Xinhua news agency said a survey of eight cities found that police officers who had worked the streets for more than 20 years were the most at risk.

Pollution was the chief culprit, but stress, traffic noise and standing long periods in the sun were also to blame.

Chinese cities are battling to clean their polluted skies, especially the capital, Beijing, a year before it hosts the Olympic Games.

More than 90 percent of the 2,746 traffic police who underwent a check-up in Guangzhou had infections, the China Daily quoted the Guangzhou Hospital of Vocational Disease Control and Prevention as saying.

The incidence of lung, heart and respiratory tract diseases and arthritis was also higher among traffic police than the public as a whole, the newspaper added.

"Vehicle emissions and excessive heat were the major contributors to the condition," Liu Yimin, vice-president of the hospital, was quoted by a newspaper as saying.

"Traffic police have to work in a polluted environment for many hours a day, so their health is bound to be affected."

Guangzhou, with a population of about 10 million, is home to 1.8 million cars and other vehicles, and the number is increasing by 16 percent annually, according to the government Web site (www.gd.gov.cn).