Alberta Health Services has issued air quality advisories for areas from the U.S. border north to the Edmonton region because of smoke from wildfires in Washington state.
The agency warns even healthy people may experience irritated eyes and throat and possibly shortness of breath.
AHS says people with respiratory conditions may notice a worsening of symptoms and that children and the elderly are at higher risk of smoke-related illness.
An air quality official says the smoke in the Calgary area is worse than cities with serious pollution problems such as Beijing and New Delhi.
On Wednesday morning, levels in Calgary had shot past the upper end of the zero-to-10 scale used to measure air quality.
The City of Calgary has imposed a fire ban to help reduce the volume of smoke in the air.
"This is the first time in my career that we've actually issued a ban due to poor air quality, even though it's in the bylaw that we have the power to do that," said Calgary fire marshal Ed Kujat.
Smoke from the U.S. wildfires has been drifting over parts of Alberta since early in the week. (CTV Calgary, CHQR, The Canadian Press)