Fans do not always lower body temperature and can instead add heat once conditions exceed certain thresholds. The switch depends on age, humidity and other variables. European heatwaves have already caused deaths, making proper use of fans a potential safety issue for those without air conditioning. Government advice in the UK indicates fans lose effectiveness above 35°C, while the World Health Organization places the changeover at 40°C. Research offers differing figures, showing no single limit applies universally. Older adults lose the ability to sweat efficiently, so fans may begin to warm them at lower temperatures than for younger people. Wetting skin or clothing can offset this effect. Without sweating, skin temperature in shade stays near 35–37°C. Air cooler than skin removes heat, while warmer air adds it. A fan accelerates whichever process occurs. Past guidance often cited 35°C as the cutoff, yet this overlooks evaporation. Sweat removes substantial heat even above skin temperature. However, very low humidity allows full evaporation already, so extra airflow mainly transfers more heat inward. At 45°C and 15 percent humidity, fans reliably increase warmth. Higher humidity slows evaporation, leaving skin wet with excess sweat. Under those conditions fans still provide cooling, for example up to 38°C at 60 percent humidity. Beyond that point evaporation drops too far for fans to help. Age further lowers the temperature at which fans become counterproductive because sweating starts later and produces less moisture. Wet clothing or misting offers added relief and reduces dehydration risk. Clothing type and heat adaptation also matter. In buildings that become extremely hot, temperatures may surpass levels where fans remain useful even with added moisture. At that stage occupants are advised to seek cooler locations.
