Millions of Americans crank a fan when the heat spikes, but the CDC and EPA warn that above the mid-90s, a fan stops cooling you and can actually push your body temperature up

Summer in the US regularly pushes past 100°F, and for most of us, the reflex is probably the same: turn on a box fan and hope for relief. According to the EPA guidance on extreme heat and indoor air quality, that reflex has a built-in limit. The agency warns that electric fans stop preventing heat-related illness once a room temperature reaches the mid-90s or higher, because a fan does not cool the air; it simply moves it around. Here's what's really happening inside your body when a fan stops working, and what current research says about it.

Why does your fan have a temperature limit
A fan cools you by helping sweat evaporate off your skin faster, not by cooling the air itself. According to the EPA, portable electric fans “do not cool air”; they just circulate it, which increases evaporation and makes you feel cooler. That only works up to a point. The CDC sets its own line a bit lower. The CDC heat and health page notes that fans should be used only when indoor temperatures are below 90°F; above that point, a fan may actually raise body temperature instead of lowering it. The exact cutoff varies a little by agency, but somewhere between 90°F and the mid-90s, the advice flips from “use a fan” to “get somewhere cooler.”

The science behind why a fan can backfire
This is not just a cautious message. This is basic thermal physics. A 2024 review published in The Lancet Planetary Health explains that a fan cools you by increasing airflow across your skin, and this is effective when the air is cooler than your body. But if the air around you is hotter than your skin temperature, roughly 95°F, the same airflow works against you. It keeps replacing the thin layer of slightly cooler air next to your skin with hotter air from the room. In simple terms, when the room is hotter than you are, a fan functions less like a cooling breeze and more like a convection oven set on low.


What the newest research on older adults found
Age changes how much this matters. Older adults sweat less efficiently, to begin with, so evaporative cooling, the whole reason fans help in milder heat, is less effective. In the 2024 JAMA clinical trial, ‘Effect of Electric Fans on Body Core Temperature in Older Adults Exposed to Extreme Indoor Heat,’ researchers tested the use of fans on adults aged 65 to 85 under conditions that simulated extreme indoor heat and found that fans did not reduce peak core body temperature in this group. Any decrease in core temperature or heart rate at the end of the exposure was so slight that the researchers questioned whether it mattered at all.

One nuance to mention here, too: separate research from the University of Sydney found fans can still help in hot, humid conditions because humidity slows sweat evaporation, and airflow helps to counteract that. Not every combination of heat and humidity paints the same picture, but the general pattern remains: fans lose their effectiveness fastest in dry, high heat.

Who's most at risk when the fan isn't enough
This is consistent with CDC-flagged populations most vulnerable to heat waves in general. The CDC’s clinical overview of heat says the most at risk are infants and young children, adults over 65, pregnant people, any person with a chronic health condition, people without reliable access to cooling, and outdoor workers or athletes training in the heat.

The same overview also points out that hot days are linked with increased emergency room visits and hospitalizations for heat illness, cardiovascular problems, respiratory conditions, and kidney disease. It also notes that city neighborhoods with less tree cover and more pavement, known as urban heat islands, tend to run hotter and are more common in lower-income communities, meaning the risk isn’t evenly distributed.

What actually works once the fan stops helping
So what should you do when your apartment crosses that line? The best thing to do, according to EPA’s guidance, is to get to an air-conditioned space, be it your own unit, a friend's home, or a public cooling center, library or mall. If you don’t have AC at home, covering windows that get direct sun with shades, blinds, or curtains can significantly limit how hot a room gets during the day. Outdoor awnings or louvres block even more heat than indoor shades. The general rule: close windows when it’s hot outside, and open them when the outside air cools, which is usually overnight.

The bottom line
Fans are not useless; they are actually useful when the room is in the 80s or low 90s. The mistake is treating a fan as your whole plan once the temperature reaches the mid-90s and beyond. But once you get past that point, moving hot air around isn’t cooling you down, and for older adults especially, a fan alone may not be enough. If you don’t have air conditioning at home, knowing your nearest cooling center now, before the next heat wave, could be a really useful thing to do this week.