During the final holiday weekend of the summer, a record 16.5 million people are expected to fly on U.S. airlines, a 3.5 percent increase over the Labor Day weekend in 2017, according to Airlines for America, the trade group for the nation’s air carriers.

The country’s airlines are expected to fly 2.36 million passengers a day over the four-day weekend, an increase of 79,000 travelers a day from the same weekend in 2017, according to the trade group. The carriers are expected to add 2.76 million additional seats a day to meet the higher demand.

Airline industry experts attribute the increase in air travel to affordable airline tickets but say the demand for travel has been increasing for several years along with the rebound of the U.S. economy and the strengthening of consumer confidence.

In the first three months of 2018, the average domestic airline ticket sold for $360, a decline of 3.7 percent from the same period last year, according to the U.S. Bureau of Transportation Statistics.

But the airline industry trade group warns that fuel prices are on the rise, which has begun pushing down the profit margins of the country’s airlines. Some airline experts have warned that such conditions may push fares up soon.

Fuel prices have increased 31.1 percent, with labor costs rising 5.7 percent in the first half of 2018 compared with the same period last year, according to Airlines for America. That has pushed profit margins for the nation’s airlines from an average of 11.5 percent in the first six months of 2017 to 7.2 percent in the same period this year, the group said.