Do you know where your pandemic flight credits are?

With vaccination rates on the rise and cities reopening to visitors, millions of travelers who were issued credit or vouchers for trips that were canceled due to the pandemic are starting to pull up those emails and read the fine print. Many hopeful flyers are discovering their credits will expire before they get to use them — and others will find that the vouchers have restrictions that make them difficult to use.

When the pandemic hit, Vanessa Mumford-Minshull of Campbell, California, scrambled to cancel a multi-leg, multi-airline European trip for four people. She asked for refunds. Airlines automatically issued vouchers. “I sent an email saying, ‘I don’t want this,’ but got no answer,” said Mumford-Minshull, who spent days calling, writing, and documenting her efforts before finally getting refund help, thanks to her credit card company.

“With the vouchers, there were too many unknowns due to Covid-19,” she said. “I could have lost almost $4,000.”

U.S. airlines, which recently received billions of dollars in a third round of federal pandemic funding relief, are holding what is estimated to be more than $10 billion in outstanding vouchers.

“Airlines don’t release the figures, but I can say with confidence there are more outstanding travel vouchers right now than have existed ever before, given the number of trips that got disrupted simultaneously because of the pandemic,” says Scott Keyes, founder of Scott’s Cheap Flights.

If you received airline travel credits or a voucher for a trip canceled due the pandemic, experts say to check the expiration date right away. If the expiration date has not passed and you do not have travel plans just yet, call and ask the airline to extend the date. Recent guidance from the Centers for Disease Control and Prevention to avoid travel, even if vaccinated, should mean airlines may be willing to do this.

“For those consumers who have unused vouchers, it’s critical that they stay on top of this and be aware of the expiration dates,” said William McGee, aviation adviser to Consumer Reports, which logged more than 700 consumer complaints about vouchers and refunds in less than a week.

To help travelers figure out their options, McGee reviewed policies being offered by 10 U.S. carriers and found that the best and most transparent policy was from Allegiant Air, which set the expiration for its travel vouchers at two years from the initial date of purchase. Other policies are “confusing, and the expirations can vary greatly based on date of booking, date of travel, date of cancellation,” McGee said.

Current rules and policies for the other nine airlines can be found here: Alaska Airlines, American Airlines, Delta Air Lines, Frontier Airlines, Hawaiian Airlines, JetBlue Airways, Southwest Airlines, Spirit Airlines, and United Airlines.

While you should be able to use your travel credits or vouchers to book the same or a different itinerary, you will be shopping for your new tickets at new prices. And, as travel picks up and pent-up demand for travel increases, you may need to throw extra money in the pot to get the flights you want.

If you know where you want to go but are not sure when you will feel comfortable flying, some experts recommend using your travel credits to book a flight on an airline that is still offering free changes on all fares. While you will still have to pay the difference if the fare rises, you will at least be able to change dates without an extra fee and you won’t lose the value of your voucher, Keyes said.