Air quality alerts remain in effect in 19 states Friday afternoon, but that number will likely drop throughout the weekend.

Air quality will gradually improve during the holiday weekend as the wildfire smoke clears out of the eastern U.S. But with no end in sight to the wildfires that have burned a record of 20 million acres across Canada so far this year, the smokey skies are likely to return.

The Centers for Disease Control and Prevention on Friday issued a health alert on wildfire smoke exposure.

People are urged to watch out for symptoms including: headache, eye irritation, trouble breathing, coughing, wheezing, chest pain, palpitations and fatigue.

The smoke on Friday is stretching from the eastern Great Lakes to the Interstate 95 travel corridor and down to parts of the Southeast.

The Air Quality Index in New York City hit 169 Thursday afternoon, which falls under the "unhealthy" category. Any number over 100 is considered unhealthy for sensitive groups.

At least 20 U.S. states are under air quality alerts on Friday morning as Canadian wildfire smoke lingers in the East for another day before it's forecast to dissipate over the weekend.

The thickest smoke on Friday will stretch from the eastern Great Lakes to the Interstate 95 travel corridor and down to parts of the Southeast.

Skies will be clearer by Saturday, but some light haze could still hang over the East Coast.

Air quality alerts are in effect in 21 states Thursday afternoon as the Canadian wildfire smoke infiltrates the Midwest, Northeast and parts of the South.

As of Wednesday morning, 20 U.S. states are under air quality alerts from Minnesota down to Georgia and as far north as western New York.

Wildfire smoke from neighboring Canada is currently blanketing large swaths of the United States, from Iowa to western Pennsylvania to North Carolina and most everywhere in between. Only Chicago is getting a small reprieve on Wednesday morning due to a lake breeze, which isn't expected to last for long.

Later on Wednesday, the smoke is expected to cover areas from Minnesota to Washington, D.C. and down to the Carolinas.

By early Thursday morning, the smoke will be seen in Minneapolis, Chicago, Indianapolis, Atlanta and Pittsburgh. But by the afternoon, it will linger from Detroit to Atlanta and east to Washington, D.C. and Philadelphia.

New York City could be impacted as well, but most of the smoke from the Canadian wildfires is expected to stay in western New York state, Pennsylvania and southern New Jersey.