Air quality alerts were issued across multiple U.S. states and cities Tuesday, as well as today, as thick, dark smoke from Canada’s hundreds of blazing wildfires blanketed the skies of the Northeast, Midwest, the Ohio Valley and the Mid-Atlantic. The haze has triggered dangerous air pollution warnings, impacting tens of millions of people, with New York City reporting the worst air quality in any major city worldwide Tuesday.

There are over 400 blazes burning across nearly all of Canada’s 10 provinces and territories, forcing tens of thousands of people to evacuate. The province of Quebec is facing some of the worst fires, many caused by lightning, covering neighboring Ottawa and Toronto with an orange haze. U.S. and Canadian officials are urging people with heart and lung issues, and other vulnerable groups, to remain indoors as much as possible.