Windows were generally wood frame units that allowed significant air passage and insulation was commonly used only in attics. More fresh air entered these older, leaky homes and stale household air leaked out. This inadvertent air flow reduced the build-up of potentially harmful fumes. Insulating and sealing of newer buildings become more complex and complete. More synthetic materials such as sealants, caulking compounds and plastic vapor barriers are used.
Interior air is retained, and re-cycled to reduce the costs of heating or cooling.
Without proper air exchange and air filtration, tight buildings are associated with increased concentrations of dust, molds, toxic gases and stale air. Proper construction of indoor spaces must include provision for cross ventilation, proper exhausting of fumes and ample intake of filtered, fresh air. In the average home there are many sources of volatile gases that can lead to symptoms. Cleaners, waxes, polishes, paints, perfumes, soaps, cosmetics and many synthetic materials are capable of out-gassing. Gas furnaces, fireplaces, water heaters and stoves are sources of carbon dioxide, carbon monoxide and oxides of nitrogen.