About 8 p.m. on a hot Thursday in July, Nicholas Gubell, a driver for UPS, was nearing the end of his route on Long Island, New York, when he started to feel woozy.

That day, Gubell, 26, had delivered about 200 packages. Temperatures had soared into the high 80s, and it was even hotter inside the metal shell of the back of the truck, where, with each stop, he would spend up to a minute or so to retrieve his cargo, sweat beading on his skin.

Now, pulled over on the side of the road, he was panting and barely able to speak, gripping his phone with his hand, which had cramped from dehydration.

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“My body was losing it,” Gubell said. Paramedics covered him in ice packs to bring down his body temperature and took him to a hospital. “I was just trying to hold on as best I could.”

As blistering heat waves swept across the United States this summer, breaking temperature records and placing millions under heat advisories and warnings, workers such as Gubell have continued to deliver America’s packages for a variety of carriers, often in trucks that have no cooling mechanisms for drivers. Some UPS workers have shared photographs that show thermometer readings of up to 150 degrees in the backs of their trucks.

Now a string of heat-related illnesses among the drivers has renewed calls to improve their working conditions.

“They’re vomiting. Their bodies are shutting down,” said Dave Reeves, president of Local 767, a Texas local of the International Brotherhood of Teamsters, which represents 350,000 UPS workers across the country. “It’s awful.”

Government records show that the problem is not isolated: Since 2015, at least 270 UPS and U.S. Postal Service drivers have been sickened and in many cases hospitalized from heat exposure. Dozens of workers for other delivery companies, including FedEx, have also suffered from heat exhaustion, according to the records, and a handful of drivers have also died in the past few years. According to the Teamsters, heat-related injuries, illnesses and deaths among drivers are severely underreported.