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The EPA has a vast web presence but offers various means of navigating it. For basic information check out their "Learn the Issues" tab. More detailed scientific information on a variety of subjects is under "Science and Technology." There's also an interactive map for finding information by geographic area. Some major subsidiary sites of EPA are:
Reports on research into the extent and nature of exposure of humans and ecosystems to environmental pollutants and other stressors. Includes links to databases of research publications as well as data on exposure itself.