A: Keeping the doors and windows closed only reduces the indoor and outdoor air exchange rate, but cannot automatically purify the indoor air. Research conducted by the School of Public Health at Fudan University China indicates that there is only a slight difference between indoor and outdoor air quality without using a room air purifier.

A: Incorrect. PM2.5 is defined as airborne particulates which have equivalent diameters below 2.5 microns. These tiny particles cannot block the visible light, so a blue sky is not a reliable indicator of good air quality. Smog is not the only pollutant in the air. Dust particles, bacteria, odours, chemical gases and so on can have adverse effects on our health too. Air purifiers can remove such harmful pollutants and contaminants.

A: Even when the outdoor air quality is good, the indoor environment is not necessarily clean. The indoor air may contain pollen, pet dander and other allergens. We can use air purifiers to remove these pollutants.