The cold that can kill: a GP speaks

Dr Sarah Jarvis, a practicing GP, medical writer and broadcaster. Watch her video which explains the medical facts behind the dangers of swimming in reservoirs.

Swimming in a reservoir might seem fun but it can, quite literally, be lethal.

Even on a warm day, the deep water in reservoirs remains very cold, sending the human body into shock.

When you first enter the water, the sudden exposure to cold will literally ‘take your breath away’ – your instinctive reaction is to take a huge breath in. You can’t hold your breath for long and start hyperventilating or over-breathing. If you can’t stop yourself breathing in, you breathe in water and start drowning, which is why you can get into trouble within seconds of going into the water. Over-breathing also makes you light-headed and deprives your brain of oxygen.

Your body can also respond very quickly to the cold by shutting down the blood supply to your skin. This increases the work your heart has to do, and can lead to dangerous irregular heart rhythms or a heart attack.

You might think you're ok, but...

Even if you don’t run into problems in the first few minutes, the so-called ‘short term immersion phase’ (the period within the first 30 minutes of getting into the water) brings its own problems. As your body cools down, your arms and legs become un-coordinated and your muscles don’t work as effectively. The effect is so great that even strong swimmers may find they can’t stay afloat or get out of the water. And remember, with reservoirs there’s no lifeguard to help.