pollution, so any cracks in pipes, backdrafts, or other problems will allow those pollutants into the home instead of venting them outside. Stoves, ovens, furnaces, water heaters, and gas fireplaces all release carbon monoxide into the home, which can kill if it is not vented.

Even more natural options, such as wood stoves and fireplaces, release similar gases, as well as potentially harmful particulate matter.

Living things

Virtually all animals add their dander (dead skin cells and fur which are sloughed off and replaced naturally) to the mix of air pollution, but so do humans. The human body constantly releases dead skin and replaces it with new layers. All living creatures release potentially harmful chemical substances as well.

Smaller living things also add to air pollution – notably mildew, mold, and bacteria. If your region suffers from humidity above 50% to 60%, these microscopic wind-travelers can cause serious trouble for both human health and the home's structural integrity.

Furnishings

Apart from the noxious chemicals used in their manufacture, carpets and upholstery have a second bad habit: they absorb yucky stuff. Toxic chemicals and other pollution floating around the house are sucked in, like water into a sponge – anything organic and easily evaporated is fair game. The pollution is then released over days, months, and years, affecting your health all the time.