It's long been understood that working outside in hot weather can be dangerous: Even ancient Egyptians worried about dehydration for workers building the pyramids.
Now, a growing body of research is quantifying that danger — and suggesting ways to better protect workers.
The risks extend beyond obvious concerns like dehydration and heatstroke.
"Heat makes people slower to react and worse at making decisions," says Adam Dean, a labor economist at George Washington University. "That means farmworkers driving a tractor or a construction worker operating equipment are more likely to have a fatal accident on a hot day."
But a suite of new analyses has found that regulations that provide basic safeguards like water, shade and rest for workers in hot conditions can help lower the numbers of heat-driven injuries, workers' compensation claims and even deaths.
The studies all use different datasets and methods but come to a similar conclusion, says Barrak Alahmad, an environmental health scientist at Harvard University and an expert on occupational health risks.
"States with heat standards have lower risk of heat injuries, of heat fatalities and other outcomes compared to states that don't have these heat standards," Alahmad says.
The most recent study, published in December in the journal Health Affairs, looked at California's rule protecting outdoor workers from heat, the oldest such rule in the country. Researchers found the regulations led to at least a 33% drop in heat-related deaths among workers after 2010 — an estimate of several dozen lives saved each year.
The outcome "delivers a clear message," says Dean, the study's lead author. "Heat standards, if they're adopted and effectively enforced, can significantly reduce worker deaths."
The new wave of studies comes as the federal government is considering creating new national rules to protect workers from excessive heat. Several states and local jurisdictions are also considering new standards.
The federal rules, first proposed under Biden, are now under review by the Trump administration. Their future is uncertain.
While the federal Occupational Safety and Health Administration (OSHA) has recognized for decades that heat poses risks to workers, there is active debate among worker advocates and business groups about how best to provide protections: via stringent, highly specific regulations, or with broader guidelines that allow employers to take the lead in crafting efforts specific to their own industries.
The new studies could help inform any new rules, says Jordan Barab, who was deputy assistant secretary of labor at OSHA under the Obama administration. Though the basic measures to protect workers have been well-known for decades, it's invaluable, he says, to "show that when a state actually implements these requirements that they actually have saved lives."
The California example
Federal regulators first noted that heat could put American workers at risk in the 1970s and '80s. But for years, OSHA prioritized regulating other workplace hazards. Heat issues were managed under the agency's more generalized rules, such as the "general duty clause," which required employers to maintain workplaces "free from recognized hazards."
But some states, like California, decided to go further. In 2005, after the highly publicized deaths of several farmworkers due to heat exposure, California passed the nation's first state-level regulations to protect outdoor workers from excessive heat. Requirements kicked off when temperatures exceeded 85 degrees Fahrenheit (the threshold has since been lowered further).
The rules set out to provide some simple protections: access to water, shade and rest on hot days.
For many years, California was the only state with such heat rules, setting up a natural experiment: Would heat-related worker deaths fall in California, compared to neighboring states with similar weather conditions but no such protections?
The new study suggests that, at first, the rules didn't make much of a difference. During the first few years, researchers did not find a decrease in heat-related death rates in California compared to neighboring states.
"When California first adopted a standard in 2005, it was ineffective," Dean says.
But that would soon change.
In 2010, the state strengthened the rule and deaths began to drop, the study found — eventually falling by more than 30%, with even more dramatic reductions in recent years.
The changes to the rule, Dean says, were critical. Though the initial rules required employers to provide water and shade, in practice, inspectors sometimes found problems — like undrinkable water.
So, the state clarified. Water had to be drinkable and free. And there needed to be enough shade for all workers during breaks. California also ramped up workplace inspections and launched an educational campaign to train the state's many outdoor workers about their rights.
"A critical lesson is that merely passing a heat standard is not enough," Dean says. "It was only after the state launched a statewide enforcement campaign that we started to see deaths decrease relative to the surrounding states."
The rules could have been even more effective with more consistent enforcement, says Garrett Brown, who until 2014 worked for Cal/OSHA, the state agency tasked with enforcing the rule. Even though the number of inspections increased, he says, limited staffing caused ongoing enforcement challenges.
It could have been "even more health protective for workers if there was an even more robust enforcement program," Brown says.
A growing body of evidence
The California study joins two other analyses with similar findings published in the past year.
Together, they provide important insights that could help in the design of future rules, says Alahmad. He led an analysis of heat-influenced worker injuries, published earlier this year, which found that states with heat rules had lower injury rates than those without.
Another recent study found workers' compensation claims were lower in states with heat standards compared to those without.
The next step for researchers is to suss out the most important parts of those regulations, Alahmad says: "What elements are actually most effective?"
That will be key information for regulators across the country. More than a dozen states and cities proposed new heat protection rules in 2025.
Edited by Rachel Waldholz
