Pauline L -

When you're body acclimates to losing a lot of heat rapidly, it seems as though your ability to conserve heat is pretty much lost.

Normal metabolic processes generate a bit of heat, so your body temperature usually is a little above the ambient air temperature. In normal temperatures this isn't too much of a problem, but basal body temperatures in excess of 105F or so can cause damage to internal organs, including the brain. To keep your body temperature at "safe" levels in high temperature, you perspire so that evaporation can add some cooling. You also tend to breathe differently so that internal evaporation in the lungs does some cooling. Your peripheral vascular system "opens up" so that more of your blood is at or near the skin surface to conduct heat out of the body.

To survive in very high ambient temperatures, you must have the ability to keep your internal body temperature below the temperature of the surrounding air. If the body is all tuned up to lose as much heat as possible, the processes that are turned on cannot be turned off rapidly. As ambient air temperature drops your body will cool down with it, and if you're very completely accomodated to losing heat you may continue to pump it out even as your basal body temperature passes through the "standard 98.6F" and drops to something less.

Most people acclimated to high ambient temperature will cease perspiring when body temperature gets down to normal, but it may take substantial time for that peripheral vascular system to get rid of the excess blood in the surface system so that the peripheral flow can be reduced, and loss of body heat will continue until it does. Until the vascular system is purged, you can't even shiver to get that extra bit of heat from the twitching about.

Most people will lose coherence at basal body temperatures below 90 - 95F or so, and will almost certainly lose consciousness by the time they're cooled to around 80F or so. The shut-down of circulation that occurs in shock, whether hypothermic shock or from trauma, prevents the body from any reaction that would further reduce heat loss.

If there's sufficient air circulation, our victim would reach ambient temperature at 68F in this case. If there is no air circulation, as is typical with falling temperatures in the desert or in the arctic, blackbody radiation can cause an "object" exposed to a clear sky to have a terminal temperature about 10 to 15F below the ambient air temperature, so once our victim collapsed in the desert his basal body temperature could easily have dropped, within an hour or two, to something like 55 - 58F, and he's dead due to shock and circulatory collapse.

Somewhat lower basal body temperatures are often survivable in normally acclimated persons when the cooling is fairly rapid, as when someone falls out of a boat into cold water; but the more slowly the hypothermal condition sets in, the more body processes degenerate, are disabled by shock, or are destroyed.

John