When the thermometer inside your home rises too high, the fan may stop providing relief and begin adding heat to your body. Establishing when this happens is less simple: the Italian Ministry of Health calls for caution already above 32 °C, while the World Health Organization indicates 40 °C as a general limit. Between the two numbers there are eight degrees, that is, the difference between a very hot room and one in which even the sofa seems to have developed a fever.
The threshold of 37 °C, circulated in recent days, therefore does not apply to everyone. Age, humidity, state of health, hydration and ability to sweat change the result. What also counts is the temperature of the environment we are in, not the maximum indicated by the weather app: it can be 38°C outside and 30°C inside, or the opposite in an attic that has been closed all day.
The fan works on the skin, not the room
A fan does not produce cold and does not appreciably lower the temperature of the room. It moves the air around the skin, takes away the already heated air from the body and accelerates the evaporation of sweat. It is this last step that gives us the sensation of freshness.
As long as the air is cooler than your skin, which is generally around 35°C, movement also helps heat dissipation. When the air exceeds that temperature, the exchange changes direction: the body absorbs heat from the environment and must rely more and more on the evaporation of sweat. The fan continues to push air, with the impeccable efficiency of a hairdryer that has undergone a course in good manners.
Sweating can still compensate for the heat received, especially in a young, healthy and well-hydrated person. The margin is reduced in the elderly, in people who are dehydrated or with chronic pathologies and in those who take drugs capable of interfering with sweating. Very high humidity also hinders evaporation: the skin remains wet, while the heat remains where it is.
Because the Ministry and WHO indicate two different thresholds
The Ministry of Health warns that, with an ambient temperature above 32°C, the fan can increase the risk of dehydration, especially in people who are bedridden or unable to drink enough fluids. The Italian indications therefore adopt a very prudent threshold, also designed to protect those who are most vulnerable.
However, the WHO recommends using fans only below 40°C. Above, according to the organization, the moving air can further heat the body. The same recommendation invites you to close windows and shutters when it is warmer outside, open them after sunset when the outside temperature drops and spend a few hours in a cool place during the heaviest days. The fan remains a support tool, not complete protection against a heat wave.
The distance between 32 and 40 °C reflects still debated scientific research and populations with very different needs. A threshold suitable for a young adult who sweats normally may be too permissive for an 80-year-old person alone at home.
Studies indicate different limits for young and old people
In 2021 a study published in The Lancet Planetary Health used a biophysical model and meteorological data collected in 108 cities between 2007 and 2019. The authors proposed simplified thresholds of 39°C for young, healthy adults, 38°C for healthy older adults, and 37°C for older adults taking anticholinergic drugs, substances that can reduce the ability to sweat. The 37 °C, therefore, concerned a specific group and came from a simulation, not from a test valid for the entire population.
In 2024, a critical review in the same journal took a more cautious line: above 35°C the improvement in evaporation produced by the fan may be too small to reduce core body temperature in a clinically significant manner, especially in the elderly and those who sweat little.
A few months later, an experimental study published in JAMA involved 18 people between 65 and 85 years old, with a median age of 72. Participants spent eight hours at 36°C and 45% humidity, first without a fan and then with air flows of different intensities. The maximum body temperature reached an average of 38.3 °C in all conditions: the fan improved the feeling of comfort and produced modest variations at the end of the exposure, without significantly lowering the peak core temperature. The sample was small and the experiment controlled, but the result urges us not to rely on a fan to protect an elderly person during extreme heat.
When is it really worth turning it off?
Below 32-35°C indoors, the fan can offer real relief to most people, as long as you drink regularly. Once beyond that range, more caution is needed, especially with the elderly, very young children, bedridden people, chronically ill people or individuals taking drugs capable of altering sweating and thermoregulation.
When the room approaches 40°C, the WHO indication is clear: the fan should be turned off and a cooler environment should be sought. Already before, if the air flow does not provide relief, the skin is very hot or malaise, weakness, nausea, headache or confusion appear, the exposure should be stopped and the body should be cooled directly with cool water, damp cloths or a shower, seeking medical assistance when necessary.
In the evening, when the temperature outside has dropped below the temperature inside, the fan can help exchange air near an open window. During the day, shutters and curtains closed, appliances turned off and staying in rooms less exposed to the sun work better. The fan remains an economical ally with low consumption. Just don’t promote it to an air conditioner.
