Does summer in a humid continental climate elicit an acclimatization of human thermoregulatory responses?

Anthony R. Bain, Ollie Jay

Many thermal physiologists follow the conventional wisdom that physiological heat adaptations occur in the summer for people living in a humid continental climate (e.g. Central Canada, North-eastern and Mid-western United States and Eastern Europe); therefore experimentation across seasons is often avoided. However, since modern behavioral adaptations, such as air conditioning, are accessible and commonplace, it is not clear whether such physiological adjustments actually do occur. It was hypothesized that despite warm weather, residing in a humid continental climate throughout a summer will not elicit any significant physiological heat adaptations since the environmental stimulus for such adjustments will be mitigated by behavioral adaptations. Eight young healthy male volunteers cycled at 60% VO(2max) for 90-min in a temperate environment before (mid-May) and at the end of (start of September) summer. Core temperature [measured in the esophagus (T (es)), rectum (T (re)) and aural canal (T (au))], mean skin temperature (T (sk)), forearm skin blood flow (SkBf), upper back sweat rate (LSR) and heart rate (HR) were measured throughout exercise. Weekly activity logs and a lifestyle questionnaire were also administered throughout the summer months. No significant differences between pre- and end-summer were observed throughout exercise for T (es) (p = 0.565), T (re) (p = 0.350), T (au) (p = 0.261), T (sk) (p = 0.955), SkBf (p = 0.112), LSR (p = 0.394) or HR (p = 0.343). Likewise, the thermosensitivity and T (es) at the onset threshold for LSR (p = 0.177, p = 0.512) and SkBf (p = 0.805, p = 0.556) were also not significantly different. The apparent lack of heat acclimatization could be due to frequent air-conditioning use and an avoidance of outdoor activity during the hottest times of day but may also be due to a lack of environmental stimulus.