Smoke from Canada’s fires has periodically engulfed the Northeast and Mid-Atlantic for more than a week, raising concerns over the harms of persistent poor air quality.

There are more than 150 active wildfires burning in Quebec this week, according to the Canadian Interagency Forest Fire Center – more than double the number of fires burning in any other Canadian province.

More than 400 wildfires have ignited across Quebec so far in 2023, twice the average for this time of year. Nearly 9 million acres have been charred by wildfires in Canada so far this year, with nearly half a million acres burned across Quebec alone.

Air quality alerts were in effect across parts of the Northeast and the Midwest on Tuesday as wildfire smoke spread west into Detroit and Chicago.

Wildfire smoke contains very tiny particulate matter, or PM2.5 —the tiniest pollutant — yet also the most dangerous. When inhaled, it can travel deep into lung tissue and enter the bloodstream. It comes from sources like the combustion of fossil fuels, dust storms and wildfires, and has been linked to a number of health problems including asthma, heart disease and other respiratory illnesses.

CNN’s Robert Shackelford, Jennifer Gray and Monica Garrett contributed to this report.