Business has soared for UPS as Americans have turned to home delivery during the pandemic, but employees say heavy workloads, COVID-19 safety measures and sweltering summer heat are pushing them to the limit.

“We’re in the middle of a pandemic,” said David Cockrel, a UPS driver and union steward in Brooklyn, New York. “It’s about 105 to 110 and hotter in the back of that truck. We’re working, 10, 11, 12, 13 hours a day. We’re tired.”

Twenty UPS workers around the country told NBC News that since spring they’ve been dealing with the volume of packages they see during their peak season, the Christmas rush, if not more. As the pandemic has forced businesses to close around the country, UPS is a shiny outlier. Company statistics show home deliveries are up two-thirds compared to the same period in 2019. But even as temperatures rise, drivers and warehouse workers said they’re pushed to work harder and pressured not to take breaks or days off.

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As the pandemic extends into the hottest days of summer, UPS employees are among thousands of essential workers in the U.S. confronting a Catch-22. To stave off infection, they’re told to wear masks and avoid clustering with others in closed, air-conditioned spaces. But those measures increase the risk of heat illness — a problem for delivery workers even before the pandemic.

Last year, NBC News found more UPS employees were hospitalized for serious heat-related injuries between 2015 and 2018 than workers at any other company except the U.S. Postal Service, which is significantly larger. Many delivery vehicles and warehouses for UPS, FedEx and the U.S. Postal Service — the nation’s three largest delivery companies — are not air conditioned, a growing challenge for workers as the climate warms.

“The challenge is compounded now that you have a pandemic risk on top of the heat risk in the summer,” said Dr. Aaron Bernstein, interim director of Harvard’s Center for Climate Health and the Global Environment. “We have to make sure, especially now that so much is dependent on package delivery, that those workers are protected — both from heat and infection.”

Both UPS and the International Brotherhood of Teamsters, which represents UPS workers, said the company has taken steps to protect them from COVID-19 and heat, including reviewing their safety policies, requiring workers with the virus to test negative before returning to work and mandating masks in the buildings.

But the mask itself makes the workers hotter.

“Wearing a mask for a long period of time makes it more difficult for your body to cool down,” said Juley Fulcher, worker health and safety advocate at Public Citizen, a nonprofit that has pushed for better heat standards. “You should be wearing a mask, but you should be taking regular mask breaks.”

Devain Campbell, a warehouse worker who had just finished loading five UPS trucks on a July morning in Brooklyn, told NBC News wearing a mask can be suffocating in the heat. “It’s like 100 degrees trapped on your face.”

Workers facing the twin challenges of heat and coronavirus have few federal protections. There are no national workplace regulations that address either heat or airborne infectious diseases, and the Occupational Safety and Health Administration has not issued emergency rules in response to the pandemic.

“It’s stunning,” said Debbie Berkowitz, director of the Workplace Safety and Health Program at the National Employment Law Project and a former top OSHA official. “It is a total failure of an agency to protect workers.”

Home delivery has grown rapidly in the past decade, pressuring warehouse workers and drivers to move ever faster to get packages to doorsteps. But during the pandemic, delivery suddenly became a lifeline.

This spring’s delivery boom somewhat cushioned pandemic-related economic losses at the Postal Service and FedEx. At UPS, it brought record growth, with revenue growing more than 13 percent and deliveries growing 65 percent.

The company increased cleaning, hired 39,000 employees and expanded weekend operations to ease the load on workers, said UPS spokesperson Matt O’Connor.

“We never want our employees to continue working to the point that they risk their health or work in an unsafe manner,” said O’Connor in an email.

But workers from Kentucky to California told NBC News said that even as business boomed, UPS was slow to address safety concerns.

In Tucson, Ariz., June saw a COVID-19 outbreak at the UPS facility, according to local union officials. More than 40 UPS employees tested positive. One died.

“We were devastated when we heard of our employee’s passing,” said O’Connor, who added the company increased cleaning at the Tucson facility and ensured that employees were using personal protective equipment properly.

Workers and union officials said the company is now falling short in adapting as summer heat has met record demand.

“You knew summer was coming,” said Karla Schumann, principal officer of Teamster Local Union 104 in Arizona. “You knew temperatures were going to go through the roof. And you knew you were going to put people in the position of working excessive hours in a pandemic. This is avoidable.”

Even before the pandemic, climate change was shaping the lives of essential workers in hot environments. This summer is on track to be one of the hottest on record, according to the National Oceanic and Atmospheric Administration.

High temperatures can have devastating effects on the body. What begins as fatigue or fainting can quickly lead to organ failure.

“Heat illnesses, heat cramps — these can be deadly,” said R. Jisung Park, an environmental and labor economist at the University of California, Los Angeles who studies heat’s effects. “But there’s evidence that finds that hotter temperatures raise the risk of other injuries, too.”

Research has increasingly linked incidents from heart attacks to accidents like ladder falls and car crashes to hot temperatures, indicating heat-related injuries are far more widespread than previously thought.

