Hypothermia Series – Part 2: Human Temperature Threshholds

Through a complex physiological process known as thermoregulation, we humans are able to maintain constant core body temperatures internally despite the many temperature fluctuations in our external environment. However, this is not an absolute phenomenon. Ongoing exposure to extreme temperatures will eventually overwhelm the body’s ability to maintain an ideal body temperature of 37°C (98.6°F). In the case of cold temperatures, this “cascade” roughly follows this process:

37°C (98.6°F) – Normal body temperature, which varies between about 36-37.5°C (96.8-99.5°F)
36°C (96.8°F) – Mild to moderate shivering (it drops this low during sleep). May be a normal body temperature.
35°C (95.0°F) – (Hypothermia) Intense shivering, numbness and bluish/grayness of the skin. There is the possibility of heart irritability.
34°C (93.2°F) – Severe shivering, loss of movement of fingers, blueness and confusion. Some behavioral changes may take place.
33°C (91.4°F) – Moderate to severe confusion, sleepiness, depressed reflexes, progressive loss of shivering, slow heart beat, shallow breathing. Shivering may stop. Subject may be unresponsive to certain stimuli.
32°C (89.6°F) – (Medical emergency) Hallucinations, delirium, complete confusion, extreme sleepiness that is progressively becoming comatose. Shivering is absent (subject may even think they are hot). Reflex may be absent or very slight.
31°C (87.8°F) – Comatose, very rarely conscious. No or slight reflexes. Very shallow breathing and slow heart rate. Possibility of serious heart rhythm problems.
28°C (82.4°F) – Severe heart rhythm disturbances are likely and breathing may stop at any time.
24-26°C (75.2-78.8°F) or less – Death usually occurs due to irregular heart beat or respiratory arrest
(Source: Wikipedia)

Bottom line: hypothermia sets in when your core body temperature drops below 35°C (95.0°F).

Researchers and medical professionals have further subdivided hypothermia into stages of severity. However, our focus during our lake swims will be on effectively managing that “gray zone” that exists between normal “yikes this water is cold” non-hypothermia swimming and actual first-stage hypothermia. The next post on this topic will examine this in greater detail.