Smoke Pollution From Canadian Wildfires Returns to Parts of U.S.

Air polluted by the Canadian wildfires is back in the United States this week.

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 5 a.m. Eastern time on Monday, several cities were reporting elevated readings of the Air Quality Index: 158 in Buffalo, 155 in Chicago and 142 in Nashville. 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.

Unhealthy conditions are 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.

Air quality health advisories were issued in New York for Monday for the entire state. 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.

“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.