Smoke from wildfires burning in the western US helped push Denver to the top of a global ranking this weekend—for world's worst air quality. The city edged out Johannesburg, 162-160, the Denver Post reports; on this scale, higher scores indicate worse air. The Colorado health department reported that Denver's fine particulates were rated "unhealthy." At the same time, for the same reason, Salt Lake City was No. 5 on Saturday afternoon. Denver was displaced later in the day, when the South African city hit 178, per CNN. IQAir, a Swiss company, ranks the air quality of 94 cities worldwide.
Officials suggest avoiding heavy exercise in the Denver area all weekend, per KMGH. On Sunday morning, Denver still had the second-worst air quality among cities, and health officials expect Monday to be much the same. Their recommendations for residents include keeping windows closed, using an air purifier, and wearing a mask. For those who must leave home, officials suggest going out in the early morning or at night, when the air is slightly better. The National Interagency Fire Center counts 107 active wildfires in the US. (Read more air quality stories.)