Purpose: We investigated the effects of humidity on regional sweating secretion and active sweat gland density on the scalp during passive heating in hot environments.
Methods: 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 60 min.
Results: (1) Total sweat rates were greater for H30% (179.4 ± 35.6 g h-1) than for H85% (148.1 ± 27.2 g h-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 ± 13 glands cm-2) than for H30% (62 ± 17 glands cm-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).
Conclusions: 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.
Keywords: Active sweat glands; Air humidity; Non-hairy head; Sweat distribution; Sweating; Total sweat rate.