CITIZENS experienced the heat of summer as the temperature rocketed to 34 Celsius degrees last Sunday. So the use of air conditioners has become a necessity, but some people are still afraid to run central air-conditioners even though the SARS crisis has almost ended.

　　The worry can be eliminated if the air inlet filter of the central air-conditioner and the filters of each room are cleaned and disinfected regularly to eliminate infectious particles and prevent the spread of harmful elements.

　　Another important aspect of healthy indoor air is good ventilation. A man breathes out about 0.17 cubic metres of carbon dioxide per exhalation, so about 30 cubic metres of fresh air are needed per person per hour.

　　In other words, a closed room with 20 square metres of floor space which is 3-metre high only contains sufficient air for two people to breathe for our hour.

　　In many modern buildings, the windows are designed to be tightly closed and people are used to keeping the doors closed. However, to maintain a healthy environment, people need to open the windows and doors for about 10 minutes every two hours to let the air flow freely by convection.