Outdoor air may be ‘cleaned’ by filtration. This is a method by which particulates and, sometimes, gaseous pollutants are removed from the air. Pollutants are intercepted by a filter while allowing clean air to pass through. This method of air cleaning is especially necessary when high concentrations of particulates are present or when the source of pollutant is derived from outside the building. Potential benefits can include improved air quality, reduced dependence on ventilation and improved energy efficiency. Filtration is not, however, a substitute for the ventilation needed to meet the metabolic requirements of occupants. Neither can filtration be used in leaky or naturally ventilated buildings. A review of particulate contamination and filtration methods is presented in the Guide to Energy Efficient Ventilation.