“We’re almost certainly undercounting what the effects are,” said Amir Jina, a University of Chicago professor and researcher with Climate Impact Lab.

UPS does not air condition its boxy brown trucks where, on hot days, drivers say they have clocked temperatures above 150 degrees in the cargo areas. The company does not consistently provide drivers with water and the ice machines at its facilities frequently run out or break, workers said. UPS said air conditioning trucks drivers constantly turn on and off is ineffective, as is air conditioning many warehouses with large, open doors. The company did not directly respond when asked about water and ice machines, but has previously said it provides drivers water in the morning but not when they are out on their routes.

Drivers often rely on public buildings and businesses to find respite. The pandemic has whittled their options.

“There are no libraries to go to. Can’t go to restaurants,” said Basil Darling, a driver in Brooklyn. “What we would somewhat have as a little break, we don’t even have that.”

Those workers who move packages in the company’s warehouses said they feel the same pressure as drivers from management to move quickly. The buildings — largely cooled by fans — can be hotter than outside in the summer, workers said. The dark brown trucks they load, which sit out in the sun, are hotter still.

“They ask, ‘Are you alright?’ but see if you can keep going,’” said a warehouse worker in Dallas, who asked not to be named for fear of retaliation. “There are days when I go home, I feel like passing out.”

Texas has reported more severe heat injuries for delivery workers to OSHA than any other state, federal data show. But some of the new workers are not being properly trained on how to protect themselves, said Debbie Franco, another Dallas warehouse worker.

“We’ve had a lot of new hires in the last two months and a lot of them are put in operations without being trained,” she said. “If they’re not trained well in hydration, they’ll just go, go, go.”

Employers can mitigate risk of both coronavirus and heat illness by giving workers time to cool off in safe places, Fulcher of Public Citizen said. During the pandemic, when more employees are pushed outdoors to do everything from curbside pickup to COVID-19 testing, essential workers are particularly vulnerable.

“We’ve all come to realize just how important these folks are, and just how much they’re putting their lives at risk for us,” said Fulcher.

But despite a growing attention to the role of essential workers, advocates said OSHA, which polices workplaces, has failed to protect them.

“It’s unthinkable to me what has been happening with OSHA,” said Terri Gerstein, senior fellow at the Economic Policy Institute and director of the State and Local Enforcement Project at Harvard Law School. “They are abdicating their duty to enforce the law.”

OSHA has been “working around the clock to protect America’s workers during the pandemic,” an agency spokesperson wrote in an email. The agency has issued guidance for employers on respirators, preparing the workplace for COVID-19 and enforcement responsibilities. It has also reminded employees about whistleblower protections, the spokesperson said.

OSHA maintains that the guidance, along with existing law, is enough to protect workers. But it has done little enforcement.

The agency has received more than 9,000 coronavirus-related complaints since March. It has issued just four citations.

Data obtained in early June showed more than 350 complaints concerned the Postal Service, FedEx and UPS inadequately protecting workers from coronavirus. Of those, OSHA conducted five inspections.

The most common complaints were an inability to social distance, bathrooms constantly without soap and minimal communication about coworkers who tested positive for the virus.

Without stronger federal regulations, some states are taking their own steps. In July, Virginia became the first state in the nation to pass safety rules to protect workers from infection.

“In the face of federal inaction, Virginia has stepped up to protect workers from COVID-19,” said Governor Ralph Northam in a press release.

More than a dozen other states have broadened worker protections. Among those is California, which is the only state that has a comprehensive standard to help prevent heat illness in outdoor workers.

Since 2018, OSHA has pushed UPS, the Postal Service and FedEx to voluntarily improve their heat protections, records show.

In written statements, both FedEx and the Postal Service said they educate workers on heat illness, encouraging hydration, rest and to recognize the signs of heat illness. The Postal Service also said it has provided all of its employees “hot-weather face coverings…to ensure their safety and well-being during the COVID-19 pandemic.”

UPS and the Postal Service were both fined twice for heat citations. But those fines may successfully be contested.

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Last month, a judge vacated five Postal Service heat citations from 2016 and 2017, ruling that OSHA failed to show that the heat-stress chart it uses to issue citations was based in science. The rulings could have widespread implications for OSHA’s ability to successfully cite employers for failing to protect workers from heat exposure, leaving workers vulnerable as temperatures continue to rise.

“If they aren’t going to defend the guidelines for heat, are they going to defend the guidelines for COVID?” asked Fulcher, the worker safety advocate.

Outside a warehouse in Brooklyn on a sticky July morning, brown-clad UPS drivers gathered before their shift on the last day of a weeklong protest.

“UPS is one of the only companies that made a large profit in the last six months, while most businesses are closing,” said Chris, who declined to give his last name for fear of retaliation. “We just don’t feel they treat their employees fairly.”

Chris said he had passed out twice from the heat during his decade working at UPS.

“I don’t think that’s safe — working in a hot truck,” he said. But in an economic crisis, he can’t afford to be out of work. “You could stand up and lose your job and end up homeless. Or you could just go with what they say.”