SHANGHAI - Heavy smog in eastern China on Tuesday made authorities in Shanghai order schools to halt outdoor activities.

The Shanghai municipal environmental protection bureau issued a yellow alert Tuesday morning, the third highest in a four-tier warning system as the air quality index passed 300.

The agency also ordered to reduced production at some factories and halted work at some construction sites.

"We paid great attention to the red alert Beijing issued days ago," said Wu Jiamin, a Shanghai resident. "So we have learned several ways to fight pollution, such as regular replacement of masks and persuading the elderly to halt outdoor exercise," she said.

At 6 am Tuesday, the environmental protection bureau in neighboring Jiangsu Province issued a blue alert, the lowest warning, saying most cities in the province would see smog in the next 24 hours. Several major cities of Zhejiang Province were also affected.

The authorities in Shanghai, Jiangsu and Zhejiang blamed pollutants from northern regions for the smog. While a cold front helped to disperse pollution in northern China, it brought the pollutants south.