Is Caution Needed When Temperatures Exceed 35°C? The Latest Research on Fans and Heatstroke

The Moment a Fan Turns from "Friend" to "Foe"—New Ways to Use It in the Era of Extreme Heat

In a summer room, what is the first thing you reach for? The air conditioner remote, the window, or the fan switch? In households concerned about electricity bills, rooms without air conditioning, old houses, dormitories, car camping, or outdoor work break areas, fans remain one of the most accessible, affordable, and immediately usable solutions against the heat.

However, recent heatwaves are challenging this common sense. It's no longer a simple matter of "if it's hot, just get some air." Once certain conditions are exceeded, a fan can become a device that pushes hot air onto your body rather than cooling it. In other words, the same fan can be a lifesaver one day and a risk factor for heatstroke on another.

Fans Do Not Cool the Air

First, it's important to remember the obvious fact that a fan is not an air conditioner. A fan does not cool the air in the room itself. It creates wind, blowing away the warm, humid air clinging to your skin and helping sweat evaporate, which makes you feel "cool."

The human body releases heat when sweat evaporates. This is the same mechanism that makes sprinkling water or using a wet towel feel cool. With wind, sweat evaporates more easily. That's why a fan can provide relief on a humid day.

However, there's a pitfall here. When the surrounding air becomes as hot as or hotter than body temperature, the wind not only "releases heat" but also "brings in heat from outside." Especially in very dry air where sweat is already evaporating rapidly, additional wind from a fan offers limited cooling effect. It can result in a state where you're continuously exposed to hot air, increasing the heat burden on your body.

The Issue Is Not Just "How Hot Is Too Hot"

A commonly mentioned guideline is "fans are dangerous when it exceeds 35°C." This is easy to understand as a public health warning. However, actual research shows the issue is a bit more complex.

Whether a fan is effective or not is not determined by temperature alone. It varies with humidity, the body's hydration state, age, medication effects, cardiovascular condition, wind speed, clothing, whether the skin is wet, and whether the room is sealed.

For example, even in high temperatures, a humid environment might allow a fan to aid sweat evaporation, reducing core body temperature and cardiovascular strain. On the other hand, in extremely hot and dry conditions, the heat intake from the wind might outweigh the cooling from sweat evaporation, worsening body temperature, heart rate, and dehydration risk.

In other words, it's not enough to just look at the thermometer. The feeling of "hot," "there's wind," and "it feels cool" is also insufficient. The safety of a fan depends on the combination of air conditions and body conditions.

"Feeling Cool" and "Being Safe" Are Not the Same

The challenge with fans is that many people feel immediate relief when exposed to wind. The temperature sensors on the skin react to the wind, making you feel "cool." However, internally, core body temperature might not have decreased. In some cases, the heart may be working harder, sweat production might increase, and dehydration might be progressing.

This is dangerous because it's easy to be deceived into thinking "I'm still okay." In an air-conditioned room, the air temperature itself decreases. However, with just a fan, there can be a discrepancy between perceived comfort and physiological safety.

This is particularly important for the elderly. With age, the ability to sweat and sense heat might decline. Even if they feel "the wind is pleasant," heat might be building up inside their body. People with heart conditions, those taking diuretics or certain anticholinergic medications, and those suffering from sleep deprivation or poor health might also have reduced heat tolerance.

Be Extra Cautious with Fans When Dehydrated

Recent research highlights the relationship between dehydration and fan use. Fans promote sweat evaporation. While this aids cooling, it also increases sweat loss. If you're well-hydrated, that's fine, but if you're already slightly dehydrated, it becomes a significant issue.

Continuously exposing yourself to a fan in a hot room, sweating without feeling thirsty, causes the body to work harder to cool down, but blood volume and sweating ability decrease. As a result, instead of providing comfort, the wind might increase cardiovascular strain.

