Wildfire smoke from hundreds of wildfires in Canada is causing significant air quality issues, prompting alerts across the Northeast and Midwest of the United States. Millions of residents are advised to minimize outdoor activities as smoke blankets areas from New York to Wisconsin, with ongoing health advisories in effect. Massachusetts is experiencing some of the worst air quality of the year, while Toronto has been identified as having among the worst air quality in the world. Many locations are seeing air quality classified as unhealthy, affecting activities and raising health concerns. Authorities advise residents to stay indoors, close windows, and monitor for symptoms related to poor air quality.