“This adds to the growing mountain of evidence of the heavy cost of China’s pollution,” said Alex L. Wang, a law professor at the University of California, Los Angeles, who studies Chinese environmental policies. “Other studies have shown significant near-term harms, in the form of illness, lost work days and even risks to children beginning in utero. This study suggests that the long-term harms of coal pollution might be worse than we thought.”

Mr. Wang said the new study could “help to build the case for more aggressive environmental regulation” — for example, a previous order by Chinese leaders to shut down coal-fired boilers in some areas could be widened, and faster shutdown times could be required.

The health statistics recorded through the two-decade period by Chinese officials and examined by the study’s researchers showed that the 5.5-year drop in life expectancy in the north was almost entirely due to a rise in deaths attributed to cardiorespiratory diseases or related health problems.

The pollution data, also recorded by officials, indicated that the concentration of particulates north of the Huai was 184 micrograms per cubic meter higher than in the south, or 55 percent greater.

Several recent scientific studies have revealed the toll that China’s outdoor air pollution is taking on humans. This spring, new data released from the 2010 Global Burden of Disease Study revealed that such pollution contributed to 1.2 million premature deaths in 2010, or nearly 40 percent of the global total.

Some Chinese officials have sought to quash reports that link premature deaths to pollution. According to news reports, Chinese officials excised parts of a 2007 report called “Cost of Pollution in China” that had concluded that 350,000 to 400,000 people die prematurely in China each year because of outdoor air pollution. The study was done by the World Bank with the help of the Chinese State Environmental Protection Administration, the precursor to the Ministry of Environmental Protection.

This year, many Chinese have expressed fury and frustration over the surging levels of air pollution, especially in the north, which in January had record levels of particulate matter. Pollution levels have remained high this summer, and many foreigners and middle- or upper-class Chinese with children are looking to leave the country rather than tolerate the health risks.

Mr. Greenstone said he did not have a basis for comparing pollution levels now with those during the period covered by the study, 1981 to 2001. During that time, the method of measuring particulate matter was different. Mr. Greenstone also said he did not know how pollution in northern China affected the life expectancy for people not living there for their entire lives, or for residents of northern China who made frequent or long trips to less polluted areas.