The advice "stay hydrated when using a fan" might seem common, but it's actually quite important. However, it's not just about drinking a lot. People with chronic conditions or those on fluid restrictions should follow their doctor's instructions. Generally, it's advisable to take small amounts of fluids before feeling thirsty and to be mindful of salt and electrolytes when sweating heavily. With fan wind drying sweat, it can be hard to tell how much you're sweating.

So, Should We Not Use Fans?

The conclusion is not "don't use them," but "don't rely on them alone."

Fans can be a very effective cooling method if conditions are right. They consume less electricity than air conditioners, are easy to purchase, and simple to install. By setting the air conditioner to a slightly higher temperature and using a fan, you can lower the perceived temperature while reducing electricity usage. When the outside temperature drops at night, fans can also help bring in cool air through windows.

The problem is relying solely on fans in a sealed room during extreme heat. Especially in environments where the room temperature exceeds 35°C, where the heat is close to body temperature, in low-humidity dry heatwaves, or in spaces with elderly or unwell individuals, ensuring safety with just a fan is difficult.

Practical measures include blocking direct sunlight during the day, closing curtains or blinds, and reducing heat entering through windows. Turn off unused lights and appliances. Open windows only during cooler times to exchange air. If possible, move to air-conditioned rooms, libraries, commercial facilities, or other cool public places.

When using a fan, it can be effective to lightly wet the skin or clothes, apply a wet towel to the neck or arms, or use a spray bottle to assist evaporative cooling. This method adds moisture from outside rather than relying solely on sweat, making cooling easier while reducing sweat loss.

However, in very humid rooms, evaporation becomes difficult, potentially increasing discomfort. The method of wetting the skin should be considered in combination with ventilation and room temperature.

On Social Media: "I Understand the Theory, But There's No Alternative"

The reason this topic gathers reactions on social media and forums is not just because it's scientifically surprising, but because it directly relates to the urgency of daily life.

On English-speaking forums, people without air conditioning or those who can't afford the electricity costs are discussing how to survive summer with just fans and ice packs. In car camping and van life communities, questions arise about how hot it can get before relying solely on fans and slightly open windows becomes dangerous. In travel-related posts, people who have experienced heatwaves in Japan or Europe talk about the necessity of combining portable fans, parasols, hydration, light clothing, and adjusting activity times.

In physics communities, fundamental questions like "why do fans feel cool?" are also posted. Here, the gap between general perception and science is evident. Many people instinctively associate "wind = cool." Therefore, explanations like "fans can be counterproductive when it's too hot" evoke surprise and skepticism.

Reactions can be broadly categorized into three types.

The first is surprise. The gap between the everyday sense of "wind feels cool" and the research finding that it can be counterproductive in extreme heat is significant.

The second is anxiety. For those without air conditioning, those wanting to save on electricity, and those living in old houses, being told "fans alone are dangerous" doesn't immediately provide alternative options.

The third is practical ingenuity. On social media, everyday measures like wet towels, spray bottles, cold water bottles, daytime shading, nighttime ventilation, and cooling the neck and limbs are shared. While scientifically mixed, the direction of "not relying solely on fans and combining multiple methods" is important.

How Should We Think About Japan's Summer?

In Japan, summers tend to be hot and humid in many regions. In high humidity, fans help sweat evaporate, but in air so humid that sweat doesn't evaporate easily, their effectiveness is limited. Additionally, during tropical nights when temperatures don't drop much, opening windows might not bring in cool air.

Therefore, the practical approach in Japan is as follows.

First, block heat before room temperatures get too high. Next, if you can use an air conditioner, don't hesitate to use it. You don't need to set the temperature extremely low; use fans or circulators to move the air. If you can't use an air conditioner, have the option to move to cool public spaces during the day. Even if enduring with just a fan, wet your body, take frequent fluids, avoid excessive activity, and move to a cool place immediately if you experience dizziness, nausea, headache, or unusual fatigue.

For the elderly, infants, people with chronic conditions, outdoor workers, and those living alone, it's better not to rely solely on personal perception. Place thermometers and hygrometers. Communicate with family and neighbors. Check room temperatures not only before bed but also during the night and early morning. Heat countermeasures are an issue of environmental management, not individual endurance.

