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

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.

Sensitivity to cutaneous warm stimuli varies greatly in the human head.

The head has been known as the most sensitive area to temperature changes but the values are limited to the face. The purpose of this study was to examine cutaneous warm thresholds on the scalp and face of young males. Eight males participated in this study (24 ± 3 yrs in age, 178.2 ± 5.3cm in height, and 90.0 ± 15.4kg in body mass). All measurements were conducted in an environmental chamber (27 ± 1°C air temperature and 53 ± 1% relative humidity). Cutaneous warm thresholds were measured on nine areas of the following regions: the frontal (two points on the right), parietal (a point on the right and the vertex, respectively), temporal (two points on the right), and occipital region (on the right) along with the forehead using a thermal stimulator (rate of temperature increase 0.1°Cs-1). Skin temperatures on the nine head regions were monitored during the threshold test. The results showed that 1) no significant differences were found in initial skin temperatures among the nine head regions; 2) cutaneous warm detecting temperatures were significantly greater on the vertex (38.2 ± 3.5°C) than on the forehead (34.8 ± 1.4°C) and the other seven scalp regions (P < 0.05); 3) subjects detected the increase of 1.2 ± 1.0°C on the forehead and 1.5 ± 1.2°C on the occipital region as the first warmth while the vertex was the most insensitive to the increase of temperature (4.0 ± 3.2°C) (P < 0.05). In summary, the scalp region of young males was less sensitive to the temperature change when compared to the forehead, and the vertex was the most insensitive among the eight scalp regions to the temperature increase. We conclude that the entire head should be considered as a binary topography with the face and the scalp in terms of cutaneous thermal sensitivity.