The effects of face cooling on the prolactin response and subjective comfort during moderate passive heating in humans.

The purpose of the present study was twofold: first, to determine the extent to which elevated skin temperature is responsible for the hormonal and perceptual responses to passive heating; and second, to determine to what extent face-cooling can override the effects of raised skin temperature. Sixteen recreationally active, non-heat-acclimated volunteers (12 male, 4 female; age, 29 +/- 9 years, [mean +/- S.D.]) underwent a passive heat exposure for 60 min in a sauna maintained at 58 degrees C (13% relative humidity), conditions under which sweating effectively maintains core temperature. Subjects were allocated to one of two experimental groups which were matched for sex, age, body mass index, body surface area and sweating response; one group received face cooling (FC) every 5 min, whilst the other control group (CON) received none. Mean skin temperatures were elevated by approximately 4 degrees C for the 60 min duration (CON, 36.5 +/- 0.1 degrees C; FC, 35.7 +/- 0.1 degrees C; P < 0.05) but core temperature rose by only approximately 0.25 degrees C with no difference between groups. Circulating prolactin remained stable and showed no increase for the FC group, whereas concentrations increased by 102 +/- 34% (P < 0.05) for the CON group. No differences were observed between groups for heart rate, but the sensation of heat was less (P < 0.05) with FC. We suggest that a significant component of the prolactin response to moderate passive heating is mediated by facial skin temperature, and selective cooling of the face is associated with improved perception of thermal comfort. These results indicate that the temperature of only a small part of the total skin area (approximately 10%) has a disproportionately large effect on the hormonal and perceptual responses to heat stress.