Updated July 17, 2026 at 8:51 AM PDT

Heavy smoke continues to spread across the Great Lakes region into New England and as far south as Maryland, leading to unhealthy and even hazardous air quality in a number of states.

Extreme heat and dry conditions in Ontario, Canada, and northern Minnesota helped fuel wildfires earlier in the week. As of Friday, nearly 900 wildfires were actively burning in Canada, according to the Canadian Wildland Fire Information System. More than 180 are burning in Ontario alone.

Where temperatures have hit 100 degrees, the fires have continued on an "unprecedented run," said Derek Mallia, a professor of atmospheric sciences at the University of Utah. "You have a firehose of smoke coming right now."

In Ontario, dramatic videos on social media show intense fires and smoke-filled skies. The air quality in certain areas is considered to be "very high risk" by Air Quality Ontario. Thousands of people in the region have been forced to evacuate.

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Minnesota, Wisconsin and Michigan are experiencing some of the worst air quality levels. Millions living further downwind, in large northeastern cities, such as Philadelphia, New York City and Baltimore, are also suffering.

Relief is still many days away. Smokey conditions are expected to last through much of the weekend.

Scientists blame climate change caused by humans for exacerbating dry conditions. Vegetation lights easily as severe droughts and prolonged heat waves affect larger and larger areas.

"The frequency and intensity of these wildfire smoke events are increasing," said Dan Westervelt, an associate research professor at Columbia University's Lamont-Doherty Earth Observatory. He warned that the fires are becoming an annual occurrence and sometimes more than just annual.

Smoke is made up of tiny particles that can be inhaled deep into a person's lungs and even enter the bloodstream.

The data is very clear that the pollution from wildfires has all kinds of health effects, said Dr. David Eisenman, a professor of medicine at the David Geffen School of Medicine at UCLA. He points to studies showing an increase in heart attacks, heart failure, stroke and respiratory illness. Chronic exposure increases the risk of pregnancy complications and the risks of possible developmental disorders in children.

He also said that air pollution is one of the top causes of premature deaths worldwide, up there with smoking and high cholesterol.

Eisenman warned that even when it doesn't look like smoke is on the horizon, people should check air quality monitors.

"These are going to be repeated events … and a lot of what they are producing is invisible," said Eisenman, who is also the co-director of the Center for Healthy Climate Solutions at UCLA. "Even if the sky is not brown, it still might be toxic for you."

With millions of Americans also under a heat advisory as temperatures climb over 100 degrees, Eisenman said both the smoke and the heat create a greater public health risk.

Eisenman recommends following the Environmental Protection Agency's guidelines for wildfire smoke such as staying up to date on air quality reports and using air purifiers and replacement filters.

The agency also recommends setting up a "cleaner air room" in your home using an air purifier and to stock up on food and medications to avoid going outside when it's too smoky.

Eisenman says depending on air quality reports, which you should check regularly, reduce outdoor activity and wear an N95 mask if you must go outside.

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