While an indoor upper temperature limit of 26 °C has been shown to protect heat-vulnerable older adults (DOI: 10.1289/EHP11651), this guideline has not been verified in young, habitually active adults. Public health recommendations during hot weather typically emphasize staying in cool environments, avoiding strenuous activity, wearing lightweight clothing, and maintaining adequate hydration. However, young adults may be less likely to follow these guidelines. They often do not reduce their physical activity during extreme heat events and may overdress for fashion, cultural, or religious reasons. These behaviors can impose an additional thermoregulatory burden and lead to greater physiological strain during heat exposure, even though young adults generally have a higher capacity for heat dissipation than older individuals. Accordingly, it is important to evaluate whether an indoor temperature limit of 26 °C is sufficient to protect young, habitually active adults. To address this gap, the investigators aim to assess changes in body temperature and cardiovascular strain in young, habitually active adults (18-29 years) during an 8-hour exposure to the recommended indoor upper temperature limit of 26 °C and 45% relative humidity (humidex of 29, considered comfortable). Participants will complete two conditions: A) seated rest while dressed in light clothing (T-shirt, shorts, and socks), and B) light exercise (stepping to simulate activities of daily living, 4-4.5 METs) performed once per hour (except for the lunch hour) while dressed in light clothing plus an additional insulating layer (sweatshirt and sweatpants). This experimental design will allow investigators to determine the effects of added clothing insulation and light activity-representative of typical daily behaviors-on physiological strain in young adults, and to assess whether refinements to the recommended 26 °C indoor temperature limit are warranted for this population.
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Core temperature (Peak) during daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Core temperature (AUC) during daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Core temperature end of daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Mean skin temperature (Peak) during daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Mean skin temperature (AUC) during daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Mean skin temperature end of daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Heat rate (Peak) during daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Heart rate (AUC) during daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Heart rate end of daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Heart rate variability: RMSSD during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Heart rate variability: SDNN during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Cardiac response to standing from supine (30:15 ratio) during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Systolic response to standing from supine during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Systolic blood pressure during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Diastolic blood pressure during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Rate pressure product during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Reaction time during daylong exposure to indoor temperature limit (cognitive function).
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Impulse control during daylong exposure to indoor temperature limit (cognitive function).
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
CDC 4-Stage Balance Test during daylong exposure to indoor temperature limit (postural stability).
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
BTrackS Balance Assessment during daylong exposure to indoor temperature limit (postural stability).
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Fluid consumption during daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Fluid loss during daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Change in plasma volume during daylong exposure to indoor temperature limit.
Timeframe: End of 8 hour daylong exposure
Thermal comfort scale during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Thirst sensation scale during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Arousal scale during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Thermal sensation scale during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Thermal sensation 2 scale during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Orthostatic intolerance symptoms assessment during daylong exposure to indoor temperature limit.
Timeframe: At end of 8 hour daylong exposure
Feel good scale during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Perceived exertion scale during daylong exposure to indoor temperature limit.
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Hydration status during daylong exposure to indoor overheating
Timeframe: At the start (hour 0) and end of 8 hour daylong exposure
Activity levels during daylong exposure to indoor overheating
Timeframe: End of 8 hour daylong exposure