As heat waves occur more frequently around the world, many cities are coping with the heat by establishing cooling centers, urging residents to find air-conditioned public buildings, and handing out free bottles of water — but they caution people to avoid using fans when temperatures exceed the high 90s

However, a preliminary study published Monday in the Annals of Internal Medicine suggests that using fans to cool off may be safer than previously thought — even when temperatures exceed 100 degrees.

Recommendations from major public health bodies — including the Centers for Disease Control and Prevention, World Health Organization, and Environmental Protection Agency — all advise against using electric fans in extreme heat. 

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When it reaches 99 degrees Fahrenheit, fans may increase the work and stress that the body has to respond to in order to cool down, and in these conditions, you shouldn’t use a fan, according to the WHO, which cites the EPA, which cites the CDC. 

“The current recommendations are not based on any evidence whatsoever,” said Ollie Jay, an associate professor in thermoregulatory physiology at the University of Sydney in Australia, who led the research team that wrote the new paper. “And [ours] is evidence, actual scientific evidence, that is supporting the use of fans in the target conditions.”

Jay and the study team compared how well 12 healthy young adult men could handle the heat when using a fan and not using a fan for two-hour trials. The team measured participants’ heart rate, core temperature, blood pressure, and rate of dehydration in climatic chambers that simulated conditions that were either very hot and dry or very hot and humid. 

They found that fans used in hot and dry conditions worsened every measured outcome, proxies for handling the heat. Except for the increased sweat rate, fans in hot and humid conditions improved all outcomes. Systolic blood pressure did not differ with fan use.

The CDC declined to comment on the findings, but said its recommendations on electric fans are based on “limited peer-reviewed evidence.”

The CDC’s and other health organizations’ recommendations use cutoffs based on a measure called the heat index — a combination of humidity and temperature — to advise when not to use a fan. The Sydney team reasoned that combining temperature and humidity doesn’t make sense. 

For example, a hot day with low humidity may yield a lower heat index than a slightly cooler but much more humid day — though using a fan in the latter conditions would pose lower health risks.  

Instead, Jay said, temperature and humidity should be considered separately. It all comes down to physics, he explained. When we sweat, the process of sweat evaporating off of our skin cools us. Evaporation occurs much faster in dry environments than humid environments. Humidity slows evaporation, leaving us feeling wet and sticky. Fans increase airflow, which can help speed up evaporation in humid environments.

However, in dry conditions, sweat evaporates quickly and fans speed up this process too much, which, according to the study, increases core temperature and cardiovascular strain. 

As a result, lead author Nathan Morris and his Sydney colleagues concluded that fans may be safely used in very hot and humid conditions — temperature of 104 degrees, humidity of 50%, and heat index of 132.8 degrees. In contrast, fans are harmful when it’s very hot and dry — temperature of 116.6 degrees, humidity of 10%, and a heat index of 114.8 degrees. 

“I think they’ve got an interesting finding that needs to be out there and people need to be thinking about,” said Mike Clarke, a professor at the School of Medicine, Dentistry and Biomedical Sciences at the Queen’s University in Belfast, Ireland, who has done research in this area but was not involved with this study. He called for more research to measure “what actually happens during heat waves rather than what happens under experimental conditions.”

Both Clarke and Jay pointed out other limitations of this study, particularly its small size and that it didn’t include children and elderly people as well as those taking medications — those at highest risk of heat-related illness during heat waves. Not to mention, the current study didn’t include any women. Jay said that follow-up experiments are currently underway that address these limitations. 

“What we would really like to see is when a heat wave happens, some people are provided with fans and some people are not,” Clarke said. In such a field study, participants would all be given good health advice about drinking water, resting, and staying away from the sun. Outcomes would be compared between those with fans and those without.

Clarke said such a study would be ethical because the outcome is unknown. 

“If we don’t know whether or not the fans will increase or decrease the chances of survival, and we’re not sure that they are a proven benefit, then I would be arguing it’s almost unethical not to actually do the trial,” he said. 

Jay said he hopes that the findings are “integrated into the guidance that we’re using to recommend certain strategies to people.” 

According to Jay, some current recommendations fail to take into account that many people can’t afford comforts such as air conditioning. 

“The idea that we should be recommending, ‘Oh, well, you should get an air conditioner,’ that’s a bit of a privileged kind of recommendation,” Jay said. “We should be thinking about more ecologically valid solutions for people who find themselves in a situation where they can’t afford these types of interventions.”

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