Wuhan, the capital city of Hubei province, has issued smog warnings for three days since Monday because of serious air pollution.

The lingering smog forced construction sites to stop work and primary and middle schools to suspend outdoor activities.

From Monday through Wednesday, the city's air quality index reading remained high, with the highest reading at 323, with PM2.5, or particulate matter smaller than 2.5 microns, as the main pollutant, according to the city's meteorological bureau. Any reading over 300 is considered "seriously polluted".

The city's meteorologists gave yellow warnings for smog each day since Monday. China uses a four-tier warning system for extreme weather and air pollution, with red the most serious, followed by orange, yellow and blue.

The city experienced at least eight heavily polluted days in the first two weeks of December, with the air quality index reading reaching a high of 440, China News Service reported.

Among those affected was a square dancing team in Jiang'an district, which had to stop daily practices after several members fell ill.

Square dancing is a common open-air fitness practice across China, with most of the participants middle-aged or elderly women.

Media reported the leader of one dancing team had to go to a hospital after she had an asthma attack on Dec 11.

Identified only as Fang, 65, the dance team leader said she decided to suspend the practices after she learned her illness was caused by smog, and after several other members also fell ill, the report said.

Gan Xuejun, deputy director of Wuhan No 8 Hospital, which treated the woman, said air pollution has a direct effect on the respiratory and cardiovascular systems and easily triggers related diseases, especially among elderly people.

On smoggy days, the hospital received two to three times as many patients with respiratory and cardiovascular complaints than on clear days, Gan said.

A random survey by China Daily found many elderly people in Wuhan have scant knowledge about the bad effects brought on by smog and few know about PM2.5.

To tackle its air pollution, the Wuhan government in early December released its clean air plan for the next five years.

The city plans to invest 28 billion yuan ($4.61 billion) in air pollution control by 2017. The plan says the city will boost the development of clean energy vehicles for public transportation and remove all of the current 140,000 heavy-polluting vehicles from Wuhan's roads by the end of 2016.

Contact the writers at liukun@chinadaily.com.cn and jinhaixing@chinadaily.com.cn.