ozone ozone

Nitrogen oxide pollution in India and China is offsetting U.S. gains in cutting emissions, researchers say. This photo from October shows road traffic, along with smoke and smog, in front of the landmark India Gate in New Delhi. Manish Swarup/AP hide caption

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Manish Swarup/AP

Environmental Protection Agency Administrator Gina McCarthy testifies at an oversight hearing of the Senate Environment and Public Works Committee on Capitol Hill in Washington on July 23. J. Scott Applewhite/AP hide caption

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J. Scott Applewhite/AP