Haze from urbanization and industrial development is a serious, persistent problem in many parts of China, especially the north, the south and in the middle and lower reaches of the Yangtze River, the National Meteorological Center (NMC) said Monday.

Buildings are shrouded in fog in Guilin City, Guangxi Zhuang Automonous Region March 20, 2009.

According to the NMC, the problem was worst in the provinces of Shanxi, Henan, Jiangxi, Guangxi and Yunnan, which got more than 20 days of haze a year. Linfen City, Shanxi, had the highest incidence of haze at 84.6 days on average annually.

The NMC statistics were released on World Weather Day, and they were accompanied by health warnings.

Haze not only caused low visibility and poor air quality, it might also induce respiratory diseases, said Qiao Lin, chief NMC forecaster.

The toxic particles in haze might cause bronchitis, allergic rhinitis and asthma, said Xiao Wenlong, doctor for respiratory diseases at the Affiliated DrumTower Hospital of Nanjing University Medical School.