When a fan helps in a heat wave — and when it hurts

An electric fan can provide instant relief on a hot day, but did you know it doesn’t actually cool the air? And that one fact is why a fan can help during one kind of heat wave and make things worse in another.

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Standing electric fans, box fans and ceiling fans are widely relied upon across Europe, the Caribbean, Latin America and the Asia-Pacific region, where fewer homes have air-conditioning, especially compared to the United States.

A fan cools you off by speeding up the evaporation of sweat, which is what cools you down. But once the air around you is hotter than your skin, a fan actually adds heat faster than your body can sweat and cool off.

That’s why the CDC, EPA  WHO and even the American Lung Association all advise against relying on fans once temperatures hit the mid 90s, roughly about 95° Fahrenheit.  The Environmental Protection Agency warns that when it’s that hot, “electric fans will not prevent heat-related illness.” 

However, newer research challenges that threshold. A small study done by the University of Sydney found that fans still helped people in hot, humid conditions, even at a heat index or “feels like” temperature of more than 100°F.  

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The reason? The high humidity. When it’s muggy, you sweat more so a fan still helps you stay cool.

Now here’s when a fan can be harmful.  Researchers in the same study and another published in Journal of the American Medical Association (JAMA), found that in dry heat, a fan can increase your heart rate and core body temperature, especially in older adults.  

Older people generally sweat less than younger ones, so the people most vulnerable in a heat wave are the ones fans help least.

So while it can be a helpful tool, you probably should not rely on a fan alone.  You can pair it with a cool shower, or misting your skin. Remember to always stay hydrated especially during a heat wave, and when in doubt get to air-conditioning or a cooling center.

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