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.