Pakistan, July 17 -- Dense smoke from hundreds of wildfires burning across Canada spread over large parts of the US on Thursday, creating hazardous air conditions across the Midwest and Northeast and prompting health officials to urge millions of residents to stay indoors.

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According to air quality monitoring company IQAir, Detroit recorded the world's worst air quality, with a pollution index of 600-twice the level classified as hazardous by the US Environmental Protection (EPA). Dangerous air quality was also reported in Minneapolis, Milwaukee, Chicago, Toronto and several other cities.

Federal monitoring data showed unhealthy air across at least 10 US states, stretching from Minnes...