Delta Air Lines canceled a flight from Las Vegas to Atlanta on Monday after extreme heat led to illnesses among passengers, KTLA sister station KLAS reports.

The number of passengers who were sick was not disclosed in a statement from the airline. Reports that the passengers were in the cabin on the ground for four hours could not be confirmed.

“We apologize for the experience our customers had on Flight 555 from Las Vegas to Atlanta on July 17, which ultimately resulted in a flight cancelation. Delta teams are looking into the circumstances that led to uncomfortable temperatures inside the cabin and we appreciate the efforts of our people and first responders at Harry Reid International.”

The apology to customers came along with a “compensatory gesture,” the airline said.

Multiple people were reportedly wheeled off the plane by paramedics, and multiple flight attendants were also said to have fallen ill.

The official high temperature in Las Vegas on Monday was 108 degrees, 9 degrees lower than on Saturday when the temperature was just a degree below the all-time record of 117.

Delta said passengers on Flight 555 were moved to other flights.

At least one customer sought treatment for heat-related discomfort, according to the airline.

“Medical teams responded to a call aboard an aircraft yesterday afternoon,” according to a statement from Reid International Airport late Tuesday afternoon. The statement directed further questions to Delta.

Planes do have air conditioning, Billy Nolen, former acting FAA administrator, told The New York Times. In most cases, there is a cooling system that works when the engines are on and an auxiliary unit that supplies cool air when the plane is on the tarmac.

But, planes can get hot, like when it is switching between cooling systems, Nolen notes. A plane sitting in warm weather will also, inevitably, get warm, just as your car does when it sits in the sun.