Some flights into New York City and Philadelphia were delayed or canceled on Wednesday due to limited visibility caused by Canadian wildfire smoke.

On Wednesday afternoon, the Federal Aviation Administration (FAA) slowed traffic but removed a groundstop on flights from the upper Midwest and East Coast destined for New York LaGuardia International Airport.

The FAA said it had also begun slowing traffic from the East Coast and Midwest bound for Philadelphia International Airport due to reduced visibility from wildfire smoke. The FAA said there were average delays of 29 minutes.

United (UAL.O), the largest carrier at Newark, said it was adjusting its schedule as needed, while Delta (DAL.N), the biggest at LaGuardia said it does not expect cancellations at the moment from the visibility issues.

Hundreds of forest fires in Canada have led to a blanket of smoky air, triggering health alarms in U.S. cities.

The FAA said Newark was currently experiencing delays averaging 82 minutes while average LaGuardia ground delays are about two hours and departure delays are about 30 minutes.

The New York City area has the most congested airspace in the United States.