Heavy, pungent wildfire smoke darkened skies in the U.S. on Friday from the Great Lakes to parts of the East Coast, reducing visibility and prompting warnings that breathing the air outside could be dangerous.

Officials in many cities this week urged residents to stay inside or wear masks outside as air quality reached unhealthy to hazardous levels, meaning it’s unhealthy for anyone, regardless of health conditions. The smoke is coming from fires that are burning primarily in Canada but also in northern Minnesota. A lingering high pressure system has trapped the smoke close to the ground, said Steven Freitag, a National Weather Service meteorologist in Detroit.

On Friday, meteorologists issued air quality warnings in Detroit, Chicago and Washington, D.C., where a hazy sky hovered over the city and suburbs, bringing a strong, acrid smell of smoke. According to monitoring site IQAir, Washington had the third-worst air quality of major cities around the world at about sunrise on Friday, behind Detroit and Chicago. The top five also included Toronto and New York City.

“It’s scary,” Omar Mitchell, 50, said Thursday as he looked to the sky while walking to his restaurant in Detroit, wearing a mask. “You don’t know necessarily what the side effects may be. That’s days or months later.”

Microscopic particles can lodge deep in the lungs and enter the bloodstream, leading to heart and lung problems and contributing to other long-term health issues.