 this review presents a brief overview of the health effects and exposures of two criteria pollutants -- ozone and particulate matter -- and two toxic air pollutants -- benzene and formaldehyde . 
 these pollutants were selected from the six criteria pollutants and from the 189 toxic air pollutants on the basis of their prevalence in the united states , their physicochemical behavior , and the magnitude of their potential health threat . 
 the health effects data included in this review primarily include results from epidemiologic studies ; however , some findings from animal studies are also discussed when no other information is available . 
 health effects findings for each pollutant are related in this review to corresponding information about outdoor , indoor , and personal exposures and pollutant sources.imagesfigure 3figure 8figure 9 
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