Smog has helped trigger a wave of respiratory ailments in the past several days in Beijing, while flu viruses such as H1N1 continue to spread across the city, experts said.

Ma Anlin, deputy director of the department of infectious diseases of China-Japan Friendship Hospital, said the hospital's fever clinics, including that in the pediatric department, are receiving 600 people a day, many more than in the same period last year.

"A large number of the patients show flu-like symptoms. It is a relatively concentrated outbreak of flu-like cases since the outbreak of H1N1 flu in the spring of 2009, but the prevalence is still at a low level," he said.

"Flu viruses tend to reach a peak in their activity every two decades and have smaller outbreaks every three to five years. Also, they tend to be more active in extreme weather, such as in this unusually cold winter."

Meanwhile, the clinics received slightly more visits around Sunday, when air pollution caused by the smog assisted the spread of viruses, Ma said.

"The most effective way to prevent flu is ventilation. However, in an environment where the air is still or moves at a slow pace, such as when there are many particles in the air, it will heighten the risk of inhaling them," he said.

"Also, too many particles inhaled may disrupt the hairlike cilia in human airways, which act to keep out dirt and micro-organisms, making it easier for the virus to contact the respiratory tract directly."

Li Yanming, a doctor at the respiration department of Beijing Hospital, agreed, and believed the damage the smog does to one's respiratory system is much greater than cold weather.

"Too many inhalable particles in the air will defy the self-cleaning function of the respiratory system, so some of the particles may enter the lower respiratory tract and even the lungs," she said.

Li said more than 300 people visit her department a day, with conditions varying from a common cold to chronic obstructive pulmonary diseases. "The number of these patients has increased by 20 percent since summer."

People with weaker immunity, such as children, elderly and people with chronic health problems, are more prone to respiratory diseases when there is less ventilation and more tiny particles in the environment, experts said.

On Tuesday, the hall of the emergency department of the Capital Institute of Pediatrics was swarmed with parents and their children taking drip infusions.

Four people told China Daily their children or grandchildren had respiratory ailments.

A nurse in the department said most of the young patients in the hall were having respiratory problems.

Lu Yuxiu, 66, said her 2-year-old grandson and 5-year-old granddaughter had both caught a cold and had a high fever last week.

"Right now they have fevers of 39 C and inflamed tonsils," she said. "I think the air may have affected their health."

On Tuesday, the Beijing Meteorological Station lifted the orange smog alert, which it had issued on Sunday. The station predicted that on Wednesday, northern winds will return to the city and drive away particles in the air.