Heat transfer from the human body in hyperthermic, low gas-velocity environments.

We studied the physics of heat transfer from the human body under conditions of low air velocity (LAV) (less than 0.1 m.s-1). For 1 ATA, we calculated values of the evaporative heat transfer coefficient (he, kcal.m-2.h-1.mm Hg-1) of 5.24 and 5.37 where ambient temperature/relative humidities were 38 degrees C/70% and 35 degrees C/80%, respectively, and ambient velocities of 0.06 m.s-1 in both cases. This is near our newly conceived "minimum he value" of 4.94, significantly greater than the 2.8 and 3.4 values that are calculated using the Lewis relation. We further described an "iso-vapor pressure temperature difference," delta Tp, which transcribes a water vapor pressure difference into a temperature difference, thus allowing use of classical nondimensional analysis to account for the increasing importance of heat loss through evaporation in LAV. Water vapor pressure differences in LAV hypobaric environments will account for increased heat losses since, for example, the minimum value of he doubles from 4.94 to 9.88 at 0.5 ATA.