“Afterdrop” is common after swimming in cold water; you get out and feel fine, and then you start to get colder, sometimes growing faint, shivering violently and feeling unwell.
Afterdrop is the phenomenon of your body temperature continue to drop even after you get out of cold water and into a warmer environment – so that you feel colder 10 or 40 minutes after you exit than you did in the water.
When you swim, your body shuts down circulation to your skin, pooling warm blood in your core. This process helps you stay in the water longer: with reduced circulation to your peripheries skin and sub-cutaneous fat is turned into a thermal layer, similar to a natural wetsuit – hence the wild swimmers’ term bioprene for fat.
But when you exit the water, the cooling process does not stop straight away. Even dry on the banks, this cold layer of skin and muscle continues to cool your core. You can lose up to 4.5°C from your core temperature (according to Golden and Tipton, Essentials of Sea Survival), bringing on shivering, hypothermia, or feeling faint and unwell.
For a while there was a popular urban myth that afterdrop happened as blood returning to the skin as you warmed up, and cooling as it travelled. In Winter 2021 Mike Tipton from the Extreme Environments Lab worked to educate the winter swimming community that that this is not what is happening: continued cooling on the banks is down to “conductive cooling” not blood flow.
The key to warming up and staying well is to warm up slowly and gradually.
How to warm up
- To minimise the risk of afterdrop, dress immediately starting with the top half of your body.
- Dry yourself off ASAP – remove all wet layers and pat yourself dry. When dealing with a cold person always remove all their wet things.
- Layer up: in thermals, wool jumpers, insulated jackets, woolly hat and gloves, long coats. Silver foil blankets do not help swimmers – unlike runners, for example, swimmers are not radiating heat post swim, so there is no escaping heat for the silver foil blanket to trap.
- Stand on something as you change if the banks are cold to avoid losing more heat from your feet – a changing mat, or wooden bath mat. Some winter swimmers take a thermos of warm water which they pour into a small tub to stand in.
- Sip a warm drink: this helps warm the body gently from the inside.
- Eat something: sugar will help raise body temperature (not suitable for hypothermic swimmers)
- Sit in a warm environment: in the absence of more salubrious spaces, cars, with heaters on full, are popular with channel swimmers.
- If you feel okay, walk around to generate body heat. It can take some time to warm properly.
- If you feel unwell at this stage, sit down somewhere warm.
- Hot baths and showers can be tricky: they may make affect blood pressure and make you feel faint and unwell. Tipton says a key is to have them warm, but not hot.
- Kate Rew is the author of The Outdoor Swimmers’ Handbook (Rider, 2022, also availablesigned in The OSS shop) and Wild Swim (Faber). Instagram: @kate_rew.
- With help from Dr Mark Harper and Heather Massey from the Extreme Environments Laboratory at the University of Portsmouth.
Kate Rew
