Shanghai air is likely to be "lightly polluted" today and tomorrow under the influence of a cold front from the north, authorities said yesterday.

Citizens walk on an overpass amid fog in Shanghai, China, Jan. 30. [File photo/CFP]

Although pollution in north China has started to disperse, the cold front may still bring airborne pollutants to the city from today, Fu Qingyan, a chief engineer at the Shanghai Environmental Monitoring Center, said.

After Wednesday's heavy pollution, the city's residents breathed a little easier yesterday with the hourly PM2.5 reading at around 40 micrograms per cubic meter, or "good," according to the center. It said this was due to warm and wet air currents from the south.

Last month, with the launch of a more accurate and stricter air quality evaluation system, the city had 17 days deemed polluted, according to the center.

January is usually the month that sees the most frequent pollution, Fu said. "However, the data this year can't compare with previous years since we adopted a more accurate and stricter air quality evaluation system. The main pollutant, PM2.5, was not calculated in the previous evaluation system."

PM2.5 refers to tiny particles that can travel deep into the lungs.

Zhong Nanshan, head of the Guangzhou Institute of Respiratory Diseases and a renowned expert in SARS, said the heavy PM2.5 density was even more dangerous than severe acute respiratory syndrome. "You can block SARS but no one can escape from air pollution," Zhong told China Central Television.

Recent research in Hong Kong showed that hospital admission rates for patients with respiratory diseases increase by 3.1 percent as PM2.5 density increases by 10 micrograms per cubic meter, and the death rate rises 11 percent if PM2.5 readings reach 200, Zhong said.

Liu Zhongmin, director of Shanghai East Hospital, said: "The emphysema rate in people above 40 years old in China has reached 8.7 percent and lung cancer carrier is increasing by 7 percent every year. The air pollution surely has a deeper and bigger influence than SARS as people may inhale pollutants with every breath."

Meanwhile, an end to the current warm weather is imminent because of the expected cold front, with maximum temperatures due to drop below 10 degrees Celsius tomorrow, forecasters at the city's meteorological bureau said.