Globally, an estimated 3.3 million annual premature deaths (5.86% of global mortality) are attributable to outdoor air pollution,1 although ambient air pollution has been regulated under national laws in many countries. In the United States under the Clean Air Act, the primary National Ambient Air Quality Standards (NAAQS) are intended to protect human health, with an adequate margin of safety, including sensitive populations such as children, older adults, and individuals with respiratory diseases. Under the Clean Air Act, the standards are reviewed every 5 years to account for new scientific evidence regarding their appropriateness and adequacy for protecting public health.