More than 100 million people across the northern and Midwestern United States are currently exposed to hazardous air as wildfire smoke from Canada drifts southward, creating a thick haze that poses significant health risks. As of Thursday, over 800 active wildfires in Canada—particularly in Ontario near the Minnesota border—are fueling the pollution, which a continent-wide heat wave is trapping and pushing into US cities.

Air quality alerts issued across multiple states

Several major US cities, including Chicago, Detroit, Milwaukee, Pittsburgh and Washington, D.C., recorded Air Quality Index (AQI) readings above 200 on Friday, according to IQAir data, placing them among the worst globally. In parts of Michigan, Ohio and Wisconsin, AQI levels reached the 300s by Friday afternoon—a threshold at which experts strongly advise against outdoor activities.

Public health authorities have responded with urgent measures. New York activated emergency operations on Wednesday, distributing free KN95 masks and opening shelter spaces, while Philadelphia suspended trash collection and closed public pools on Friday due to deteriorating air conditions.

Health risks and protective measures

Wildfire smoke contains microscopic particulate matter (PM2.5), ash and trace minerals that, when combined with water vapor, create visible haze. These particles, smaller than a grain of sand and 30 times finer than a human hair, can penetrate deep into the lungs and even enter the bloodstream, according to Serap Erdal, a professor at the University of Illinois, Chicago.

Experts warn that such pollution exacerbates health issues for vulnerable groups, including children, pregnant individuals, older adults and those with asthma or heart conditions. Suzanne Paulson, director of the Center for Clean Air at UCLA, noted that wildfire particles may be more toxic than typical urban pollutants. She advised treating the AQI color code as one level higher when wildfire smoke is present.

To mitigate exposure, officials recommend staying indoors, using air purifiers or setting air conditioning to recirculate, and wearing high-quality particle masks such as N95 or KN95 when outside. Surgical masks, however, offer limited protection against fine particles. As Erdal cautioned, indoor spaces are not entirely safe, as outdoor air can still infiltrate.