Suitability of a 26 °C Indoor Environment for Mitigating Heat Strain in Young Adults

Evaluating the Effectiveness of a 26 °C Indoor Temperature Limit to Mitigate Physiological Strain in Young, Habitually Active Adults

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.

Study Overview

• Status: Recruiting
• Conditions: Cognitive Change; Physiological Stress; Heat Stress
• Intervention / Treatment: Other: Simulated indoor temperature upper limit

• Study Type: Interventional
• Enrollment (Estimated): 10
• Phase: Not Applicable

Contacts and Locations

• Study Contact: Name: Glen P Kenny, PhD; Phone Number: 4282 6133029267; Email: gkenny@uottawa.ca

Study Locations

• Canada
  • Ontario
    • Ottawa, Ontario, Canada, K1N 6N5
      • Recruiting
      • University of Ottawa
      • Contact: Glen P Kenny, PhD; Phone Number: 4282 613-562-5800; Email: gkenny@uottawa.ca

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult
• Accepts Healthy Volunteers: No

Description

Inclusion Criteria:

• Non-smoking.
• English or French speaking.
• Ability to provide informed consent.
• Healthy young adults without chronic disease (e.g., type 1 diabetes, hypertension)

Exclusion Criteria:

• Endurance trained (i.e., engaged in regular physical training activities more than 3 times a week for more than 1 hour per session).
• Undergoing passive heat exposures such as regular of saunas, warm-water immersion, other.
• Use of or changes in medication judged by the patient or investigators to make participation in this study inadvisable.
• Current pregnancy

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Basic Science
• Allocation: Randomized
• Interventional Model: Crossover Assignment
• Masking: Single

