Some flights into the New York City area and Philadelphia on Wednesday were delayed and others briefly halted because of reduced visibility from wildfire smoke from Canada.

The Federal Aviation Administration (FAA) was slowing traffic but lifted a groundstop on flights from the upper Midwest and East Coast bound for New York LaGuardia International Airport on Wednesday afternoon.

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. (Reuters: Text, Excluding Headline)