and strategies to reduce driving or children’s time in and near motor vehicles. These strategies all relate directly to features of the built environment.

Air Quality

In environments where automobiles and trucks are the principal means of transportation, the emissions from these mobile sources figure prominently as a source of air pollution. Although vehicle engines have become far cleaner in recent decades, the sheer quantity of vehicle miles releases large amounts of carbon monoxide, carbon dioxide, particulate matter, nitrogen oxides, and hydrocarbons into the air. Nitrogen oxides and hydrocarbons, combined with sunlight, form ozone. Cars and trucks account for a substantial amount of the emissions of such chemicals.

Ozone levels do not vary over a small scale, from block to block. A child in the suburbs may sustain ozone exposure that is as high as, or even higher than, the exposures of an inner-city child. In contrast, particulate matter less than 2.5 microns diameter (Pm2.5), which can affect respiratory function, may vary from block to block. A child living near a busy intersection or near a heavily traveled truck route may sustain considerably more particulate exposure than a child living in a quieter neighborhood several blocks away.

Children who live, attend school, or play near busy roads or in crowded urban areas, where they are exposed to the exhaust from automobiles and trucks, may experience acute and chronic respiratory effects. In addition, children who live in metropolitan areas with heavy traffic, especially in parts of the country conducive to ozone formation for biogeophysical and meteorological reasons, may be exposed to high levels of ozone during the warm months of the year.

Physical Activity

The built environment plays a major role in promoting or hampering physical activity in children. Schools, parks, and even sidewalks that are integrated into the design of a community can encourage physical activity. For example, physical activity among youth increases when schools offer such facilities as basketball courts and sports fields (Frumkin, 2003). A considerable body of research shows that sprawl—as measured by low residential density, low employment, low “connectivity”—is associated with less walking and bicycling and with more automobile travel than denser communities (U.S. Environmental Protection Agency, 2001; Holtzclaw et al., 2002; Cervero, 2002; Cervero and Ewing, 2001).

SOCIAL ENVIRONMENT

Humans are social creatures. While social influences are important for children of all ages, their nature and form change over the course of childhood. In



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