Anyone who’s had to go to work on a hot summer day knows how exhausting it can be. Brain function slows, commuting is more uncomfortable and, for people who work outdoors, simply staying safe becomes a challenge. All of these factors combined add up to a heat-related hit on worker productivity, which stands to get more pronounced as climate change drives more intense heat waves.

“We used to think, ‘Well, people get hot and they sweat and they’re fine.’ But now we know that’s not necessarily the case,” says Jill Rosenthal, director of public health at the Center for American Progress, which this month released a report on the threat extreme heat conditions pose to workers in the U.S.

Quantifying that threat can be difficult. Broadly speaking, it includes everything from weather-related commuter disruptions to diminished cognitive performance, poor sleep and added time off when kids are kept out of schools that lack air conditioning.

But there are some hard numbers: During a heat wave last summer, President Joe Biden said extreme heat is costing the U.S. $100 billion a year, citing a report from the Atlantic Council that warned those losses could double by 2030. Between 1992 and 2017, heat stress killed more than 800 workers in the U.S., and injured more than 70,000, according to the Bureau of Labor Statistics.

The most severe of these impacts are felt by people whose jobs are outdoors. In the U.S., just five states guarantee workers access to rest, shade and water. Federally, the Occupational Safety and Health Administration’s general duty clause requires employers to provide a safe and healthy workplace, but it doesn’t set clear standards for heat hazards. OSHA is working on such standards, which would be implemented anytime the outdoor temperature crosses 80°F. But they aren’t expected to take effect until next summer.

Some other countries have clearer guidelines. In China, for example, employers are required to train workers on heat-related illnesses and provide rest areas, free cool drinks and air conditioning indoors. Work hours and intensity must be cut or suspended on hot days, and workplaces that can’t keep temperatures below 95°F outdoors and 91.4°F indoors must pay their workers High-Temperature Subsidies ranging from the equivalent of $1.24 to $30.90 per day.

The greatest heat-related labor losses are born by outdoor industries such as construction, mining and agriculture, according to the Federal Reserve Bank of San Francisco. By 2200, researchers there estimate heat-induced labor losses will reduce U.S. capital stock, a measure of accumulated investment, by 5.4 percent, and consumption by 1.8 percent. In workplaces with limited AC, including restaurants and warehouses, safety and productivity will also take a hit.

White-collar workers, who are most likely to have an air-conditioned office at their disposal, aren’t entirely spared from the effects of extreme heat. That’s in part because heat affects cognitive performance. The body’s process of cooling itself down saps energy that is normally used for the complex brain functions, says Clayton Page Aldern, neuroscientist and author of “The Weight of Nature: How a Changing Climate Affects Our Brains.”

“What do we happen to see disappear in those instances? Well, it’s the finely honed executive control functions and higher-level attention networks — the stuff that happens in the most newly evolved areas in the brain,” Aldern says. “The stuff that defines humans as humans is some of the first stuff to go in the heat.”

Hot weather also impairs cognition by causing inflammation to brain tissue, weakening the connection of neural networks and disrupting sleep. Though air conditioning can rescue people from these deficits, Aldern says heat exposure at night and on the way to work can still have lingering cognitive effects.

Then there’s the way heat disrupts infrastructure. Railroad tracks, airport runways and roads are all susceptible to heat, which can melt asphalt and weigh down overhead wires. Some of the biggest transit corridors — including the Northeast Corridor that links up much of the U.S. East Coast — experience delays when temperatures climb, preventing commuters from getting to work on time.

Gabrielle Guarneri, who commutes from New Jersey to Manhattan for work via New Jersey Transit, says she was “consistently late” during the heat wave that struck the U.S. last July. The memory felt particularly fresh as Guarneri waited on a stalled New Jersey Transit train outside Penn Station on June 18, when her commute was delayed by over an hour due to a disabled train.

In places where AC is less common, arriving at work doesn’t necessarily offer relief. Across Europe, many businesses, schools and homes lack air conditioning entirely. Even in the U.S., known for its high levels of AC adoption, more than 40 percent of schools need new or updated heating, ventilation and air conditioning systems.

When an office does have AC, extreme heat can have the ironic impact of increasing employees’ affinity for it — a reversal from the pandemic-driven shift toward working from home. “For a lot of people, it’s more comfortable in the office than home,” says Mark Ein, chief executive officer of Kastle Systems, an office security company that monitors how often employees in over 2,600 buildings across 138 US cities swipe into their offices.

When Kastle examined four heat waves across the U.S. cities it monitors between 2022 and 2023, it found no correlation between a rise in temperature and employee absenteeism. But other events do have clearer fallout. “During adverse weather like the flooding in Texas, we saw those have a huge impact on our data,” Ein says.

Mansoor Soomro, a future of work lead and professor at Teesside University Business School, says even companies with air-conditioned offices are working to increase employee awareness of heat. Some promote “hydration stations,” while others mandate risk assessments, focusing on the employees most vulnerable to heat.

Although productivity is a factor, Soomro says mitigating any adverse effects from heat — like medical emergencies — is a bigger one. “Reputation is a risk far higher than the financial,” he says.