More than 800 flights canceled as FAA cuts traffic at 40 major airports. Here's what to know.

Hundreds of flights were delayed or canceled across the country on Friday after the Federal Aviation Administration said it would reduce air traffic at dozens of the nation’s busiest airports due to the ongoing U.S. government shutdown.

According to the flight tracking website FlightAware, there were at least 800 flight cancellations and more than 1,000 delays within, into or out of the United States as of 9 a.m. ET.

Transportation Secretary Sean Duffy announced earlier this week that the FAA would cut traffic by 10% at airports in 40 “high-volume” markets amid staffing shortages triggered by the shutdown, now the longest ever.

The list of affected airports includes some of the country's largest airports, such as New York’s John F. Kennedy and LaGuardia, Newark Liberty International in New Jersey, Los Angeles International, Atlanta’s Hartsfield-Jackson International, Boston’s Logan International, and Chicago’s O’Hare. (See the full list below.)

Why are the cuts being made now?

A plane takes off from San Francisco International Airport on Nov. 5. (Tayfun Coskun/Anadolu via Getty Images)

Duffy said the move was an attempt to “alleviate the pressure” on air traffic controllers, who have been working without pay since the shutdown began. FAA Administrator Bryan Bedford said that the reductions were necessary to keep air travel safe.

“We’re not going to wait for a safety problem to truly manifest itself when the early indicators are telling us we can take action today to prevent things from deteriorating,” Bedford said. “The system is extremely safe today and will be extremely safe tomorrow.”

He added: “If the pressures continue to build even after we take these measures, we’ll come back and take additional measures.”

The shutdown has resulted in furloughs for an estimated 750,000 nonessential government workers. Air traffic controllers are considered essential workers, so they have been expected to continue working without pay.

Absences at air traffic control towers have already resulted in delays and cancellations. Last weekend, at least 39 air traffic control facilities reported shortages, according to the Associated Press, leading to flight disruptions.

How many customers will be impacted?

The cuts could result in hundreds if not thousands of flight cancellations per day. According to an estimate from aviation analytics firm Cirium, as many as 1,800 flights, or about 268,000 passengers, could be impacted.

At least two major U.S. airlines, United and Delta, said they would offer refunds to passengers even if they purchased tickets that aren't normally refundable, per the AP.

“Any customer traveling during this period is eligible for a refund if they do not wish to fly — even if their flight isn't impacted,” Delta CEO Scott Kirby said in a memo to employees.

Earlier this week, Duffy warned of “mass chaos” at airports if the shutdown drags on.

“You will see mass flight delays,” the transportation secretary said. “You will see mass cancellations.”