According to the U.S. Environmental Protection Agency, items inside the home that release gas are the primary cause of indoor air problems. The list includes carpeting, upholstered furniture, gas appliances, paints and solvents, cleaning products, air fresheners, dry-cleaned clothing and pesticides. If you have an attached garage, fumes from the gasoline, oil and antifreeze in your car can find their way into your home's air. Harsh chemicals can also come from cigarette smoke and wood stoves.

Insufficient ventilation can worsen the problem because pollutants get trapped inside. Tightly sealed and well-insulated homes keep out fresher outdoor air, which can dilute the pollutants. High temperature and humidity levels can also increase concentrations of some pollutants.