COLD INJURIES - Medicine for Mountaineering and Other Wilderness Activities - page 384

Perspirationiscontinuouslyproducedinsmallamounts,evenincoldclimates.This“in-
sensible” perspiration evaporates from the skin, extracting approximately 575 calories of
heat for each cubic centimeter lost. Additional heat is lost from the respiratory passages as
inspired air is warmed to body temperature and moistened to 100 percent relative humid-
ity.Intemperate conditions20to30percentofallheatlossresultsfromevaporation, about
twothirds of which takes place on the skin.
Heat and water losses from the lungs become much larger at high elevations where
breathing is deeper and more rapid to compensate for the lower quantity of oxygen in the
atmosphere. As much as four liters of water and 2300 kilocalories of heat can be lost daily
through the lungs at high altitudes (1000 calories = 1 kilocalorie; the calories listed for
foods are actually kilocalories).
Heat loss from the respiratory tract cannot be reduced in any practical manner. Mouth
breathing increases fluid and heat loss somewhat, but the amount is insignificant in com-
parison with the quantity of heat lost through other sources. Outdoor recreationalists must
beawarethatthisheatandwaterlossareoccurring,musteatenoughfoodtoregeneratethe
heat, and must drink enough liquids to replace the water.
Heat loss from insensible perspiration also cannot be effectively limited. Vapor barrier
systems, which consist of a layer of material impermeable to water vapor (usually plastic)
between layers of insulation, have been tried. Since the barrier prevents water vapor loss,
the air beneath the barrier quickly becomes saturated, perspiration cannot evaporate, and
heat loss by that route should be eliminated. However, perspiration does not cease, and the
clothing underneath the barrier becomes saturated with water. Wet clothing is hazardous
because it no longer limits convective heat loss.