The smog that has shrouded the northern parts of the country since last week will persist until Thursday, China's environmental watchdog said.

Special: Fight air pollution

In Beijing, the measurement of PM2.5 - particulate matter with a diameter less than 2.5 micrograms, which can go deep into the lungs and is extremely harmful to people's health - soared to the hazardous level of 276 on Monday, the Ministry of Environmental Protection said.

Tianjin and many cities in Hebei and Shandong provinces were also hit by heavy air pollution, with Xingtai and Dongying having PM2.5 readings above 250.

The environmental watchdog advised that children, the elderly and people with respiratory or cardiovascular diseases stay indoors to reduce the risk from polluted air.

The Beijing Environmental Protection Bureau said it will send more water-spraying vehicles on the road, and strictly monitor polluting enterprises and construction sites to reduce dust and emissions.

It also suggested that residents use public transportation.

The Beijing Environmental Monitoring Center asked the public to put on protective masks when going out.

Meanwhile, as people become more aware of air pollution, consumers are racing to get air purifiers in their homes.

Huang Xiaoping, manager of J&D Activated Carbon Filter Co, a company that produces filters for household air purifiers, said its business is booming in China, and the market is a lot bigger than that in the US.

"We started the business in 2002, and the recent two years witnessed rapid growth," he said at an international air purifier exhibition in Beijing on Monday.

The three-day exhibition, held by the Chinese Society for Environmental Sciences, kicked off at the National Agriculture Exhibition Center on Monday.

More than 20 percent of the households in the US are equipped with an air purifier, and the figure is similar in Japan, in sharp contrast with China, where only a small percentage of families are using a purification system, he said.

Zhao Yusen, sales director of Beijing Greentech Co, a dealer of a US-brand air purifier located in Beijing, said he was confident about the future of the air purification market in China.

"We are confident about our sales, both due to our technology and the great potential of the market," he said.

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