Sweating distribution and active sweat glands on the scalp of young males in hot-dry and hot-humid environments

Abstract

We investigated the effects of humidity on regional sweating secretion and active sweat gland density on the scalp during passive heating in hot environments. Eight male subjects shaved their heads prior to expose to dry (30%RH; H30%) and humid (85%RH; H85%) conditions at an air temperature of 32°C. Total sweat rate, local sweat rates (frontal, vertex, temporal, and occipital regions), active sweat glands on the scalp (2 frontal, 2 parietal, 2 temporal, 1 occipital, and 1 vertex), and rectal and skin temperatures were measured during leg immersion in 42°C water for 60min. (1) Total sweat rates were greater for H30% (179.4 ± 35.6gh-1) than for H85% (148.1 ± 27.2gh-1) (P < 0.05). (2) Scalp sweat secretion tended to be greater in the H85% than the H30%. (3) Head sweat rates were greater on the frontal than on the vertex for both humidity conditions (P < 0.05). (4) Active sweat gland density on the scalp was greater for H85% (82 ± 13glandscm-2) than for H30% (62 ± 17glandscm-2) (P < 0.05). (5) No significant difference was found in rectal temperature between H30% and H85%, whereas mean skin temperature was significantly lower for H30% (34.8 ± 0.7°C) than for the H85% condition (36.0 ± 0.3°C) (P < 0.05). These results indicate that the thermoregulatory sweating responses for the scalp region were significantly increased in the hot-humid condition compared to the hot-dry condition. Among the regions on the scalp surface, the vertex was the least sensitive to the change in humidity.