Wildfire smoke affecting millions of Canadians expected to linger for at least 2 days

Be patient, but don't take a deep breath.

Smoke from Canadian wildfires will make breathing outdoors difficult today for millions of people across the six largest provinces — and in some cases this bout of bad air is expected to last days.

Environment Canada has released air quality statements for parts of the Northwest Territories and every province outside Atlantic Canada, including huge swaths of Ontario and Quebec.

"Stop or reduce your activity level if breathing becomes uncomfortable or you or someone in your care feel unwell," the weather agency says. "Contact your health-care provider or local health authority if you develop severe symptoms or need advice."

• Check the CBC News Climate Dashboard for information on air quality and live updates on active fires across the country. You can also set your location to find out how today's temperatures compare to historical trends.

People with lung disease, including asthma, or heart disease, older adults, children, pregnant people and people who work outdoors are at higher risk of smoke-caused health effects, the agency warns.

The statement covers a large continuous area that stretches from Windsor, Ont., all the way to Northern Quebec, and includes a smog warning for an area of Quebec that includes Rouyn-Noranda and Val-d'Or. That warning says "high concentrations of fine particulate matter" will return Wednesday and could continue for the next few days.

In Ontario, Environment Canada's Air Quality Health Index is warning of a "high risk" for the entire corridor of southern Ontario from Windsor to Ottawa.

The agency says wildfire pollution levels can fluctuate over time and can vary depending on the location, but air quality is only expected to improve for some areas in Ontario and Quebec on Thursday night.

The Canadian Interagency Forest Fire Centre's website shows there are 487 active fires burning across the country as of Wednesday morning, with 253 of them classified as out of control.

Smoke affecting the U.S., too

The unhealthy haze travelled south of the border, too, settling over most of the Great Lakes region and spreading as far as Missouri and Kentucky.

The U.S. Environmental Protection Agency's AirNow.gov site showed Detroit in the "hazardous" range and warned that "everyone should stay indoors and reduce activity levels."

Drifting smoke from the wildfires has lowered curtains of haze on broad swaths of the United States, pushing into southern Illinois, Indiana and Ohio, and moving into parts of West Virginia. The AirNow.gov site listed air quality Wednesday in Cleveland, Chicago and Pittsburgh as "very unhealthy." A wider circle of unhealthy air spread into St. Louis and Louisville, Ky.

The smoke has even reached western Europe.

Earlier this week, the.U.S. National Aeronautics and Space Administration (NASA) said satellite imagery showed smoke extending across the North Atlantic Ocean to the Iberian Peninsula in France and other parts of western Europe.

Air quality in Europe has not deteriorated to the extent seen in North America because of the height of the smoke in the atmosphere, NASA said.