A new study links heavy air pollution from coal burning to shorter lives in northern China. Researchers estimate that the half-billion people alive there in the 1990s will live an average of 5{ years less than their southern counterparts because they breathed dirtier air.

China itself made the comparison possible: for decades, a now-discontinued government policy provided free coal for heating, but only in the colder north. Researchers found significant differences in both particle pollution of the air and life expectancy in the two regions, and said the results could be used to extrapolate the effects of such pollution on lifespans elsewhere in the world.

The study by researchers from China, Israel and the United States was published Tuesday in the Proceedings of the National Academy of Sciences.

While previous studies have found that pollution affects human health, ``the deeper and ultimately more important question is the impact on life expectancy,'' said one of the authors, Michael Greenstone, a professor of environmental economics at Massachusetts Institute of Technology.

``This study provides a unique setting to answer the life expectancy question because the (heating) policy dramatically alters pollution concentrations for people who appear to be of otherwise identical health,'' Greenstone said in an email. ``Further, due to the low rates of migration in China in this period, we can know people's exposure over long time periods,'' he said.

The policy gave free coal for fuel boilers to heat homes and offices to cities north of the Huai River, which divides China into north and south. It was in effect for much of the 1950-1980 period of central planning, and, though discontinued after 1980, it has left a legacy in the north of heavy coal burning, which releases particulate pollutants into the air that can harm human health. Researchers found no other government policies that treated China's north differently from the south.

The researchers collected data for 90 cities, from 1981 to 2000, on the annual daily average concentration of total suspended particulates. In China, those are considered to be particles that are 100 micrometers or less in diameter, emitted from sources including power stations, construction sites and vehicles.

The researchers estimated the impact on life expectancies using mortality data from 1991-2000. They found that in the north, the concentration of particulates was 184 micrograms per cubic meter _ or 55 percent _ higher than in the south, and life expectancies were 5.5 years lower on average across all age ranges.

The researchers said the difference in life expectancies was almost entirely due to an increased incidence of deaths classified as cardiorespiratory _ those from causes that have previously been linked to air quality, including heart disease, stroke, lung cancer and respiratory illnesses.

Total suspended particulates include fine particulate matter called PM2.5 _ particles with diameters of no more than 2.5 micrometers. PM2.5 is of especially great health concern because it can penetrate deep into the lungs, but the researchers lacked the data to analyze those tiny particles separately.

The authors said their research can be used to estimate the effect of total suspended particulates on other countries and time periods. Their analysis suggests that every additional 100 micrograms of particulate matter per cubic meter in the atmosphere lowers life expectancy at birth by about three years.

The study also noted that there was a large difference in particulate matter between the north and south, but not in other forms of air pollution such as sulfur dioxide and nitrous oxide.

Francesca Dominici, a professor of biostatistics at Harvard School of Public Health who has researched the health effects of fine particulate matter in the U.S., said the study was ``fascinating.''

China's different treatment of north and south allowed researchers to get pollution data that would be impossible in a scientific setting.

Dominici said the quasi-experimental approach was a good approximation of a randomized experiment, ``especially in this situation where a randomized experiment is not possible.''

She said she wasn't surprised by the findings, given China's high levels of pollution.

``In the U.S. I think it's pretty much been accepted that even small changes in PM2.5, much, much, much smaller than what they are observing in China, are affecting life expectancy,'' said Dominici, who was not involved in the study. (AP)