Your normal cooling solution might not cut the mustard during this heatwave
Trying to get a good night of sleep this week can feel near impossible for many households, with an extreme heatwave bringing soaring night-time temperatures to much of the country.
The nocturnal discomfort can push people towards quick fixes like electric fans. But the government has now warned that one of the most common heatwave sleep habits may not be as harmless as it seems.
The UK Health Security Agency (UKHSA) has launched a ‘Beat the Heat’ campaign to provide advice for everyone on how to stay safe during hot weather.
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As part of its guidance, the UKHSA warns that using mechanical electric fans to cool down can actually be dangerous once indoor temperatures reach 35°C (95°F) or higher.
While switching on a fan is an automatic response for many during a heatwave, health, education, and consumer authorities warn that doing so in extreme heat can accelerate dehydration, exacerbate respiratory issues, and fail to prevent severe heat-related illnesses.
The primary reason for the 35°C threshold comes down to basic human biology and physics. Electric fans do not actually cool the air in a room, they simply move it around.
When the ambient room temperature is lower than your body temperature, the moving air helps sweat evaporate from your skin, which cools you down. However, when the surrounding air temperature reaches 35°C – approaching the human body’s natural core temperature – the fan ceases to cool.
Instead, it begins blowing air that is hotter than or equal to your skin temperature directly at you.
Rather than relieving heat, this creates a “convection oven” effect, rapidly heating the body and accelerating the risk of severe dehydration. Home climate experts note that sitting directly in front of a fan in these conditions is equivalent to being blasted by a hair dryer, forcing the body to sweat at an unsustainable rate without providing any actual cooling benefit.
The dangers of using fans during a 35°C heatwave extend into the night, where many households rely on them to get through stifling sleep conditions.
Medical experts warn that leaving an electric fan running throughout the night can also introduce separate health risks, particularly for those with respiratory sensitivities, Wales Online reports. The constant airflow from a fan acts like an invisible broom, disturbing settled dust, pollen, and pet dander, and circulating these airborne particles around an enclosed bedroom.
For allergy and hay fever sufferers, this can significantly aggravate irritation in the eyes, throat, and nasal passages. Furthermore, the continuous flow of moving air creates a severe drying effect, which can dehydrate the skin, throat, and nasal linings overnight, compounding the overall fluid loss caused by the heat.
The Department for Education (DfE) explicitly highlighted these physical limitations in its heatwave guidance for schools and early years settings, instructing staff on the exact limits of mechanical cooling.
“Mechanical fans can be used to increase air movement if temperatures are below 35°C – at temperatures above 35°C fans may not prevent heat-related illness and could worsen dehydration.”
In high-heat scenarios, authorities emphasise that standard fan usage must be halted, and alternative cooling methods must be prioritised to keep indoor spaces safe.





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