WASHINGTON (AFP) - A decades-old Chinese policy of giving free coal for winter heating to residents in the north has shaved more than five years off life expectancy there, a study said on Monday.

The practice of providing no-cost coal to homes and offices north of the Huai River began prior to 1980 during China's era of central planning, said lead study author Michael Greenstone, professor of environmental economics at the Massachusetts Institute of Technology (MIT).

The result has been much heavier pollution in the north, up to 55 per cent more than in the south, and life expectancies that are 5.5 years lower, said the study in the Proceedings of the National Academy of Sciences.

The earlier deaths are "almost entirely due to an increased incidence of cardiorespiratory mortality," said the study, carried out by researchers from China and Israel.

Researchers looked at the increase in a type of pollution called total suspended particulates (TSPs) found in soot and smoke, which were 55 per cent higher in the north during the period studied,1981-2001.

The difference equaled about 184 micrograms of particulate matter per cubic meter.

The researchers then analysed mortality statistics from 1991 to 2000, both north and south of the river, and found evidence of shorter life expectancy in the free-coal areas.

"We can now say with more confidence that long-run exposure to pollution, especially particulates, has dramatic consequences for life expectancy," said Prof Greenstone.

In all, the estimates suggest that the policy may cost the 500 million people living in northern China some 2.5 billion life years, said the study.

"It's not that the Chinese government set out to cause this," Prof Greenstone said.

"This was the unintended consequence of a policy that must have appeared quite sensible." The researchers said their analysis can also be extended to other countries, showing that long term exposure of an additional 100 micrograms of particulate matter per cubic metre is linked to a lower life expectancy at birth of about three years.

Chinese government agencies have described particulate-matter levels of more than 400 micrograms per cubic metre between 1981 and 2001, the researchers said.

In comparison, the United States had total suspended particulates of about 45 micrograms per cubic meter in the 1990s, the authors said.

"The analysis suggests that the Huai River policy, which had the laudable goal of providing indoor heat, had disastrous consequences for health, presumably due to the failure to require the installation of sufficient pollution abatement equipment," the study concluded.