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FILE - This combination of Dec. 13 and 17, 2018 photos shows the Utah State Capitol during clear and an inversion day in Salt Lake City. Inversions hover over Salt Lake City as cold, stagnant air settles in the bowl-shaped mountain basins, trapping tailpipe and other emissions that have no way of escaping to create a brown, murky haze the engulfs the metro area. After decades of getting ever cleaner, AmericaâÄ™s air quality seems to be stagnating. In 2017 and 2018, the nation had more polluted air days than just a few years earlier, federal data shows. While it remains unclear whether this is the beginning of a trend, health experts say itâÄ™s a troubling development. (AP Photo/Rick Bowmer)