Map shows which US cities, states are being impacted by Canadian wildfires

Wildfires burning in Canada are continuing to create poor air quality conditions in the U.S. as the smoke makes its way south.

On Wednesday, New York City had the worst air quality rating of any major city in the world.

Other cities with low air quality on Wednesday included Washington, D.C., Philadelphia and Columbus, Ohio, according to AirNow , a website that publishes air quality data.

On Thursday, air quality alerts have been issued in 20 states from Missouri to Massachusetts.

Waves of smoke will continue to move south Thursday from New York City to Philadelphia to Washington, D.C.

At-risk populations, such as young children, the elderly or those with lung and heart disease, are urged to avoid the outdoors.

By Friday morning the AQI levels will drop for New York City. Smaller smoke plumes will linger from Detroit to Pittsburgh to D.C.

By Saturday and Sunday, the winds will begin to switch and blow smoke away from the U.S.

The smoke originated from wildfires in Quebec, where more than 160 forest fires are burning. Some of the fires were ignited by lightning strikes, according to NASA , and unusually dry and warm weather is fueling the blazes. The intensity of the fires has exceeded the capacity of water bombers, Quebec's wildfire prevention agency announced on Monday, CTV reported.