Women and exertional heat illness: identification of gender specific risk factors.

OBJECTIVE: With the expanding role of women into previously closed combat military occupational specialties, women will likely be exposed more to challenging and extreme conditions. Physical work or exercise in extreme environments could increase the risk for exertional heat illness (EHI) and exertional heat stroke (EHS), the most severe type of EHI. Although men have higher rates of EHS than women, women have slightly higher rates of other EHI. Women may respond differently to exertion in the heat than men, as they typically have higher percentage of body fat (BF%) and lower aerobic power. Further, published pilot-data using the Israeli heat tolerance test (HTT) indicate that women are more likely to be classified as heat intolerant than men. The objectives of the present study were to (1) compare male and female classification patterns of heat tolerance, and (2) identify EHI risk factors that might account for the relationship between heat tolerance classification and sex.
METHODS: Fifty-five male and 20 female participants were recruited from military and university communities to participate in a standardized HTT. Subjects underwent measures to calculate anthropometric variables (BF%, body surface area, and waist circumference), a maximal oxygen uptake test to assess aerobic power (VO₂max), and a standardized HTT, which consisted of treadmill walking at 5 km/h at a 2% grade for 120 minutes at 40°C and 40% relative humidity. Heat intolerance was defined as attaining a maximum heart rate (HR) greater than 150 bpm or a core body temperature (Tc) more than 38.5°C. Separate hierarchical regressions were conducted using categorical (heat tolerant/intolerant) and continuous (physiological strain index, maximum HR, Tc) HTT outcomes. Risk factors were identified with and without controlling for sex.
RESULTS: Women were 3.7 (95% CI, 1.21-11.24) times more likely to be heat intolerant than men (χ²=6.85, P<.01). Compared to men, women had significantly higher BF% and lower body surface area, waist circumference, and VO₂max. All heat intolerant participants had lower VO₂max and higher BF% than those who were classified as heat tolerant. When VO₂max and BF% were entered into regression equations to predict HTT outcomes, sex became nonsignificant; VO₂max predicted maximum HR and physiological strain index after controlling for sex.
CONCLUSION: The present study found that differences between men and women in heat tolerance classification are largely explained by VO₂max. The higher rates of heat intolerance among women likely correlate with higher EHI risk, and underscore the need to understand the physiological and thermoregulatory differences between men and women. As lower aerobic power is a major risk factor for EHI, maximizing the aerobic power of women will be critical to force health protection and readiness as they integrate into combat military occupational specialties.