Low levels of radioactive iodine-131 were detected in the air above Beijing, Tianjin, Ningxia Hui Autonomous Region and the provinces of Shandong, Hebei, Henan and Shanxi yesterday.

And caesium-137 and caesium-134, again at very low levels, was detected in Anhui and Guangdong provinces, Guangxi Zhuang Autonomous Region and Ningxia Hui Autonomous Region, China's National Nuclear Emergency Coordination Committee said.

The committee said the levels of radioactive material were below one-hundred-thousandth of the average annual exposure level.

The findings follow the discovery of iodine-131 in the air above Shanghai on Monday and again yesterday. The Shanghai Environmental Protection Bureau said yesterday's levels were still too low to have any effect on the environment or people's health.

The traces could take two to three months to disappear from the city, experts said.

The radioactive material from Japan's earthquake-damaged Fukushima Dai-ichi nuclear plant spread to the city through atmospheric diffusion not the wind, officials from Shanghai Radiation Environment Supervision Agency said.

A senior engineer at the agency, Wang Mingxia, added that the discovery of plutonium in the soil near the Japanese plant would be of no concern locally. "Plutonium, though much more harmful than iodine-131, only impacts the areas close to the plant," he said.

Low levels of iodine-131 were detected in Shanghai and other southeastern coastal areas including the provinces of Jiangsu, Zhejiang, Guangdong, Anhui and Guangxi Zhuang Autonomous Region on Monday after its detection in Heilongjiang Province at the weekend.

Wang said the agency was checking for all radioactive substances in the air, including iodine-131 and caesium-137, the current two major substances directly related to the nuclear plant.

"Our equipment can report any abnormal detection immediately," he said.

Gao Linfeng, an official with the Shanghai Center for Disease Control and Prevention, said the public needn't take any protective measures against iodine-131.

"The detected levels were also too low to contaminate local food and drinking water," Gao said, adding: "It is not helpful to take iodine tablets at present."

Hong Kong also detected minimum radioactive iodine-131 in the air over the weekend.