Smoke from hundreds of Canadian wildfires is blanketing a vast stretch of the United States, prompting air quality alerts from North Dakota all the way down to South Carolina and reigniting political frustration north of the border. The National Weather Service has warned millions of Americans to monitor local air conditions as haze drifts across states in the Midwest, Mid-Atlantic, and Northeast. The situation is most dire in Michigan, where air quality has been classified as hazardous — the most extreme category on the index. Wisconsin, Minnesota, Illinois, and Indiana are not far behind, with pollution levels deemed very unhealthy. Meanwhile, residents across New York, New Jersey, Pennsylvania, Ohio, Connecticut, Rhode Island, Delaware, and Maryland are contending with unhealthy air and widespread haze that has settled over major cities.

The smoke is flowing south from roughly 850 wildfires burning across Canada, many of the largest concentrated in Ontario, along with more than a dozen fires raging in northern Minnesota. As the smoke has spread, so has political anger. Republican lawmakers have seized on the crisis to criticize the Canadian government, accusing officials of failing to do enough to prevent and contain the blazes. Four Republican representatives from Michigan sent a letter this week arguing that Canada has the tools to stop the smoke from pouring into American communities but has chosen not to use them. Senator Bernie Moreno of Ohio went further, announcing plans to introduce legislation that would sanction Canada and hold specific government officials accountable for what he called an atrocity.

Health officials are urging people in the hardest-hit areas to take the warnings seriously. In places where air quality has reached very unhealthy or hazardous levels, the guidance is straightforward: stay indoors with windows closed and limit any outdoor activity. Doctors are particularly concerned about people with heart or lung disease, older adults, children, teenagers, and pregnant individuals, all of whom face elevated risk from exposure to wildfire smoke. For those who must go outside — especially people who work outdoors — health experts recommend wearing an N95 or P100 respirator mask, which can filter out at least 95 percent of airborne particles. Masks offer some protection against fine particulate matter but cannot block harmful gases like carbon monoxide, making indoor avoidance the safest strategy when conditions deteriorate.

Wildfire smoke is dangerous because it carries a toxic mix of water vapor, pollutants, fine particulate matter, and gases that can penetrate deep into the lungs and enter the bloodstream. Once there, these particles can trigger systemic inflammation, worsen chronic conditions such as asthma, and increase the risk of heart attacks and strokes. Scientists note that climate change is fueling hotter and drier conditions across North America, extending fire seasons by more than a month in some regions compared with 35 years ago. NASA reports that human-caused warming has driven more frequent and severe fire activity worldwide, with extreme wildfire behavior more than doubling over the past two decades. As fires grow larger and more intense, the smoke they produce can travel hundreds or even thousands of miles, exposing populations far from the flames to hazardous air pollution.