Monday, November 24, 2014

Hypothermia, or "Aren't You Cold?"

To see teenage boys at a school bus stop on a winter morning, dressed in no more than shorts a hoodie and sneakers is to believe that some magical cold-proofing has been discovered. One wants to ask “Aren’t you cold? Does your mother know you are out in that?” On both counts, apparently not.

Perhaps what the boys are demonstrating is the distinction between feeling cold and being cold. Men of a tender age tend to have a low percent body fat compared to girls, say 10 to 15 percent compared to 18 to 22 percent. Part of the difference is boys having less subcutaneous (under the skin) fat. This means boys will have better blood circulation to and from skin. Also, underlying muscle will be closer to the skin. Both differences warm skin at the expense of lowering core body temperature faster. The end result is girls feel cold faster than boys, while doing better at preserving core body warmth. In a similar fashion, alcohol relaxes surface blood vessels, so drinkers feel the cold less while at the same time losing heat faster.

Bicyclist dressed for winter ride
The distinction between sensing cold on skin and losing core body temperature is important to anyone planning outdoor activities ranging from walking, biking, skiing, ice fishing, kayaking or snowmobiling. Exposed skin is prone to frostbite in very cold weather, especially when moving fast or exposed to strong winds. At temperatures above freezing the frostbite risk is gone, but hypothermia can still occur at air temperatures well into the 50’s, especially if clothing becomes wet from rain or sweat.

Shivering sets in once core body temperature drops below 95 degrees. Shivering, i.e., involuntary muscle twitching, burns fuel to generate heat. Whenever muscle cells contract, more than half the energy used is “wasted” on heat production rather than contraction. In cool weather, bumblebees pre-warm their flight muscles via shivering. As core temperatures head south towards 90 degrees our shivering response becomes stronger and the “umbles” set in, as in having the fumbles, stumbles, grumbles and mumbles. Emergency medical treatment is warranted when core temperature fall below 90 degrees.

Paradoxically, vigorous exercise in cold weather increases risk of hypothermia, and thus can be described as an exercise trap. First, exercise blunts perception of cold. A university study of people exposed to cold and either exercising or not exercising found that the ones who were exercising “felt” less cold even though their core and surface temperatures were declining at the same rate as when not exercising.

Exercise also generates heat, and thus sweat. Sweat, just like rain or immersion in water, reduces the insulating quality of protective clothing. When this water evaporates, it cools. Finally, prolonged endurance exercise can result in depletion of muscle fuel, and either delay or compromise the shivering response.

In the US, hypothermia accounts for about 1,300 deaths per year (1/3 female, 2/3 male). That figure includes deaths due to alcohol-related exposure and homelessness, but not boating accidents in cold water, which are instead logged as drowning.

Winter scene
Locally, wooded areas are large enough to get into trouble in cold weather. Don’t go out alone, especially in the fast-fading light of late afternoon. If heading out alone, someone should know where you intend to be and when you expect to return. A cell phone doubles as a means to call for help and a GPS device, aiding emergency rescue to home in on your location.

Getting back to our teenage boys, deliberately under-dressing for cold weather may be a form of preening for members of the opposite sex. Males of many species demonstrate their superior fitness by growing the largest horns, or brightest feathers, or showing physical prowess over other males. Being (apparently) impervious to cold is one more way of claiming to be a physically prime specimen of manhood. Is it also demonstrating mental superiority? Not so much.

This column was first published in the Beacon-Villager in November 2010. Winter is upon us again, so worth a reminder that hopothermia is a careless way to die.

1 comment:

  1. Another excellent article. By chance we sedldom have in Europe those very low temperatures you have in the US.