With broiling hot weather baking much of the country, airline passengers are finding yet another reason to dread flying: airplane cabins that feel like saunas.

When planes are waiting at the gate or sitting on an airport runway, temperatures inside can soar, and the air-conditioning may barely function.

In 2018, the Association of Flight Attendants, a union representing those workers, petitioned the U.S. Department of Transportation to set minimum standards for cabin temperatures.

But the department said this week that it was still studying the issue, and the complaints from passengers and crew members have not stopped. Some of them warn of the dangers of heatstroke and other heat-related illnesses, not to mention the anxiety and the discomfort of feeling trapped in an overheated airplane cabin.

“I can tell you as an active flight attendant, this is a major problem,” said Sara Nelson, the international president of the Association of Flight Attendants. “Everyone has a story about being too hot on an aircraft and the horrible things that happen as a result.”

How is the plane supposed to stay cool?

Most planes have a cooling system that works when the engines are roaring and the plane is in flight. When the plane is sitting on a hot runway and the engines are off, an auxiliary power unit, which uses jet fuel, can push cold air into the cabin. At the gate, airport workers can connect the plane to an external air-conditioning unit powered by the airport’s electrical system, which can pump cool air into the plane through a hose.