In this Aug. 19, 2020, file photo, smoke from California wildfires up to 200 miles away obscures the view of traffic traveling on Interstate 80 looking west in Sparks, Nev. A study released by the Desert Research Institute, July 13, 2021, suggests a correlation between exposure to the particulate matter in wildfire smoke and risk of contracting the coronavirus. Scientists found that test positivity rates increased last year in Washoe County when the region was blanketed with wildfire smoke from fires in neighboring California. (AP Photo/Scott Sonner, File)