Over the past two decades, as part of the effort to develop the National Ambient Air Quality Standards (NAAQS), researchers have been using real human exposure data to help analyze the magnitude and extent of the risks from specific or multiple pollutants. Surrogates for exposure have also been used, such as the ambient air quality measured at fixed monitoring sites. These approaches are based on available science. Meanwhile, during the past decade, researchers at the U.S. EPA, universities, and institutes have been developing better scientific approaches for measuring and modeling real or potential human exposures that explain hazardous exposure situations. In parallel, the President, Congress, and the public have set higher expectations for government programs, such as NAAQS. These expectations were first defined through the evolution of the "risk assessment-risk management paradigm" and then by the requirements of the Government Performance and Results Act (GPRA), which define accountability for government programs.
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