"The Courage to Turn Off the Fan" Is Also Necessary

What is needed for future summers is not to deny fans but to question them correctly.

Just because the wind feels good doesn't mean it's safe. It's not just about feeling cool because sweat dries. When the temperature approaches body temperature, the room traps heat, water becomes scarce, and the body sweats less, a fan can quietly increase risk.

Turning on a fan on a hot day is natural. However, in the era of extreme heat, "when to turn it on" is as important as "when to turn it off" and "what to combine it with." Don't just rely on the wind; prevent the room from heating up, wet your body, rest, drink, move, and use air conditioning in combination. By layering these multiple measures, fans can once again become a reliable ally.

Summer safety cannot be ensured by a single appliance. Don't over-rely on the coolness of the wind; listen to your body's signals and watch the numbers on the thermometer. This is the new way to use fans in an era where heatwaves are becoming the norm.

List of Source URLs

Referenced as a basis for the theme "You Should Actually Turn Off Fans When It Gets Too Hot" from ZME Science. Confirmed in search results as "You Should Actually Turn Off Fans When It Gets Too Hot. But When Do They Start Making Heat Worse?"
https://www.zmescience.com/feature-post/health/human-body/temperature-cutoff-when-fans-make-heat-worse/

Introduction of research from the University of Sydney. A 2019 experiment explained that fans improved core body temperature, cardiovascular strain, and comfort in hot and humid conditions, while worsening them in extremely hot and dry conditions.
https://www.sydney.edu.au/news-opinion/news/2019/08/06/is-it-safe-to-use-an-electric-fan-for-cooling.html

EurekAlert's release of the same research. Referenced to confirm the context that it's necessary to look at climate conditions, not just temperature, against traditional guidance like WHO's "caution over 35°C."
https://www.eurekalert.org/news-releases/793005

Scientific American article. Referenced for the explanation that fans can be useful in high humidity but may worsen heart rate, body temperature, and dehydration risk in extreme dry heat.
https://www.scientificamerican.com/article/fans-may-be-okay-for-muggy-days-but-avoid-them-in-extreme-dry-heat/

2025 study from JAMA Network Open. Referenced to confirm that in conditions around 39.2°C and 49% humidity, fan use in a dehydrated state worsened cardiovascular strain and increased sweat loss.
https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2837486

Medical Xpress article. Referenced for the 2025 study overview showing that fans were somewhat effective at 38°C and 60% humidity for the elderly, but increased core body temperature and discomfort at 45°C and 15% humidity.
https://medicalxpress.com/news/2025-07-fan-older-adults-hotter-dry.html

GOV.UK "Beat the heat" guidance. Referenced for household measures like preventing room temperature rise, night ventilation, fan use below 35°C, and caution against continuous direct exposure.
https://www.gov.uk/government/publications/beat-the-heat-hot-weather-advice/beat-the-heat-staying-safe-in-hot-weather

Reddit r/lifehacks post. Referenced as an example of a sincere reaction from people living without air conditioning, trying to endure 40°C heat with fans and ice packs.
https://www.reddit.com/r/lifehacks/comments/13tcvx7/how_do_u_guys_survive_in_summers_i_only_have_a/

Reddit r/vandwellers post. Referenced as an example of a reaction asking about the limits of relying solely on fans and ventilation in car camping and van life.
https://www.reddit.com/r/vandwellers/comments/1iyfatm/when_does_a_fan_not_become_enough_during_summer/

Reddit r/JapanTravelTips post. Referenced as a traveler's perspective sharing multiple countermeasures against Japan's extreme heat, such as parasols, hydration, clothing, and activity time adjustments.
https://www.reddit.com/r/JapanTravelTips/comments/1eq6m2m/its_brutally_hot_in_japan_heres_what_i_learned/

Reddit r/AskPhysics post. Referenced as an example of general curiosity and interest in the mechanism of why fans feel cool.
https://www.reddit.com/r/AskPhysics/comments/133ku9a/why_do_fans_help/