In the midst of a climate crisis, the world must prepare for an onslaught of heatwaves like never before. New research now suggests that concerns over using electric fans are largely unfounded.

Air conditioners and electric fans both have their perks and pitfalls, but the former is recommended far more than the latter. Today, many leading public health authorities, including the World Health Organization, advise against relying on electric fans on days exceeding 35 degrees Celsius (95 degrees Fahrenheit). The fear is that these contraptions will wick away sweat from the skin and make heat stress worse, especially among older individuals.

A newly published study finds the picture isn't quite so dire. Electric fans are mostly detrimental to human health when humidity is extremely low or extremely high, researchers say, and that's not the case for the majority of the world.

Between 2007 and 2019, models suggest fans could have been safely used for both young and old adults across large swathes of northern Europe, northeastern regions of the United States, Canada, all of South America, and much of southeast Asia.

Only in exceptionally arid areas, like the Middle East and southwestern parts of the US, and exceptionally humid areas, like northern India and Pakistan, were electric fans inadvisable.

"We have shown that for younger adults fan use could be recommended most of the time in most regions around the world, except for those living in extremely hot and arid areas," says physiologist Ollie Jay from the University of Sydney.

"This is extremely important as air conditioning, both directly and indirectly contributes to greenhouse gas emissions. Electric fans on the other hand require 30 to 50 times less energy to operate. They are more sustainable and more accessible, particularly in low- and middle-income countries."

Unfortunately, that message is getting muddied by current health guidelines. The US Centers for Disease Control and Prevention and the UK National Health Service, for instance, both advise against the use of electric fans in extreme heatwaves – even though there is no experimental evidence to support such measures.

Nevertheless, the public is listening. By 2030, studies predict 700 million new air conditioning units will be installed worldwide, despite there being a cheaper and more accessible solution.

If all people under the age of 65 were to switch from air conditioning to electric fans, estimates show there would be nearly a 10 percent reduction in global CO2 emissions and nearly a 60 percent reduction in global hydrofluorocarbon emissions, which erode the ozone layer.

Clearly, fans are a 'greener' alternative to air conditioning, but are they a safer one, too? To figure out when electric fans actually do become dangerous to public health, an international team of researchers has conducted several lab-based physiological studies.

On a 40 °C (104 °F) day with around 50 percent humidity, the team found fans were quite effective at cooling down young adults. Only when temperatures exceeded 47 °C and humidity dropped below 10 percent did fans appear to accelerate heat stress in those under the age of 65.

As expected, people older than that had a lower tolerance for aridity and heat. In the lab, this group experienced accelerated heat stress when they used fans on days above 42 °C with more than 50 percent humidity.

The results are remarkably similar to what other recent studies have found as well, and it suggests most young people around the world are okay to use fans during most heatwaves.

Based on these experiments the authors are recommending new fan guidelines. Regardless of humidity, they say electric fans are beneficial for young people as long as temperatures stay below 39 °C (102.2 °F). Meanwhile, healthy older adults are okay to use fans on days below 38 °C, and older adults taking medication that reduces their sweat are okay to use fans when temperatures stay below 37 °C.

Applying these new thresholds to 108 major cities, it appears we are seriously underutilizing electric fans worldwide.

Between 2007 and 2019, which includes eight of the ten hottest years on record, electric fans would have been safe for young adults to use on 97 percent of all hot weather days and for healthy older adults to use on 95 percent of all hot weather days.

In the 12 countries with the greatest potential for growth in air conditioners, including India, China, Brazil, the US, and Mexico, fan use would have been beneficial and safe for young adults to use on nearly 99 percent of all hot weather days.

Under current WHO recommendations, only 88 percent of those days would have been considered fan-worthy.

"[T]here are many locations on Earth where fan use could be safely recommended as an alternative to air conditioning all of the time despite air temperature exceeding the currently recommended threshold of 35 °C," the authors write.

In the 2018 UK heatwave, for instance, public health authorities and the media regularly warned against the use of fans for particularly hot days.

According to these new models, however, fans could have been universally recommended for both young and old individuals during this time. Only those taking anticholinergic medication would have required an air conditioner at particularly hot moments.

"Our hope is that public health authorities will modify their current heatwave recommendations to align with the latest scientific evidence," says environmental physiologist Nathan Morris from the University of Copenhagen, Denmark.

"Broadening the use of these devices during heatwave conditions has the potential to both drastically reduce peak power demands and offer a low-resource alternative for those who do not have access to air conditioning."

The study was published in Lancet Planetary Health.