BEIJING - Smog in parts of north China, including Beijing, from Sunday, has prompted Lisa Wu and parents of her son's classmates to buy a high-end air purifier for their kids.

"We raised over 10,000 yuan ($1,500) for the purifier," said Wu, mother of a third-grader in Beijing. "It will be delivered next week.

"Too many kids fell ill during the recent smog and we are left with no other choice," she said.

Monday evening's first red alert ever for smog in Beijing has been followed by online discussions on masks and air purifiers. Like Wu, many are asking about the effectiveness of the products before joining the shopping rush.

Alibaba searches for masks and air purifiers have surged 148.4 percent and 56.5 percent month on month in the past 30 days and many sellers ran out of stock.

A shop owner at Taobao told Xinhua that his shop has sold all imported air purifiers in stock. "We normally sell 20 to 30 purifiers a month," he said, "but we have sold over 80 just this week. I have sold out of all my stock, but customers can place orders and we will deliver within two weeks."

Some shops are even profiteering, hiking prices as people rush to buy.

Liu Youwei bought an air purifier on Singles' Day on Nov 11 and said the shop owner has been regularly raising prices since then as the smog season kicked in.

Beijing has banned two million private cars from roads, closed factories and suspended construction work. Neighboring regions have taken similar measures. To ensure the effectiveness of anti-pollution measures, the environmental protection authorities of Beijing, Hebei and Tianjin are collaborating on a joint law enforcement action for the first time, including surprise night inspections.

Heavy industry is the focus of the inspections, said Zhong Chonglei, head of Beijing environmental inspection squadron.

Coal-burning contributes most of the pollutants, with vehicle emissions and construction work also being blamed.

China's cabinet decided on Dec. 2 to upgrade coal-burning power plants nationwide and reduce pollutants by 60 percent in the next five years.