Marathon runners consistently finished slower in cities with higher levels of dangerous particles in the air, researchers found.

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Forrest Smith was forced to step down last month as the National Park Service’s chief petroleum engineer. He wasn’t replaced.

© Mark Makela for The New York Times

Officials experimented with cloud seeding to try and reduce environmental pollutants as residents choke on filthy air.

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President Xi Jinping told a U.N. climate summit that China will reduce emissions across its economy, expand renewables sixfold and make electric cars “mainstream.”