Suddenly plunging into cold water, especially when it's hot, can trigger dangerous health responses, especially in people with underlying conditions, says Sarah Jefferies, lead trainer at Emergency First Aid at Work Course.
“Sudden immersion in cold water, especially during a heatwave, can cause cold water shock. This isn’t just an uncomfortable surprise – it can be fatal,” Jefferies explained. “And for people with heart conditions, asthma or high blood pressure, the risks are even higher.”
What is cold water shock?
Cold water shock happens when the body reacts to an abrupt drop in skin temperature – usually below 15°C.
What are the symptoms of cold water shock?
Cold water shock can cause uncontrollable gasping, rapid breathing, panic, and in some cases, heart failure or drowning – even in experienced swimmers.
Who is most at risk of cold water shock?
- Those with heart conditions or high blood pressure
- Asthma sufferers (cold water can trigger an attack)
- Older adults and young children
- Anyone who’s been drinking alcohol (which impairs judgement and body temperature regulation)
How to stay safe
- Never jump or dive straight into cold water – enter slowly and let your body adjust
- Avoid swimming alone, especially in open water
- Check for hidden hazards – strong currents, sudden depth changes or slippery banks
- Wear a buoyancy aid if you're paddleboarding or wild swimming
- Know the signs of cold water shock and how to respond in an emergency
The experts at St John Ambulance say: "If you start to feel panicked in the water or experience difficulty breathing, roll onto your back. Although the natural instinct may be to swim to shore, rolling onto your back and lengthening your exhales will help you relax and call for help. If possible, exit the water and warm up quickly to avoid hypothermia.
"Lastly, it's important to bring warm clothes and a hot drink for after. And remember, alcohol can worsen the situation, so it's best to avoid it."