A northerly to northwesterly flow over dozens of wildfires in southern Canada will help to usher in smoke over the Northeast and Great Lakes on Thursday, with health alerts already in effect for many communities.

More than 120 million Americans were under air quality alerts on Wednesday that stretched over 1,000 miles from Iowa to Massachusetts and down through the Carolinas and Georgia.

Cities such as Chicago, Indianapolis, Cleveland, Pittsburgh and Philadelphia all experienced poor air quality and reduced visibility.

Some of the poorest Air Quality Index readings were in Northeast Ohio, where several communities reported an AQI value above 300.

The AQI scale starts at 0 and runs to 500, with values of 50 or below representing cleaner air and values over 300 considered to be hazardous.

Reductions in visibility couldn’t come at a worse time for passengers, as AAA expects record travel for the Independence Day weekend.

If visibilities are reduced enough, air traffic control towers can change operations to allow more space between aircraft, leading to delays.

Canada’s Toronto Pearson International Airport reported delays of more than an hour on Wednesday, which included many flights to the US.

The Statue of Liberty appears in haze from Canadian wildfires on June 6, NYC's air quality is expected to be reduced Thursday.
The Statue of Liberty appears in haze from Canadian wildfires on June 6, NYC’s air quality is expected to be reduced Thursday.
Getty Images
Cities such as Chicago(pictured,) Indianapolis, Cleveland, Pittsburgh and Philadelphia all experienced poor air quality and reduced visibility.
Cities such as Chicago(pictured,) Indianapolis, Cleveland, Pittsburgh and Philadelphia all experienced poor air quality and reduced visibility.
Getty Images

Officials in New York warned that air quality levels could reach poor values on Thursday, but the FOX Forecast Center said the invasion of smoke would not be as extensive as when the Big Apple set records earlier in the month.

Observation sites in the NYC metro reported an AQI of around 405 in June, setting a new modern record for the poorest air.

New York Governor Kathy Hochul’s office warned emergency cell phone alerts would be sent out once a community sees an extended AQI reading of at least 200.

States expected to see unhealthy air values on Thursday include Indiana, Ohio, West Virginia, Virginia, Maryland, Pennsylvania, New York, New Jersey, Vermont and the District of Columbia.

When air quality is as poor as anticipated, health departments warn individuals to limit outdoor activity to reduce health effects.

Observation sites in the NYC metro reported an AQI of around 405 in June, setting a new modern record for the poorest air.
Observation sites in the NYC metro reported an AQI of around 405 in June, setting a new modern record for the poorest air.
AFP via Getty Images

Sensitive health groups may experience respiratory problems and difficulty breathing.

The FOX Forecast Center expects a series of frontal boundaries will limit the extent of the smoke and haze through the weekend and into July 4th holiday.