The hazy skies in the Chicago area are the visible sign of the poor air quality on Thursday as smoke from Canadian wildfires sent Chicago to the top of the list of most polluted cities in the world.

Behind the cold front that finally brought temperatures down from the suffocating heat and humidity that have enveloped the Chicago area for the last few weeks, hazy and smoky skies have filtered back in.

The Canadian wildfire smoke has traveled down to the surface, creating unhealthy air quality for everyone, and especially for people with health conditions or who are very young or elderly. Visibility is also poor, as low as two miles in some area.

An Air Quality Alert is in place for the entire Chicago area through Friday until midnight. According to IQAir, Chicago's air quality index on Thursday was, for a time, standing at 174, the worst of any major city in the world.

The wildfire smoke is expected to linger Thursday night and into Friday. At times you may even be able to smell the smoke in the air.

All groups, especially sensitive groups, should limit their time outdoors Thursday, and keep windows and doors closed in these conditions.

The air quality will improve slightly Friday, from unhealthy for all to just unhealthy for sensitive groups.