Physiological responses of men and women to humid and dry heat

Sex-related differences were evaluated in 10 males and 9 females under hot-wet and hot-dry conditions. Preacclimatized subjects were exposed to a comfortable climate (20°C, 40% rh), mild-wet weather (32°C, 80% rh), two hot-wet conditions (35°C, 90% rh; 37°C, 80% rh), and two hot-dry conditions (49°C, 20% rh; 54°C, 10% rh). Exposures lasted 120 min: 10 min rest, 50 min walk (1.34 m.s^-1), 10 min rest, 50 min walk. During hot-dry exposures, heart rate (HR) and rectal temperature (T(re)) were significantly lower for males than females by 13 and 20 beats.min^-1 and by 0.25 and 0.32°C for the two conditions; no significant differences in sweat loss (ṁ(sw)) were observed. During hot-wet exposures, both final T(re) and (Ṁ(an)) were lower in females than males by 0.34 and 0.24°C and by 106 and 159 g.m^-2.h^-1, respectively (males sweated 25 and 40% more than females). None of these differences correlated with maximal O₂ uptake, body weight, skin surface area, or percentage of body fat. During hot-wet exposures, a negative relationship between surface area-to-mass ratio (A(D)/wt) and T(re), mean skin temperature, HR, and change in heat storage was found. It was suggested that three major factors are involved in these differences: 1) higher A(D)/wt for females than for males, 2) better sweat suppression from skin wettedness for women, and 3) higher thermoregulatory set point for women than for men.