Wildfire smoke is stoking a new challenge for cleaner air. A NASA-supported study published Thursday found that, over the last decade, wildfires have worsened ground-level ozone pollution across much of the contiguous United States, creating unhealthy air far from active flames. Wildfires have become an increasingly important contributor to ground-level ozone, or smog, across much of the United States, researchers report June 4 in the journal Science.