• Smoke from wildfires in Canada delayed flights at Northeast airports as haze blanketed the region.
• The FAA slowed flights in New York, Newark and Philadelphia.
• As of 9:10 p.m. ET, more than 4,800 U.S. flights were delayed, according to flight-tracking site FlightAware.

Smoke from wildfires in Canada delayed hundreds flights at New York-area airports and in Philadelphia on Wednesday as haze blanketed the region and cut visibility.

Earlier, the FAA briefly halted flights bound for New York's LaGuardia Airport.

"The agency will adjust the volume of traffic to account for the rapidly changing conditions," the FAA said.

Smoke drifting south from wildfires in Canada covered the New York City area on Tuesday into Wednesday, tainting air quality and sending residents indoors. As of noon Wednesday, the city ranked fourth in the world for worst air quality, with an IQAir World Air Quality Index of 158, a level considered unhealthy for all residents.

As of about 9:20 p.m. ET, more than 4,800 U.S. flights were delayed, according to flight-tracking site FlightAware.

At LaGuardia Airport, 230 departures, 40% of the day's schedule, were delayed as were 264 inbound flights, or 46%, according to FlightAware. Nearly a third of flights bound for Newark Liberty International Airport were delayed, or 204 flights, and another 186 departures were running behind.

Flights to LaGuardia were delayed an average of about two hours during the afternoon, and flights to Newark were delayed an average of 82 minutes, the FAA said. New York's John F. Kennedy International Airport also reported both arrival and departure delays.

Nearly 670 United Airlines flights, 24% of its schedule were delayed. Newark is one of the carrier's major hubs.

United said in a statement it was "monitoring the situation closely and looking out for the safety of our employees who work outside, and adjusting our schedule where needed."

A spokesman for Delta Air Lines told CNBC that the carrier has protective equipment for staff.

"And for those who work on the tarmac, we're having them come inside to where breakrooms are in between aircraft turns," the spokesman said. "We are also watching the forecasts which call for rain in NYC in the days ahead which should improve the air quality."

An American Airlines spokesperson said the company is making sure ramp workers have access to vehicles in between flights as well as other supplies like water and masks.

— CNBC's Emma Newburger contributed to this report.