"They may not have an attack of asthma for breathing smog, but they usually experience symptoms such as a runny nose and chest tightness."

Long-term exposure to air pollution is extremely adverse to controlling child asthma, and frequent attacks increase the risks of someone suffering the disease after growing up, and even developing chronic obstructive pulmonary diseases, he warned.

Zhang Yinping, a professor at Tsinghua University's School of Architecture, said he believes that household decorations are also to blame for the increase in child asthma.

In January he released findings in the Chinese Science Bulletin in which he surveyed more than 48,000 children in 10 cities.

The findings showed that Shanghai had a higher prevalence of asthma among children than any of the other nine cities - 9.8 in every 100 kids aged 3 to 6 in Shanghai were diagnosed with asthma in 2011.

However, it had the lowest concentration of PM10, particles whose diameter is at most 10 micrometers and can be inhaled, in 2010.

"Decorations, including semi-volatile organic compounds such as plastics and flame retardant in furniture, may have a greater influence on causing asthma than outdoor air pollution of PM10," he said.

"We speculate the decorations in apartments are probably the reason why Shanghai has the highest doctor-diagnosed asthma prevalence.

"About 90 percent of the air people breathe is indoors. It's not sound enough to attribute the increase of respiratory diseases directly to outdoor atmospheric pollution."

But the results do not mean air pollution is irrelevant to the cause of the disease, given that PM10 is only part of the pollutants, he admitted.

Wu Yonghong is taking no chances, however. Her 12-year-old daughter relieves her asthma by swimming regularly, and Wu said she does not let her daughter do outdoor exercises when the air is bad.

"I have a treadmill and an air purifier for her in our house, so she can do some exercise during smoggy days," she said. "The air pollution, surely, is bad for her recovery.

"All I can do is try my best to lower the exposure by having her wear masks, and shutting doors and windows and turning on the purifier."

wangqingyun@chinadaily.com.cn