Residents in Fuyang, Anhui province continued morning exercises in spite of thick smog which had reduced visibility to less than 50 meters on Jan 14. An Xin / For China Daily

That's the highest level recorded in Beijing.

The World Health Organization considers the safe daily level to be 25 micrograms per cubic meter.

The problem has now become a national phenomenon, rather than one unique to Beijing. On Saturday, levels of PM2.5 passed 300 micrograms per cubic meter in 33 of the 74 cities with systems equipped to monitor the particles.

"Larger particles can be filtered by the nose and expelled through the respiratory tract, PM2.5 and viruses can become lodged deep in the lungs and produce harmful effects," says China-Japan's Zhang.

"PM2.5 exposure is linked to a variety of both upper and lower respiratory illnesses, including flu, cold, asthma, bronchitis and chronic obstructive pulmonary disease, as well as to worsening cardiovascular diseases."

Zhang says PM2.5 is more hazardous to people with weaker immune systems, such as children and the elderly.

The people's skyline

In Chongqing, often nicknamed the "fog city", the haze has also increased the number of patients in the emergency room.

Geng Zhaohua, a cardiovascular diseases specialist with Xinqiao Hospital, says they've seen at least a 20-percent increase of patients compared to last year.

"Particles in the air are not only harmful to respiratory tracts, but are also likely to produce irritation and diseases in cardiovascular system, because of the reduced oxygen intake.

"Besides, when it is cold, the blood vessels contract and result in higher blood pressure, which will contribute to such cardiovascular diseases as heart attack and stroke," Geng says.

Despite all the problems that air pollution, cold weather and the flu can create, doctors interviewed by China Daily say there is no need to panic. They highly recommend people receive vaccinations and stay indoors while the smog lingers.

"Many people are confused if they should continue to exercise in such weather. It is absolutely not wise to exercise in such cold and smoggy days. The loss overwhelms the gain," China-Japan hospital's Zhang says.

Doctor Richard Saint Cyr of Beijing's United Family Hospital says that now is the perfect time to get the flu vaccine, especially for the elderly, pregnant women and children.

"No one is saying it's a serious pandemic, they're just saying it's a relatively bad year. I don't think it's any more worrying than the usual flu season concerns, certainly nothing like an H1N1 pandemic scare.

 "The situation can change quickly, of course, but right now it just appears to be a bad but not serious flu season," he says

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