Canada Wildfires Smoke Returns to Pollute U.S. Air

Air polluted by the Canadian wildfires is back in the United States this week, and state officials urged residents in some cities on Monday to take precautions by limiting outdoor activities and wearing masks.

There are nearly 900 wildfires burning across Canada, but the smoke this week will be coming from the western part of the country.

“Unfortunately, the wildfire smoke will begin to make a return to the region to start the new week,” according to the National Weather Service in the Philadelphia area.

Air quality alerts were issued for multiple areas, including the Midwest, the Great Lakes, central Tennessee and North Carolina, as well as the Northeast.

By 10 a.m. Eastern time on Monday, cities across the Midwest were reporting some of the worst air quality in the country, according to the Environmental Protection Agency’s Air Quality Index. The index runs from 0 to 500; the higher the number, the greater the level of air pollution. An A.Q.I. of 201 or more is considered very unhealthy. Billings, Mont., and Fort Wayne, Ind., had an A.Q.I. of 161, while the Cleveland area was at 157.

Local officials advised residents to limit outdoor activity, use public transportation or wear masks. The message was repeated from Buffalo — where Mayor Byron W. Brown told residents to take precautions — to Chicago, where an air quality alert was in effect through Sunday night.

“We are acutely aware that the recent weather events prominently impacting our City this summer are the direct result of the climate crisis,” Chicago’s mayor, Brandon Johnson, said on Sunday, warning children, older residents and those with heart or lung disease to limit outdoor activity.

Unhealthy conditions were forecast for upstate New York on Monday, according to AirNow, a website run by the U.S. Environmental Protection Agency, which oversees air quality across the country. The forecast is expected to cause “unhealthy for all” conditions in the area closest to the Canadian border, according to AirNow. In other parts of the state, conditions will reach “unhealthy for sensitive groups” status.

The same status was applied on Monday to Pennsylvania, where the environmental protection department issued a statewide “code orange” alert. It suggested residents and businesses help by limiting burning of leaves, trash, and other materials, and avoiding the use of gas-powered lawn and garden equipment.

“Canadian wildfire smoke will remain in the picture through Tuesday as northwesterly winds aloft that originate out of the Canadian Prairies continues to direct more smoke into the Lower 48,” the Weather Service said.

Early last month, the level of particulate matter in the air from smoke became so unhealthy that many U.S. cities set records. At points, it was hazardous to breathe everywhere from Minnesota and Indiana to sections of the Mid-Atlantic and the South.

Visibility decreased to startling degrees in cities, including New York, Toronto and Cincinnati. In some places, smoke from the fires blanketed the sky in an orange haze. That smoke could be traced to wildfires burning in Quebec.

Here’s a guide to understanding air quality readings.