Chilled to the Bone

The normal body core temperature is around 37°C. Hypothermia refers to a condition where the body core temperature drops below 35°C and as a result manifests with different signs and symptoms. Hypothermia can be classified into mild (34°C - 36,5°C), moderate (28°C - 33,5°C) and severe (below 28°C). At this point, the victim is in shock and can die suddenly from irregular heart rhythms.

Water conducts heat 25 times faster than air. Because of rapid heat loss (by conduction) into the water and minimal natural insulation, the human body cannot defend against low water temperatures as well as low air temperatures. Most of the heat loss occurs through the head, neck, flanks and groin. This makes hypothermia particularly important to divers when it comes to the selection of protective clothing (wetsuits and accessories).

Initially, the body tries to preserve heat by shutting down blood circulation to the extremities and rather shunting all the blood to the "core" of the body. This system is used with great success in whales and is called the counter-current mechanism. This explains why lips and fingernails become blue and the skin pale. Movement starts to slow and heart rate decreases, breathing becomes slow and shallow. The shivering occurs early and is the body's way of trying to generate internal heat. Other signs that divers will notice rather early are loss of manual dexterity, chattering of the teeth and the urge to urinate more frequently! What divers don't know is that their air consumption is likely to increase slightly and their risk for decompression sickness increases as they retain more nitrogen in their tissues. A dive to 20 meters in Cape Town should have a slightly shorter bottom time than the same dive profile in Sodwana as a result!

Often divers are seen in the warmer waters of Sodwana diving with just a farmer john wetsuit. What they don't realize is that they too are at risk of mild hypothermia if they do more than one dive in the day and don't allow themselves to re-heat between dives. Divers who keep a wet wetsuit on between dives are also likely to feel the cold more on the second dive.

A good fitting wetsuit keeps a diver warm by preventing heat loss through the neoprene (those nitrogen bubbles) and to a lesser extent, by preventing water flowing over the diver's skin. The 'titanium lining' of wetsuits is a sales gimmick and has never been proved. Thicker wetsuits or dry suits (if water < 10°C) are required for cold water diving remembering that the deeper the diver goes, the thinner the protective layer of the wetsuit and the more heat that is lost through the wetsuit material. The purchase of hoods, gloves and chicken vests are a sound investment, it's always possible to cool down on the dive rather than abort a dive due to the cold.

 

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