On average, Americans spend more than 90 percent of their time indoors -- 65 percent of that time at home. That means daily exposure to levels of indoor air pollution that can be two to five and sometimes more than 100 times higher than outdoor levels. Naturally, those most at risk are those who spend their time indoors -- children, pregnant women, the elderly and people with lung disease such as asthma, emphysema and chronic bronchitis.

Unfortunately, it's often difficult to detect exposure to indoor air pollution because many of the contaminating substances give no warning and produce vague or sometimes similar symptoms that are difficult to pinpoint to a specific cause. Or, they produce symptoms years later when it's even more difficult to identify the source.

According to a report from the US Government Accounting Office, an estimated 20 percent of all schools in the United States have indoor air problems and 25 percent have unsatisfactory ventilation. Recent research has shown that indoor pollutants such as dust mites, molds, mildew, animal dander, insect waste and body parts (especially from cockroaches), and tobacco smoke have been strongly associated with increased respiratory problems for people with asthma, as well as people with allergies. This is especially alarming in light of the fact that asthma is on the rise, affecting an estimated 14.6 million Americans -- 4.8 million of whom are under the age of 18. Today, asthma is the sixth-ranking chronic condition in the country and the leading serious chronic illness of children in the U.S.

Those with asthma, allergies and other forms of lung disease are not the only ones affected by poor air quality. Many more may be at risk for health problems related to indoor air pollution. Some of the most prevalent and harmful pollutants include: