The Ministry of Environmental Protection said on Wednesday that a recent study about the link between air pollution and life expectancy is unconvincing due to the limited samples on which it is based.

Liu Zhiquan, an official with the ministry, told China News Service on Wednesday that the pollutants emitted in coal burning include heavy metals, small particles and sulfur dioxide, which can affect human health, but whether the pollution can shorten lives or not is still unknown.

It will take a long-term observation to get such a result, Liu said.

The study, published in a US multidisciplinary scientific serial called the Proceedings of the National Academy of Sciences on Tuesday, showed that air pollution from coal burning caused people in northern China to live an average of 5.5 years shorter than those who live in the south. Coal burning has been providing heat to the north for decades.

It analyzed the total suspended particulate matter and deaths in 90 cities across the country from 1981 to 2000.