People with respiratory issues usually get a break in the summer, but after Beijing's spell of smog in June, more and more are finding anything but relief.

Li Yanming, a doctor at the respiratory department at Beijing Hospital, said she has received calls from four asthma patients over the past three days, all complaining that their chronic disease is worsening. She said the heavy pollution in the capital could be exacerbating problems for asthma sufferers.

"Usually asthma becomes active during spring and autumn. Summer is a season where asthma can be controlled without medication. Their problems may have been caused by the pollution," Li said.

Zhang Shunan, vice-director of the lung diseases department at the China-Japan Friendship Hospital, said the number of patients he has seen has recently increased by 10 to 20 percent.

"Summer should be a season when there are fewer people with respiratory symptoms," he said.

People with sensitive and weak respiratory systems or respiratory diseases are more prone to weather changes, he said. Coughing, a scratchy throat and chest tightness usually occur for asthma sufferers when weather conditions worsen suddenly. Over the long run, he said, smog may cause severe chronic diseases such as lung cancer and chronic obstructive pulmonary disease.

Both Li and Zhang advise using masks, but added that only medical workers should wear the N95 mask made by 3M for long periods of time. The N95 mask can block fine particulate matter such as PM2.5, particles with a diameter of 2.5 micrometers or less, from entering the respiratory system.

"The mask makes it more difficult to breathe, and a majority of people can't bear it for a long time," Li said. "The most effective way is to cut down on outdoor activities."

Zhang warned that PM2.5 particulates is not the only pollutant in the atmosphere. Masks, he said, can help keep away pollutants larger than PM2.5.

Zhang Jinliang, a researcher with the Chinese Research Academy of Environmental Sciences, explained that besides PM2.5, the density of pollutants that are greater in size all rise when the atmosphere is heavily polluted.