Underground Hazards
The basement is not a part of the house often frequented but it can be the source of some of the most polluted air in the home. Closest to the ground, the basement is susceptible to radon, a natural radioactive gas linked to lung cancer. Test your home for radon. Often ventilating the area is a good fix for a radon problem.

Improperly functioning heating systems and gas-fired dryers not well vented are sources of carbon monoxide. "Carbon monoxide kills you and does so quickly," says The Healthy House Institute's John Bower. A carbon monoxide detector is a good inexpensive investment.

Also, the basement is one of the most vulnerable areas for moisture, leading to mold growth and other allergens. Inspect for condensation on walls, standing water on the floor or sewage leaks, according to the Healthy Indoor Air for America's Homes initiative.

Keeping gutters cleaned, grading soil away from the house, applying waterproofing sealants in the basement can prevent outside water from getting in. If water gets in the basement, you can kill and remove mold and mildew by applying one cup of bleach to one gallon of water, according to the initiative.

Finally, if you use the basement as an office, playroom or a shop area, make sure it is heated, has air conditioning and is ventilated like the rest of the house, Bower advises.