Indoor air pollution is among the top five environmental health risks. Usually the best way to address this risk is to control or eliminate the sources of pollutants and ventilate a home with clean outdoor air. But opportunities for ventilation may be limited by weather conditions or by contaminants in the outdoor air. If the usual methods of addressing indoor air pollution are insufficient, air-cleaning devices may be useful. Air filters and other air-cleaning devices are designed to remove pollutants from indoor air. Some are installed in the ductwork of a homes central heating, ventilating, and air-conditioning (HVAC) system to clean the air in the entire house. Portable room air cleaners can be used to clean the air in a single room or in specific areas, but they are not intended to filter the air in the whole house. Air-cleaning devices are categorized by the type of pollutants--particulate and gaseous--that the device is designed to remove or destroy. Two types of air-cleaning devices can remove particles from the air: mechanical air filters and electronic air cleaners.