the south was not.

"It's not that the Chinese government set out to cause this," said Michael Greenstone, co-author of the study and the 3M professor of environmental economics at the Massachusetts Institute of Technology in Cambridge, Massachusetts. "This was the unintended consequence of a policy that must have appeared quite sensible."

He noted in passing that China doesn't generally require home owners to install equipment to abate air pollution on their coal-burning heaters.

Thanks to the arbitrary use of the Huaihe River as a boundary, the researchers were able to approximate a scientific experiment, or what the study calls a "quasi experiment".

"We will never, thank goodness, have a randomized controlled trial where we expose some people to more pollution and other people to less pollution over the course of their lifetimes," Greenstone said. But that's what happened and the scientists looked in.

Greenstone was joined in the study, which was published in the Proceedings of the National Academy of Sciences this week, by Chen Yuyu of the Guanghua School of Management at Peking University, Avraham Ebenstein of the Hebrew University of Jerusalem and Li Hongbin of the School of Economics and Management at Tsingua University.

"With the heating policy, the northern areas have been exposed to more pollution than the southern areas, which makes the study possible," Li told China Daily.

The study notes that the policy produced "dramatic differences in air quality within China." They were already noticeable in 1999, when then Chinese Premier Zhu Rongji, who had spent most of his career in the southern China city of Shanghai, quipped, "If I work in your Beijing [in Northern China], I would shorten my life at least five years," according to the study.

What also helped the study was that the span of time examined was before people started to move around, or become "extensively mobile", which would have made it difficult to draw cause-and-effect conclusions about the health effects of regional pollution. "In this period," Greenstone explained, "migration was quite limited. If someone is in one place, the odds are high they [had always] lived there and they would have been exposed to the pollution there."

Further bolstering the study's scientific validity was that, as far as the researchers could tell, there were "no other policies that are different north or south of the river", Greenstone explained. And since other pollutants - such as sulfur dioxide and nitrous oxides - are spread similarly north and south of the river, the particulate matter from coal burning appears guilty as charged for the diminished life expectancies.

Greenstone said none of the research team was sure what kind of results they would find when they set out to examine the data, which they assembled from a variety of sources, including hand entry Chinese-language publications and access to electronic files. The mortality data - 500,000 deaths between 1991 and 2000 - came from China's Disease Surveillance Points system, which records all deaths at 145 collection points and produces annual nationwide representative numbers.

Still, Greenstone said, "I was surprised by the magnitude - both in terms of [the quantity of] particulates, and in terms of human health."

Greenstone said he hopes the study will have an impact on policy not only in China, but also in other rapidly growing countries that are increasing the use of coal.

"The study gives a clear answer to the link between life expectancy and air pollution," Li said.

Greenstone concluded: "What this paper helps reveal is that there may be immediate, local reasons for China and other developing countries to rely less on fossil fuels. The planet's not going to solve the greenhouse-gas problem without the active participation of China. This might give them a reason to act today."