Carlos Solis
With Europe reeling from heat, few homes have air conditioning. Most people use fans, so the question of when a fan helps and when it makes things worse matters to millions.
How a fan affects the body
A fan does not cool the air the way an air conditioner does. It simply moves air across the skin. Whether that movement helps depends on two competing forces: the heat energy flowing into the body from warm air and the heat removed when sweat evaporates.
Skin temperature in the shade usually runs about 35°C to 37°C. If the air is cooler than the skin, moving air removes heat and provides relief. If the air is hotter, the fan can just blow extra heat onto the body.
That is why people often point to a rough cutoff around 35°C. The body, however, sweats, and sweating complicates the simple rule “air hotter than skin = bad.” Evaporating sweat pulls heat off the skin, and a fan can speed that evaporation by moving the warm, humid air away from the body. In humid weather, sweat tends to remain on the skin, and that effect matters a lot.
What the research shows
The World Health Organization recommends limiting fan use above 40°C in some guidance and warns in other materials that above 35°C a fan “may not relieve heat.”
In a 2015 JAMA study, eight healthy young men sat in hot chambers at 36°C and 42°C while researchers gradually raised the humidity. The fan allowed them to maintain a normal heart rate and core temperature at higher humidity levels than without a fan. At 36°C the fan pushed the sudden spike in heart rate out to about 83% relative humidity; without a fan that spike occurred at about 62%. At 42°C the fan helped up to roughly 50% humidity.
So for healthy, young, well-hydrated people who sit still, a fan sometimes helps even when the air is hotter than the skin. The key question is when the benefit from evaporation stops outweighing the extra heat the fan delivers. Humidity is the critical factor.
In dry air, sweat can evaporate as fast as it forms, so a fan will do little to increase evaporation and may simply push more hot air against the skin. In humid air, sweat pools on the skin, and a fan that moves that saturated air away lets evaporation resume.
The picture is different for older adults. With age, sweat production falls, blood vessels become less able to move heat to the skin, and cardiovascular and kidney disease plus some medications can disrupt temperature regulation. A temperature that a 25-year-old tolerates safely can be dangerous for a 75-year-old.
In a 2016 JAMA trial, older participants endured 42°C at elevated humidity. Unlike the younger study, the fan did not prevent rises in heart rate or core temperature; overall the heart rate and temperature were higher with the fan. A 2024 eight-hour exposure study at 36°C and 45% humidity concluded that fans at normal or higher speeds did not lower peak core temperature in older people and cannot be relied on as the sole cooling method in rooms above roughly 33–35°C.
A 2025 JAMA Network Open study found that in young adults at 39°C and 49% humidity, a fan increased sweat loss by about 125 grams per hour. If participants were dehydrated, the fan raised heart rate and did not improve core temperature or comfort.
How to make a fan safer: water and hydration
A low-tech way to make a fan more useful is to add water. Evaporation works best when moisture is available to evaporate. If the body cannot produce enough sweat, or if sweating would worsen dehydration, wetting the skin allows cooling without additional fluid losses.
Useful in the heat: Mist your skin, wear damp clothing, use a wet towel, take cool showers, or frequently wet your hands and feet.
Studies of older adults showed that at 38°C and 60% humidity wetting the skin made the heat more tolerable and reduced sweating, which can lower the risk of dehydration.
Drinking 1.5–2 liters of water is critical. If a person is dehydrated, a fan can speed sweat losses and increase strain on the heart without improving core temperature or how the person feels.
When to turn a fan on and what to do instead
| Situation | What studies show |
|---|---|
| Indoor temperature below 32°C | Fans are usually appropriate for most people, but vulnerable groups need extra measures: hydration, shade, and monitoring. |
| 32°C–35°C | A fan can improve comfort and sometimes cooling, but older adults, infants, and people with chronic illnesses should not rely on it alone. |
| 35°C–39°C | Conditions fall into a gray zone. A fan helps more in humid heat when the person is well hydrated and can sweat. Wet the skin and use caution with older adults. |
| Around 40°C and above | Do not rely on a fan as your main cooling method. Seek air conditioning, a cooler room, a cooling center, a cool shower, or medical help if symptoms appear. |
| Very hot and dry weather (for example, 45°C with low humidity) | Fans are most likely to increase heat load, especially for older adults. |
What to do at home during a prolonged heat wave
- Open windows in the evening and at night if the outdoor air is cooler to flush out hot indoor air.
- Close shutters or curtains during the day to limit solar heating.
- Move to lower floors or into a shaded room if possible.
- Spend a few hours each day in cooler public buildings, such as malls or libraries.
- Do not run a fan next to your bed in a very hot room; if the room stays hot overnight, use other measures from this list instead.
A fan is neither inherently bad nor a universal replacement for air conditioning. It is an affordable, energy-efficient tool for coping with heat. If you use a fan smartly, taking temperature, humidity, age, and hydration into account, it can provide relief. If a room is very hot and dry or the person is dehydrated or elderly, a fan can worsen the situation.
Simple practical rule: if the room is under 35°C, a fan usually helps; above 35°C check the humidity and the person’s condition, wet the skin, and seek a cooler place if needed.
Based on reporting from ZME Science
