The report, by experts in China, the US and Israel, said a government policy of giving out free coal for home and office heating everywhere north of the Huai River in central China between 1950 and 1980 resulted in an increase in the burning of coal and an associated increase in heart and lung disease.

The study is based on analyses of health and air-quality data from 1981 to 2000.

Even though coal is no longer free in the north, the policy has had a long-lasting impact because so many of the heating systems remain in use.

There are very few cities in the south with heating systems like those in the north, providing an ideal control group for the research.

The study shows that though the government’s policy was well intended, it ended up doing more harm than good.

The air concentration of total suspended particulates in the north was 55% higher than in the south and life expectancies were 5.52 years lower, the researchers found.

"Life expectancies are about 5.5 years lower in the north owing to an increased incidence of cardio-respiratory mortality," the study said.

"The analysis suggests that the Huai River policy, which had the laudable goal of providing indoor heat, had disastrous consequences for health.”

Since there are 500 million residents in northern China, the air pollution was associated with the loss of more than 2.5 billion life years of life expectancy, the study said.

The researchers were able to determine that an extra 100 micrograms per cubic meter of total suspended particulates was associated with a drop in life expectancy of about three years.

“These results may help explain why China’s explosive economic growth has led to relatively anemic growth in life expectancy,” it concluded.

The paper’s co-authors are Michael Greenstone, Yuyu Chen, Avraham Ebenstein and Hongbin Li.

It was published in Tuesday’s edition of the Proceedings of the National Academies of Sciences.

"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,” Greenstone told National Geographic.