July 16 (Reuters) - Heavy smoke from hundreds of wildfires in Canada enveloped a swath of the U.S. from the Midwest to the Northeast on Thursday, prompting warnings from officials that residents should stay indoors wherever possible and avoid the acrid, unhealthy air.
Detroit had the worst air quality of any city in the world on Thursday, according to monitoring company IQAir, with a reading of 600, twice the level considered "hazardous" by the U.S. Environmental Protection Agency.
Federal data showed dangerous levels of smoke across Minnesota, Michigan, northern Illinois, northern Ohio and into Ontario, with Minneapolis, Milwaukee and Toronto all recording hazardous readings. Ten states had at least some locations with an "unhealthy" reading, from Minnesota all the way down to Maryland.
The smoke was expected to worsen throughout the day. In New York, where the sky was tinged with an orange haze and the air smelled acrid, local officials urged residents to limit their outdoor time and cautioned those with risk factors, such as heart and lung disease, the pregnant or the elderly, to remain inside.
The dangerous conditions in the New York area came just days before the FIFA World Cup final is set to be played in nearby New Jersey on Sunday in front of more than 80,000 fans.
"Today is expected to be the worst day of this event," New York City Mayor Zohran Mamdani said at an event on Thursday. "At 'unhealthy' levels, everyone - not just people with asthma or heart conditions, not just older adults - everyone may feel health effects. So today, every New Yorker should take precautions."
The city is handing out free KN95 face masks at hundreds of libraries, police precincts and firehouses, the mayor said.
As of Thursday morning, there were 858 active fires across Canada, including 111 considered out of control, according to government data. Most of the fires were in the central provinces of Manitoba, Saskatchewan and Ontario.
So far, about 2.4 million hectares (5.9 million acres) have burned during this wildfire season in Canada. Climate experts say rising global temperatures are driving an increase in the frequency and intensity of wildfires around the world.
Wildfire smoke, which can linger in the air for weeks and travel thousands of miles, is more toxic than normal air pollution. Studies have linked wildfire smoke with higher rates of heart attacks, strokes, cancer, pregnancy complications and weakened immune defenses.
(Reporting by Joseph Ax in New Jersey, Andrew Hay in New Mexico, and Caroline Stauffer and Wa Lone in Toronto; Editing by Mark Porter)
