It’s not uncommon for nasty winter and summer weather to disrupt flight schedules and other operations at Canada’s biggest and busiest airport, but officials at Toronto Pearson Airport in Mississauga say a relatively new threat has also emerged.

Smoke impacts began in northern Wisconsin Tuesday morning and will spread south through the state by July 31. Air quality may briefly improve on July 31 before smoke returns to northern areas on Aug. 1, the DNR reports.