By Li Hongmei

China's Health Ministry will set up a national network within five years to monitor the long-term impact of chronic air pollution on human health.

The network will gather data on PM2.5, or particulate matter with a diameter of 2.5 micrometers, in different locations around the country, the ministry said in a statement.

The absence of a long-term, systematic monitoring system has prevented the country from uncovering the link between air pollution and human health, the ministry said.

The network will first cover cities where smog is most prevalent, it added.

The evaluation will be based on the integrated and long-term analysis of PM2.5 data, weather information and cases of local residents' diseases and deaths, according to the ministry.

An international study published in July showed that air pollution is shortening the lives of people in northern China by about 5.5 years compared to the south, a legacy of a policy that provided free heating, mainly using coal, in the north.

Meanwhile, the National Meteorological Center said yesterday large parts of northern China will be blanketed by smog over the next three days. Several regions including Tianjin and Beijing received a yellow alert for heavy fog yesterday, after suffering from lingering smog over the past two months.

The density of PM 2.5 exceeded 150 micrograms per cubic meter early yesterday in most parts of northern and northeastern China.

The density of PM 2.5 surpassed 245.3 micrograms per cubic meter in Tianjin, according to NMC monitoring data.

The center warned of a decline in air quality and advised residents in the regions to stay indoors or take precautions.

Last week, the PM2.5 index reached a reading of 1,000 in some parts of Harbin, capital of northeastern Heilongjiang Province and home to some 11 million people, virtually shutting it down.

A level above 300 is considered hazardous, while the World Health Organization recommends a daily level of no more than 20.