Indoor air quality can be compromised by outdoor air pollutants penetrating into homes through gaps in walls, windows and doors. Burning coal, smoking, using cleaning detergents and low-quality construction materials also sources of indoor pollution. In order to improve indoor air quality, use air purifiers with HEPA (High Efficiency Particulate Air) that is able to filter out over 99.9% of PM2.5. If the air outside is clean, indoor air quality can be improved by ventilating your space with outside air. Remember to check outside air quality first.