India, July 17 -- A thick blanket of smoke drifting from massive Canadian wildfires has turned Chicago's skyline hazy and pushed air pollution to dangerous levels, prompting health officials to urge residents to stay indoors. On Thursday, Chicago recorded a hazardous Air Quality Index (AQI), placing it among the most polluted major cities in the world.

The smoke has spread across the Midwest and Northeast, affecting more than 125 million people and disrupting sporting events, travel and daily life. Authorities say conditions could persist through Friday before gradually improving.

Chicago Air Quality Today: Why is Chicago's Air Quality So Bad Today?

The dramatic drop in Chicago's air quality is being driven by smoke from hundreds of ac...