the time-activity pattern. People spend more than one
half of their time indoors, with variations attributable
to age, gender, and place. The National Human
Activity Pattern Survey (NHAPS) showed that people
in the United States and Canada spent an average of
87% of their time in enclosed buildings, and about
6% of their time in enclosed vehicles.4 The average
proportion of time spent indoors in rural areas of developing
countries is, for women, 70% in Kenya and
75% in Mexico.5,6 Although the fraction of time spent
indoors is less in rural than in urban areas, individual
exposures are often huge due to high concentrations
of pollutants in indoor air. Therefore, indoor exposure,
including that in occupational settings, dominates
total exposure for many pollutants.
Indoor sources of air pollution can be categorized
by type of source and by pollutant group, as shown
in Table 1, which also depicts the main health effects