Women wearing masks and other residents dance during their daily exercise amid the heavy smog in Beijing on Monday, the day the capital issued its first-ever red alert for pollution from Tuesday until Thursday. JASON LEE/REUTERS

Liu Chuanhe, a doctor in the respiratory department of the Capital Institute of Pediatrics, said he has received more patients in the past several days.

"There are indeed more patients, including some whose syndromes have worsened, during smoggy days," Liu said.

Liu said he examined more than 50 children on Tuesday, most of them suffering from respiratory conditions such as asthma.

"Some of the patients came to the hospital because symptoms of their ailments, such as asthma and rhinitis, reappeared in the past several days," he said.

Since Beijing is shrouded in thick smog, doctors also warn patients, especially children and the elderly, to take measures to protect themselves from the pollution.

Li Yanming, a doctor of respiratory diseases at Beijing Hospital, said smog harms all people, but groups such as children, whose disease resistance has not matured, the elderly and those with diseases will suffer most.

"Healthy young people usually are more resilient, but smog may produce more serious consequences for children and the elderly, who are more vulnerable to diseases," she said.

Li suggested that people avoid outside activities as much as possible during smoggy days and wear am ask when going outside.

"In particular, children should wear masks when going out," she said.

Air purifiers should be used with doors and windows closed to reduce particles indoors, she said.

Smog can harm health in many ways, such as leading to lung cancer and cardiac and vascular diseases, Li said.

"Almost all those with lung or respiratory diseases will feel discomfort during smoggy days," she said.

Bernhard Schwartlander, the World Health Organization representative in China, said in a news release: "Air pollution is a serious health hazard. In the short term, high levels of air pollution can cause health problems such as respiratory and breathing problems and asthma attacks.

"People already prone to these kinds of health conditions, such as the very young and the very old, need to take particular care. In the longer term, air pollution can cause heart disease, cancer (especially lung cancer), and chronic respiratory problems," he said.

Song Li, a 31-year-old mother who was at Beijing Children's Hospital, said her 18-month-old child began to cough two or three days ago.

"She never coughed before," she said. "The doctor told me she has tracheitis and it might be linked to the smog.

"The doctor suggested I get a mask for my baby, but her face is too small. I will see if I can buy a small mask for her."