Heavy air pollution in 31 provincial capitals and municipalities in the Chinese mainland could have caused some 257,000 premature deaths in 2013, according to a report released on Wednesday.

The report on the health effects of PM2.5 in urban China, jointly released by Peking University and environmental group Greenpeace, found that an average of 90 out of every 100,000 people living in the 31 cities may have died prematurely from long-term exposure to the high levels of pollution in 2013.

Shijiazhuang in Hebei Province saw the highest rate of 134 premature deaths per 100,000, while Hebei is considered the most polluted province in the country.

Pan Xiaochuan, the lead Peking University researcher, urged local authorities to base their pollution control policies on public health effects evaluation rather than merely reducing the density of PM2.5, news portal thepaper.cn reported.

"These findings prove once again the paradox of China's old development pattern which relied on dirty industry," said Greenpeace East Asia campaigner Fang Yuan. "It need not be so. The fact that China's coal consumption drops as the economy expands shows that business activity and growth do not necessarily lead to smog."

The premature deaths were projected by using PM2.5 data from 2013 and the methodology of the World Health Organization's Global Burden of Diseases. Researchers said the smog-related deaths in 2013 could recur within the next 10 years.