Wildfire smoke from Canada is blanketing the skies in the Midwest, leaving millions with dangerous breathing conditions.

Smoke from wildfires blazing across Canada is once again wafting over huge portions of the Midwest and worsening air quality, with conditions particularly bad on August 1 in Minnesota, Iowa and Illinois.

The Iowa Department of Natural Resources reported Friday that air quality in Iowa will be at levels “unhealthy for sensitive groups” through Saturday, due to smoke from wildfires in Canada. The National Ambient Air Quality Standards advise that fine particulate matter should not exceed averages of 35 micrograms per cubic meter in a 24-hour period.