Importance of Indoor Air Quality to Human Health
Indoor air quality is important to human health because individuals spend a large fraction of their time indoors at their residences, schools and workplaces. In addition, there are numerous sources of airborne toxic pollutants in these indoor environments where outdoor air ventilation provides the only primary means to dilute pollutant concentrations.

In 1987 and 1988, the California Air Resources Board (ARB) conducted a statewide survey of activity patterns of individuals over 11 years of age (Jenkins et al., 1992). Participants completed recall diaries of activities and locations and responded to questions regarding their use of and proximity to potential pollutant sources. The results showed that Californians spent, on average, 87% of their time indoors. This was broken down into 62% of the time at home and 25% of the time indoors, but not at home including someone else's home, work, school, shopping, church, restaurant, and various other microenvironments. The ARB concluded that consideration of indoor exposures to toxic contaminants, especially residential exposures, is critical to accurate risk assessments for these chemicals.

From 1992 through 1994, the U.S. EPA conducted the probability-based National Human Activity Pattern Survey (NHAPS). Telephone interviews were conducted with over 9,000 respondents across the ten EPA regions in 48 states. The national results were generally consistent with the California study (Klepeis et al., 2001). Again, the mean percentage of time spent indoors was 87%. This was broken down into 69% of time spent in a residence and 18% of the time spent in other indoor locations.