Dangerous smoke from hundreds of Canadian wildfires has seeped down to the U.S., impacting over 100 million Americans across more than a dozen states.
Air quality alerts on Friday stretch from the Midwest to the Great Lakes to the Northeast to the Mid-Atlantic, impacting major cities including Chicago, Detroit, Washington, D.C., and Philadelphia.
Canada has nearly 900 wildfires burning, with over 200 categorized as out of control.
Heavy rain by the Minnesota/Canada border on Friday may help with some fires, but the storms could also bring strong winds and lightning, which can spark new fires and create more erratic fire activity.
Passing showers and thunderstorms are also possible in Ontario Friday, but that rain won't be enough to put the wildfires out, and the winds may make conditions worse and lightning could spark new wildfires. There are more chances for rain over Ontario on Saturday and again on Tuesday.
In the U.S., the smoke Friday spans from Duluth, Minnesota, to Richmond, Virginia, impacting states including Wisconsin, Michigan, Ohio, New York, Pennsylvania, New Jersey, Maryland and Delaware.
Philadelphia issued a "Code Purple Air Quality Emergency Day" Friday.
"Everyone may experience negative health effects from particulate matter in the air," Philadelphia officials warned. "Members of sensitive groups may experience more serious health effects."
The worst air quality Friday afternoon is in northern Minnesota and Wisconsin.
In Milwaukee, where the air is in the hazardous category, the Milwaukee Brewers are allowing ticket holders to exchange tickets for Friday night's game for another regular season home game.
In Cleveland, the Guardians postponed Friday night's home game due to the air quality. The game against the Pittsburgh Pirates will be rescheduled as part of a split double header on Saturday, the team said.
The air is expected to improve on Saturday in the Ohio Valley, Northeast and the Mid-Atlantic, when rain moves in. But parts of the Midwest and Great Lakes will continue to see dangerous air quality.
Smoke is probable over New York City on Sunday when Spain plays Argentina in the FIFA World Cup Final at MetLife Stadium in New Jersey, though it is still too early to know exactly where the smoke will be. Sources familiar with the situation told ABC News that "all involved authorities are monitoring."
Smoke contains fine particles that can travel deep into the lungs. For those who need to work outdoors, are more sensitive to smoke or are in a high-risk group, Kai Chen, an associate professor of epidemiology at Yale School of Public Health, told ABC News that the best type of mask to wear is an N95, which is designed to filter at least 95% of airborne particles.
In New York City, Mayor Zohran Mamdani said free KN95 masks are available at hundreds of locations.
Click here to read more on how to stay safe.
ABC News' Michelle Simmons contributed to this report.
