Swimming in cold water – Part 2

Why can cold water swimming be dangerous? Losing consciousness while in the water is always treacherous. Although rescue teams are on alert during a triathlon, they will always take some time to reach the person in trouble.

By Jonny Hisdal, PhD, associate professor of physiology, and a six time Norseman finisher, and Jørgen Melau, Norseman’s Safety Director, a rescue paramedic with Norway’s helicopter air ambulance service, and a six time Ironman veteran. 

The water in Eidfjord is always cold

This article, the second in a series of three, looks at some of the issues with cold water swimming, and at which physiological responses may be induced by cold water. By the way, the definition varies, but with regards to swimming, cold water is often defined as water temperatures below 15°C. However, some of the issues we describe here may occur at considerably warmer temperatures, both because individual responses to cold water vary greatly, and because one person’s response may vary from one day to the next.

Diving reflex

You may associate «diving reflex» primarily with diving birds and mammals. But humans also have a diving reflex – though to varying degree. The diving reflex is triggered mainly by a fall in blood oxygen. Your arteries have receptors that continuously gage your blood’s oxygen content. If the oxygen level falls, the receptors trigger a reflex that slows the heart rate frequency (diver’s bradycardia) and reduces the blood stream to most body organs. Signals sent through the parasympathetic part of the autonomous nervous system, which functions as the heart’s brake systems, cause the heart rate to slow down.

The purpose of this response is to reduce the oxygen consumption of the diving animal’s body, in order to prioritize the most vital organs, i.e. the brain and heart. Human infants have a strong diver’s reflex, and during birth the child’s pulse can be monitored, as a drop can indicate insufficient oxygen. Humans retain their diver’s reflex, and some may find it to be triggered by e.g. cold water against the face. Some people actually risk fainting from low blood pressure when taking a cold shower, as a result of an overactive diver’s reflex.

Cold shock

When you have plunged into cold water, you most probably experienced some form of cold shock. This stress response to cold water increases activity in the sympathetic part of the autonomous nervous system, which is activated in crises or stress situations. The heart will beat faster and harder, and the blood vessels in your skin will contract sharply, resulting in increased blood pressure.

Simultaneously, your breathing rate increases, and you begin to hyperventilate. If you have done your training laps in heated pools, you’ll most likely feel a need to breathe more frequently when you start swimming in colder water outdoors. The hyperventilation will be particularly forceful during the first few minutes in cold water, during which you’ll also be far less able to hold your breath. If, while in a pool, you could hold your breath for one minute, you’ll most likely manage no more than 10 seconds when you jump into cold water. Hyperventilation also means you reduce your CO2 levels by exhaling heavily, which is not only bad for global warming, but may have the more immediate effect of giving you cramps.

Diver’s reflex and cold shock – a dangerous mix

As you’ve probably noted, diver’s reflex and cold shock trigger opposite effects. As, during the first minutes in cold water, the heart receives opposite signals to slow and increase heart rate, there is an elevated risk of arrhythmia.  In a person with an undiagnosed heart disease, the condition can be dangerous and induce loss of consciousness, or, in worst case, cardiac arrest.

Even a state of reduced consciousness is treacherous, as you may swallow large amounts of cold water if you draw a breath underwater. If you are not wearing a wetsuit, you may lose enough buoyancy to sink below the surface. Another dangerous response to cold water, even in very small amounts, is triggering involuntary contractions in the respiratory tract, causing the tract to close, and stopping air from reaching the lungs. As this condition will simultaneously affect the vocal chords (laryngospasms), the person affected may be unable to call for help. Lack of oxygen may also rapidly lead to cardiac arrest.

Acute dizziness due to cold water in the ears

When cold water enters your ears, it will cool your ear and eardrums. The ear is also where our sense of balance is located, and if cold water enters and cools this area, it could cause acute dizziness. This state of discomfort could make you disoriented, or, in a worst case scenario, even exacerbate the symptoms of cold shock and increase the risk of swallowing cold water, or of breathing cold water into the lungs.

Reduced coordination and cramping

To swim well, muscle coordination is a prerequisite. Muscles are controlled by nervous signals from the brain’s motor center in cortex. The nervous system’s speed of transmission is reduced in cold temperatures. When your body cools off, you’ll soon notice that both muscle coordination and muscle power diminish.  Cooling off of the body may also cause muscle cramps, which can be lethal. So put on that wetsuit, will you?


When swimming in cold water, you need to be aware that water has great heat (or cold) potential, and may rapidly induce hypothermia, a drop in core body temperature. The effect of cold water on your body depends on insulation (wetsuit, subcutaneous fat), your body’s heat production capacity, and body size. Larger individuals have a greater body volume to body surface ratio, compared with smaller persons.

Hypothermia is often classified as mild, moderate, or severe. Body temperatures approaching 32°C are considered mildly hypothermic; 32-30°C moderately hypothermic, and below 30°C severely hypothermic. Normally the brain regulates the body’s core temperature to 37.5°C, which is optimum for our body’s cells to thrive.

Normally, bodily functions are rapidly affected by hypothermia. Even a one degree drop in core temperature will diminish concentration and sense of judgment. If a person’s core body temperature approaches 30°C, he may lose consciousness and drown.

How can you prepare – stay tuned!

In this article we have listed issues you should be aware of when you swim cold waters. Don’t be alarmed! In our next article, we’ll look at what you can do to make your swim practice and triathlon swim legs as safe as possible, even in cold water.