KUALA LUMPUR, Aug. 21 -- As the haze returns to Malaysia, staying indoors is often the simplest way to reduce exposure to polluted air.

But how much protection do closed doors and windows really provide against microscopic particulate matter such as PM10 and PM2.5 that can enter a home?

According to experts, keeping doors and windows closed can significantly reduce the amount of outdoor particulate matter entering a home, particularly in residences with air-conditioning systems.

Applied Air Quality Specialists Sdn Bhd chief executive Tan Zhi Shen said particles such as PM10 that enter an indoor space will eventually settle on surfaces, while finer particulate matter such as PM2.5 may remain airborne longer before settling.

Tan, whose ...