August’s scorching weather has made even short trips outside exhausting and turned nights into sweaty, sleepless ordeals. With no air conditioning in many homes, people are using ice packs, wet towels, frozen water bottles and electric fans to cope. But a fan might not be doing you any favours; in fact, if you use it the wrong way, it could be raising your risk of a heart attack.What the study saysA new study from the University of Sydney has found that while fans can make you feel blissful during a heatwave, they can also push your body into dangerous territory, particularly if you’re dehydrated. Researchers wanted to understand exactly how fans affect our bodies in hot and humid conditions, so they put 20 volunteers into a climate-controlled space set to 39.2 degrees Celsius with 49 per cent humidity.The participants weren’t just asked to sit there and suffer. Scientists tracked their heart rate, core temperature, sweating, and comfort levels, both when they were properly hydrated and when they had been deliberately dehydrated (by avoiding fluids and water-rich foods for 24 hours). Each hydration state was tested twice — once with a fan blowing and once without.Sweat, strain, and a shocking findingThe study revealed that using a fan while dehydrated increased sweat losses by around 60 per cent. Now, sweating might sound like a good thing when you’re overheating, but in this case, it pushed the body into a dangerous loop: more sweat loss means more dehydration, which means more cardiovascular strain. That extra strain can, in extreme cases, trigger heart attacks, particularly in vulnerable individuals.Connor Graham, PhD, who led the study, explained that fans can help keep you cooler at temperatures up to around 39–40 degrees Celsius. But when the air gets hotter than your skin, the fan can actually heat your body faster than it can cool itself. Most extreme heat decedents do not have air conditioning but often own electric fans, he said. In hotter conditions, fans should be turned off, as they can worsen heat stress.Why hydration is importantThe study says that hydration is a game-changer. When the volunteers were well hydrated, fans were far less risky, even in the extreme heat chamber. But when hydration levels were low, the fan’s effects tipped from helpful to harmful, sending heart rate and body strain higher.This is because sweat is our body’s primary cooling mechanism. When you’re hydrated, you can produce enough sweat to evaporate and take heat away from your body. But if you’re already running on empty, a fan simply accelerates fluid loss without actually cooling you enough. That’s like trying to run your car on fumes while flooring the accelerator.Heatwaves, Britain, and fan habitsIn the UK, people are not exactly built for this kind of weather. Their homes are designed to trap warmth, not keep it out, which is why fans are practically flying off the shelves in high street stores right now. But unlike in countries where air conditioning is the norm, we often rely on fans as our only cooling option and that’s where this warning matters.It’s easy to assume that “more fan” means “more cool”. The reality is a bit more complicated. In moderately hot weather, a fan can help your sweat evaporate and keep you feeling comfortable. In extreme heat, particularly if your flat feels like a slow cooker, a fan can just push hot air onto your body, speeding up dehydration and heart strain.How to use fans safely in a heatwaveDon’t chuck your fan in the bin just yet. Here are some science-backed ways to stay safe while keeping cool:Stay hydrated first — Drink water regularly throughout the day, not just when you feel thirsty. Herbal teas and diluted juices also count.Use fans strategically — Aim them across your body, not directly at your face, and combine them with open windows to improve airflow.Cool the room, not just yourself — Place a bowl of ice in front of the fan so it blows chilled air.Turn it off during extreme heat — If indoor temperatures are hotter than 40 degrees Celsius (or feel like it), skip the fan and switch to other cooling methods.Know your limits — Older adults, people with heart conditions, and those on certain medications are more vulnerable to heat strain.