Sweating efficiency in acclimated men and women exercising in humid and dry heat

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Abstract
Four acclimated men and 4 acclimated women exercised at 30% .ovrhdot.VO2 max in a hot-humid environment [dry-bulb temperature (Tdb)/wet-bulb temperature (Twb) = 37.degree./30.degree. C] and a hot-dry (Tdb = 48.degree. C, Twb = 25.degree. C) environment. Variables recorded during heat stress tests, rectal temperature (Tre), mean skin temperature, heart rate (HR), total body sweat rate (.ovrhdot.Msw), chest sweat rate (chest .ovrhdot.msw), sweat gland activity per unit surface area (.rho.SGA) on the chest, and sweat gland flow (SGF) on the chest were recorded. Sweating efficiency (.eta.sw) was determined as the ratio of required to observed sweating. Maximum sweat gland activity per unit surface area (.rho.SGAmax) was determined with the aid of methacholine. There were no differences between sexes or environments in Tre or HR. Both sexes had significantly lower .ovrhdot.Msw and chest .ovrhdot.msw in the humid heat compared with the dry heat. The women maintained significantly lower .ovrhdot.Msw and chest .ovrhdot.msw than the men in the humid heat, with significantly higher .eta.sw. There were no differences in sweating rates or efficiency between sexes in the dry heat. Among the women, .rho.SGA relative to .rho.SGAmax (%.rho.SGAmax) and .eta.sw were significantly higher in the dry heat than in the humid heat, but SGF was similar in both environments. Among the men, SGF was significantly higher in the dry heat than in the humid heat, and %.rho.SGAmax and .eta.sw were similar in the 2 environments. In both environments, the men recruited a significantly lower percentage of their available sweat glands than did the women. The reduction in .rho.SGA in the humid heat among the women allowed the women to conserve body water through improved sweating efficiency. The men had a larger apparent reserve to increase sweating in more severe dry heat.