Hypothermia can occur in surprisingly warm conditions. In fact, hypothermia occurs as often in summer as it does in winter and the belief that hypothermia is related to cold weather can leave runners unprepared and vulnerable.
1 The first casualty
One thing that makes hypothermia particularly dangerous for a runner is captured in the saying "The first casualty of hypothermia is good judgment." If you are hypothermic your decision making is likely to be impaired, which leads to further dangers. 20-50% of deaths due to hypothermia involve the victim undressing, a symptom related to severe hypothermia. Poor decision making when running can be extremely dangerous, such as when crossing the road or navigating in the back country.
2 Risk Factors
There are a number of risk factors for hypothermia in runners.
- Air Temperature This is the obvious factor, but it is the most deceiving. Hypothermia is likely at 50f, and can occur at much warmer conditions.
- Wet As the water evaporates, it cools your body making the effective temperature much colder. Wet clothes also have far less insulation value, though wool and good synthetic fabrics are better than cotton. When running in cool conditions, it is important not to overdress and sweat through your clothing. Instead, try to carry excess clothing, or wear clothing that can be adjusted by opening zippers.
- Wind The movement of air will suck the heat from your body, and if you are wet, it will increase the evaporative cooling. Remember that running in still conditions will create air movement over your body.
- Rain Rain will obviously make you wet, but heavy rain will directly chill you and torrential rain can rapidly trigger hypothermia. I've had moderate hypothermia hiking on a 95f day when a summer storm occurred, soaking me cold rain.
- Exhaustion If your body as low energy, you will struggle to keep warm in conditions where you would otherwise be comfortable. This means longer runs and races increase the risk for hypothermia.
- Slowing up Running generates a lot of heat, and generally makes the perceived temperature 20f warmer than the actual temperature. This means that slowing up or stopping can cause you to become chilled, especially if you've been running at a faster pace. I have been comfortable while running in cool rain, and become mildly hypothermic within a couple of minutes of stopping, then moderately hypothermic within 10 minutes.
- Sun down The risk of hypothermia increases when the sun goes down, partly because the heat from the sun is lost, but also this is the time that the temperature drops the fastest. In a long race, this is often a psychological and physiological low point, creating a perfect storm.
- If you are with someone in potentially hypothermic conditions, the first symptoms may be the "umbles" – stumbles, grumbles, mumbles, fumbles. Remember that you may be doing fine when your companion is suffering. Beware doing anything that puts you at risk from hypothermia when helping someone else; never create a second victim. You can’t help someone if you are also impaired.
- If you are shivering, but can stop if you make an effort, you are suffering from mild hypothermia (core 96-98f). This will reduce your coordination, which could be a problem on technical trails. The biggest problem is that mild hypothermia will make you mildly stupid, and less lightly to make good decisions. It is important at this stage to correct the problem as soon as possible.
- If you are shivering and cannot stop even if you try, you have moderate hypothermia (core 91 - 95). Coordination is likely to be obviously impaired and your skin is likely to be pale, possibly with blue lips, ears and fingers. This is a dangerous condition; you need to get warm and dry urgently.
- If the shivering comes in waves, this is probably severe hypothermia. Other symptoms include difficulty speaking, very poor coordination and inability to use the hands. Collapse is likely at this stage and the victim may appear to be dead. (If you pull the victims arm away from their body and it curls back up, they are still alive.)
- There is some evidence that hypothermia may increase the oxygen consumption for a given exercise load, as well as dramatically reduce muscular endurance, muscular force, and muscular power.
To avoid hypothermia it is important to be able to adjust your clothing to stay warm.
- A hat can be tucked into your waist band and carried easily
- A light waterproof jacket can be wrapped around your waist and it will boost the insulation value of your other clothing dramatically. This is particularly valuable if heavy summer storms are possible.
- Zippers on tops allows for quick and easy adjustment.
5 Dealing with Hypothermia
If in doubt, seek shelter, get warm and dry quickly. A delay is likely to result in impaired judgment, further increasing your risk. If you are with someone, especially if you are pacing them on a long race, monitor them for symptoms of hypothermia. Medical help is required for the more significant levels of hypothermia. Remember the saying "You're not dead until you're warm and dead". Even if someone appears to be dead, seek medical help as revival may be possible.
6 See also
- AP. Campbell, BD. Sykes, Effects of internal motions on the development of the two-dimensional transferred nuclear Overhauser effect., J Biomol NMR, volume 1, issue 4, pages 391-402, Nov 1991, PMID 1841707
- The duration of sustained contractions of the human forearm at different muscle temperatures http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1356726/
- Anthony J. Sargeant, Effect of muscle temperature on leg extension force and short-term power output in humans, European Journal of Applied Physiology and Occupational Physiology, volume 56, issue 6, 1987, pages 693–698, ISSN 0301-5548, doi 10.1007/BF00424812