Experts Say Fans Still Cool Above 35C as Heatwave Myths Spread

Q: At what point does your electric fan stop cooling you and actually start making you hotter?

There is no single cutoff, but a fan can start doing more harm than good when the air is hotter than your skin and sweat can no longer evaporate enough to offset that heat. In practice, public-health guidance often treats about 35C (95F) as the caution point, while some agencies advise not relying on room fans once indoor temperatures rise above about 32C to 35C (90F to 95F). The key factor is humidity. Experts including Prof Mike Tipton and building specialist Becci Taylor stress that fans still help in many hot conditions because moving air boosts sweat evaporation. That is why the old idea that fans stop working above 35C is too simplistic. In humid heat, a fan may still cool you even above that level because it helps sweat evaporate instead of sitting on the skin. Fans are more likely to make you hotter in very hot, very dry conditions, when sweat is already evaporating as fast as it can and the fan is mainly blowing hotter air onto you. Research cited in the wider evidence suggests this can happen around 45C (113F) in very low humidity, while some studies find fans remain useful below about 40C (104F), depending on the person and conditions. Age, hydration and health matter. Older adults, young children and dehydrated people may get less benefit and more risk. If you are sweating, hydrated and using the fan properly, it will usually still help; if the room is extremely hot, especially above 40C, do not rely on a fan alone.

3 articles · Updated · bbc.co.uk · Jul 8

Fans can still help in extreme heat above 35C, experts said, rejecting social-media claims that they become useless once air temperatures top skin temperature.
Daytime use works best with windows, curtains and blinds closed to trap cooler indoor air; at night, a fan placed by an open window and pointed inward can pull in cooler air.
Whole-body airflow matters more than blasting the face, physiologists said, because moving air boosts sweat evaporation; young children should not have fans pointed directly at them.
A bowl of water or ice in front of a fan can add modest cooling through evaporation, but a wet or frozen towel over the fan risks electrical hazards and heavy misting can raise humidity.
The advice comes as UK scientists warn extreme heat is becoming more likely and the chance of temperatures above 40C is accelerating.