tor producing detrimental health effects on millions of

peopleacrosstheworld. IntheUnitedStates, theEnviron-mental Protection Agency has set National Ambient Air QualityStandards for6criteriapollutants routinelymea-suredacross thecountryandconsideredharmful topub-lichealth:particulatematter(PM),ozone,nitrogenandsul-fur dioxides, lead, and carbonmonoxide. Outdoor air pollutant exposures include complex mixtures of PM, gases, and organic (including polycy-clic aromatic hydrocarbons [PAHs]) and inorganic com-pounds emitted directly into the air mainly from com-bustion of fossil fuels from gas, oil, coal, industry, and fires. Airborne PM of less than 2.5 μm and the ultrafine PM (particles <100 nm) are of special interest regard-ing effects on the brain because of their ability to cross epithelial, endothelial, and neural barriers, producing direct and indirect brain damage. Forty-two million people residing in cities across the United States are exposed to harmful PM of less than 2.5 μm. Ozone is the most important secondary air pollutant—a product of complex photochemical reactions of nitrogen oxides and volatile organic components. In the United States, more than 123 million people are exposed to ozone levels that are greater than the National Ambi-ent Air Quality Standards. Air pollution components have significant variabil-ity in rural areas, fromcity to city, andwithin specific re-gions in the samecity. Spatial air pollutant variations re-latetofixedandmobilepollutionsources,meteorological factors, andmassiveexposures fromevents suchas the 9/11 World Trade Center attack. Because most people spend more than 90 % of their time indoors, control-ling the effects of indoor air pollution is also critical for brainhealth.Smoking, cooking, cleaning, theuseofplas-ticsandconglomerates, and, inaddition, in-vehicle (cars, heavyduty trucks,buses, constructionequipment, sub-way) concentrations of pollutants are of deep concern