Shanghai city has suffered slight air pollution for three consecutive days - a first for this year.

And more of the same is forecast for today.

Pudong New Area has had the poorest air pollution index in recent days, according to officials.

Experts from the Shanghai Environmental Protection Bureau yesterday began an urgent monitoring network, incorporating the meteorological authorities and universities, to study the cause and a possible solution to the air pollution.

Hu Guoliang from the bureau's pollution control department, said cities along the Yangtze River suffered similar air pollution in recent days which may be related to a cold front from the north.

"This recent air pollution is strange as people haven't felt discomfort even though the air pollution index, especially particulate material, is high," he said.

"A similar situation took place in 2005. We are cooperating with neighboring cities and experts to closely monitor the situation."

He said the bureau would carry out measures after determining the cause.

City air quality has five categories - excellent, good, slightly polluted, moderately polluted and seriously polluted.

Shanghai has taken many measures, such as shutting highly polluting plants, to improve air quality and has seen more than 85 percent of the days in a year with excellent and good quality air since 2003.

City medical experts have urged people, especially those with a history of allergies, respiratory diseases and cardiovascular problems, to take preventive measures during this period of pollution.

Yue Mengyuan, a doctor from the Shanghai Children's Medical Center, said there had been an increase in patients with respiratory ailments in recent days.

"People with ailments like asthma should try and avoid going out," Yue said. "If they have to go out, they should wear masks."