Not all heatwaves are made equal.

Some—like the ones that shattered high-temperature records in Europe and swept the United States in late July—are a mix of heat and high humidity. Others in regions like the Southwestern U. S., the Middle East, and parts of Australia instead strike with an intense, dry heat.

Both can be dangerous, and an initial human reaction to either is often to blast a fan. But according to a new study published Monday in Annals of Internal Medicine, in arid heatwaves, flipping on an electric fan may not help cool you down—and may even spike your body temperature.

“There’s a lot of conventional wisdom around how the body works—what makes us hot, what makes us cold,” says Ollie Jay, a health science associate professor at the University of Sydney in Australia and co-author of the new study. “But sometimes what we experience might not mirror what’s happening physiologically.”

For the study, Jay and his colleagues used a climate chamber which allows them to control temperature, humidity, air flow, and “even imitate solar radiation, if we want,” to recreate the peak conditions of several historic events: Chicago’s 1995 heatwave, Shanghai’s 2017 July heatwave, a California heatwave in July of 2018, and an Ahmedabad heatwave in May of the same year.

Over the course of several days, 12 male research subjects withstood two types of heat waves for two hours at a time: hot and dry, and cooler but very humid. They then returned to the climate chamber and sat for two more “heat waves”—this time with a standard air fan placed about four feet away from them. Each time, researchers monitored the subject’s internal temperature, their cardiovascular strain (determined by measuring changes in heart rate and blood pressure), and their overall sweat lost (measured by their change in body mass during the experiment). The participants also provided a subjective rating of their comfort level.

When the researchers compared the baseline physiological responses, they discovered the fans successfully lowered core body temperature, lessened cardiovascular strain, and improved comfort—but not in every case.

Jay and his colleagues found that in a hot, humid condition with a heat index (which combines air temperature and humidity to better represent how hot it feels out) of 132 degrees Fahrenheit, fans did a good job of cooling down the participants. However, in an extreme, dry heat (and relatively lower, 114-degree Fahrenheit heat index) they found fans left subject worse off across all measures. The men got hot and dehydrated faster and their hearts had to work harder, Jay says, even though the heat index was lower in the second condition.

“(The fan) makes it quite drastically worse,” Jay says. “It’s a compelling way to demonstrate this case.”

As Jay explains it, fans work so well in humid conditions because they help accelerate the evaporation of sweat or moisture on our skin. But when it’s arid, the fan works almost like an oven by simply blasting the body with more hot air.

Major public health groups—the World Health Organization and Centers for Disease Control and Prevention among them—warn that electric fans could exacerbate dehydration and quicken the onset of heat exhaustion when temperatures rise above 95 degrees Fahrenheit. But prior to this study, there’s been little scientific evidence backing those recommendations, Jay says.

In 2012, researchers in London set out to analyze previous peer-reviewed studies to determine whether fans helped people cool down during heat waves. But when they searched for relevant studies, they couldn’t find any.

Jay’s findings are among the first to suggest climate, and not temperature alone, should dictate whether it’s safe to use fans during heat swells.

Still, the study’s results are limited. Researchers used healthy young men as subjects and only exposed them to 120 minutes of brutal heat (natural heat waves typically last a day or longer).

All kinds of factors—including age, heart health, and medical prescriptions—can affect how the human body responds and adapts to hot conditions.

After the age of 60, for example, the body’s ability to sweat becomes impaired.To that end, Jay says his team has already begun repeating their experiments on older adults up to the age of 85. He adds they also plan to test how people taking psychiatric medications respond to heat wave conditions. Growing evidence suggests that people with mental illnesses could be at an elevated risk if they take psychiatric medications that interfere with the body’s self cooling.

As the planet warms (the past two months have been Earth’s hottest June and July on record) it will be increasingly crucial for doctors and public health officials to understand how heat and physiology interact. Recommendations, particularly for vulnerable populations who can’t afford air conditioning or are not mobile, need to be backed by science and specific in their prescriptions, Jay says.

“Stay cool—well how do you stay cool? Stay hydrated—well what does that mean?” Jay says. “They’re really vague. They should be based on actual evidence and not conventional wisdom. And there’s no time to waste: Heatwaves are already a major health hazard.

When the body’s internal temperature rises to 104 degrees Fahrenheit, heat stroke sets in, and organs and the central nervous system can start to dysfunction or fail. For heat stroke, the mortality rate is around 50 percent.

In 2018, a total of 108 people died from heat-related complications—more than the number killed by lightning, tornadoes, hurricanes, extreme cold, and winter conditions combined. It has consistently been the most lethal category of weather fatalities in the U.S. for the last 30 years, according to statistics released by the National Weather Service. A single wave can be unfathomably devastating, as in the case of a 2003 European heat wave that killed at least 30,000 people or the 2010 Moscow heat wave responsible for nearly 11,000 deaths.

“This problem is not going away,” Jay says. “We need to adapt to a hotter future.”