Other contaminants deliberately introduced into the indoor environment. These can include tobacco smoke from pipes, cigars and cigarettes, cosmetic powders, baby powder and some powdered laundry detergents, aerosols such as air fresheners, and cleaning products with strong odors. Problems with Buildings and Building Systems Ventilation systems are designed and installed in buildings to replace stale, contaminated air with fresh air from the outside. Air enters and leaves a house in several other ways too. It can enter by infiltration, through construction joints and cracks around windows and doors, through the foundation, and crawl spaces.
Air also enters through natural ventilation openings, such as open doors and windows.
It measures how many times the complete volume of air inside the house is replaced with fresh outside air. In typical U.S. homes, the average exchange rate is 0.7 to 1 complete air exchanges per hour. In tight homes, the exchange rate can be as low as 0.02 complete air exchanges per hour. In leaky homes it may be as high as 2 complete air changes per hour. Some current construction practices can trap pollutants that normally form inside the house along with those brought inside with everyday traffic.