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Exposure to indoor temperature upper limit without physical activity and added clothing.; Participants, dressed in light clothing (t-shirt, shorts and socks), perform seated rest while exposed daylong (8 hours) to an indoor temperature maintained at 26°C and 45% relative humidity (humidex equivalent of 29).	Other: Simulated indoor temperature upper limit; Young adults exposed to an 8-hour simulated exposure.
Experimental: Exposure to indoor temperature upper limit with physical activity and added clothing.; Participants, dressed in light clothing (t-shirt, shorts and socks) and an added clothing layer (sweatshirt and sweatpants), perform light exercise (i.e., stepping, representing activities of daily living, 4-4.5 METS) every hour (with exception of lunch hour) while exposed daylong (8 hours) to an indoor temperature maintained at 26°C and 45% relative humidity (humidex equivalent of 29).	Other: Simulated indoor temperature upper limit; Young adults exposed to an 8-hour simulated exposure.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Core temperature (Peak) during daylong exposure to indoor temperature limit.	Peak rectal temperature (15 min average) during exposure. Rectal temperature is measured continuously throughout the 8 hour exposure to the simulated indoor temperature upper limit.	End of 8 hour daylong exposure
Core temperature (AUC) during daylong exposure to indoor temperature limit.	Areas under the curve (AUC) of rectal temperature during the 8 hour exposure to the simulate indoor temperature upper limit.	End of 8 hour daylong exposure
Core temperature end of daylong exposure to indoor temperature limit.	Rectal temperature measured at hour 8 of exposure to indoor temperature limit (15-min average).	End of 8 hour daylong exposure
Heat rate (Peak) during daylong exposure to indoor temperature limit.	Peak heart rate (15 min average) during exposure. Heart rate is measured continuously throughout the 8 hour exposure to the simulated indoor temperature upper limit.	End of 8 hour daylong exposure
Heart rate (AUC) during daylong exposure to indoor temperature limit.	Areas under the curve (AUC) of heart rate during the 8 hour exposure to the simulate indoor temperature upper limit.	End of 8 hour daylong exposure
Heart rate variability: RMSSD during daylong exposure to indoor temperature limit.	Root mean squared standard deviation of normal-to-normal R-wave to R-wave intervals (RMSSD) measured during 5 minutes of paced breathing (15 breaths/min) with participants in the supine position. RMSSD will be evaluated twice, during two paced breathing periods (separated by 4 min of supine rest).	At the start (hour 0) and end of 8 hour daylong exposure
Heart rate variability: SDNN during daylong exposure to indoor temperature limit.	Standard deviation of normal-to-normal R-wave to R-wave intervals (SDNN) measured during 5 minutes of paced breathing (15 breaths/min) with participants in the supine position. SDNN will be evaluated twice, during two paced breathing periods (separated by 4 min of supine rest).	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.	Cardiac response to standing evaluated as the ratio between the highest R-wave to R-wave interval (lowest heart rate) measured at the 30th heart beat after standing from supine (+/- 5 beats) and the lowest R-wave to R-wave interval (highest heart rate) measured at the 15th heart beat after standing (+/- 5 beats). Cardiac response to standing will be evaluated twice, during two lying-to-standing tests (separated by 10 min of supine rest).	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.	Systolic blood pressure response to standing evaluated as the difference in blood pressure measured between the standing and supine. Standing systolic blood pressure will be taken as the lowest value of those measured after 60 and 120 seconds of standing. Systolic response to standing will be evaluated twice, during two lying-to-standing tests (separated by 10 min of supine rest).	At the start (hour 0) and end of 8 hour daylong exposure
Systolic blood pressure during daylong exposure to indoor temperature limit.	Systolic blood pressure measured in triplicate via automated oscillometry (~60 seconds between measures).	At the start (hour 0) and end of 8 hour daylong exposure
Diastolic blood pressure during daylong exposure to indoor temperature limit.	Diastolic blood pressure measured in triplicate via automated oscillometry (~60 seconds between measures).	At the start (hour 0) and end of 8 hour daylong exposure
Rate pressure product during daylong exposure to indoor temperature limit.	Rate pressure product, an index of myocardial work and strain, calculated as systolic blood pressure x heart rate.	At the start (hour 0) and end of 8 hour daylong exposure
Reaction time during daylong exposure to indoor temperature limit (cognitive function).	Participants will be provided with a tablet device with the Sway Medical testing platform for the assessment of reaction time. Participants will be asked to initiate a movement of the device in response to a visual cue.	At the start (hour 0) and end of 8 hour daylong exposure
Impulse control during daylong exposure to indoor temperature limit (cognitive function).	Participants will be provided with a tablet device with the Sway Medical testing platform for the assessment of impulse control. Participants will be asked to respond to both "go" and "no-go" visual cues. In response to the "go" cue, participants will initiate a movement of the device.	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).	To assess postural stability, participants hold a tablet to their chest, then auditory cues guide participants through four consecutive stances, feet side by side, instep of one foot touching the big toe of the other foot, tandem stand with one foot in front of the other, heel touching toe, and stand on one foot. The balance assessment will be evaluated based on movement detected by an accelerometer integrated into the hardware of the tablet device (Sway Medical Inc).	At the start (hour 0) and end of 8 hour daylong exposure
Arousal scale during daylong exposure to indoor temperature limit.	Felt arousal scale assessed via a visual analog scale ("How worked up are you?") ranging from "high arousal" to "low arousal".	At the start (hour 0) and end of 8 hour daylong exposure
Thermal sensation scale during daylong exposure to indoor temperature limit.	Thermal sensation assessed via a visual analog scale ("How hot do you feel?") ranging from "extremely hot" to "neutral"(midpoint: hot)	At the start (hour 0) and end of 8 hour daylong exposure
Mean skin temperature (Peak) during daylong exposure to indoor temperature limit.	Peak mean skin temperature (15 min average) during exposure. Mean skin temperature as calculated from skin temperature measured across 7 body regions is measured continuously throughout the 8 hour exposure to the simulated indoor temperature upper limit.	End of 8 hour daylong exposure
Mean skin temperature (AUC) during daylong exposure to indoor temperature limit.	Areas under the curve (AUC) of mean skin temperature as calculated over 7 body regions during the 8 hour exposure to the simulate indoor temperature upper limit.	End of 8 hour daylong exposure
Mean skin temperature end of daylong exposure to indoor temperature limit.	Mean skin temperature measured over 7 body regions at hour 8 of exposure to indoor temperature limit (15-min average).	End of 8 hour daylong exposure
Heart rate end of daylong exposure to indoor temperature limit.	Heart rate measured at hour 8 of exposure to indoor temperature limit (15-min average).	End of 8 hour daylong exposure
BTrackS Balance Assessment during daylong exposure to indoor temperature limit (postural stability).	To assess postural stability, participants will be asked to stand on a BTracks force plate with their feet spread out to shoulder width while looking straight ahead. The assessment will comprise of one trial lasting 120 seconds in length. Center of pressure (COP) vector data along vertical (y) and horizontal (x) axes will be summed for the total path excursion length (cm) during each trial.	At the start (hour 0) and end of 8 hour daylong exposure
Thermal comfort scale during daylong exposure to indoor temperature limit.	Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") ranging from "very uncomfortable" to "comfortable".	At the start (hour 0) and end of 8 hour daylong exposure
Thirst sensation scale during daylong exposure to indoor temperature limit.	Thirst sensation assessed via a visual analog scale ranging from "very, very thirsty" to "not thirsty at all".	At the start (hour 0) and end of 8 hour daylong exposure
Thermal sensation 2 scale during daylong exposure to indoor temperature limit.	Thermal sensation assessed via a visual analog scale ("How do you feel?") ranging from "hot" to "cold"(midpoint: neutral)	At the start (hour 0) and end of 8 hour daylong exposure
Hydration status during daylong exposure to indoor overheating	Hydration status measured via assessing the urine specific gravity of a urine sample.	At the start (hour 0) and end of 8 hour daylong exposure
Activity levels during daylong exposure to indoor overheating	Activity levels assessed via a wearable monitor (Actical) containing an accelerometer worn on the wrist and hip.	End of 8 hour daylong exposure
Fluid consumption during daylong exposure to indoor temperature limit.	Average hourly fluid consumption calculated by weighing participant water intake at the start and end of each hour of exposure (normalized to the exposure duration).	End of 8 hour daylong exposure
Fluid loss during daylong exposure to indoor temperature limit.	Fluid loss calculated as the change in body mass during each exposure presented as a percentage of baseline body mass (corrected for food consumption).	End of 8 hour daylong exposure
Change in plasma volume during daylong exposure to indoor temperature limit.	Change in plasma volume from baseline values calculated from duplicate measurements of hemoglobin and hematocrit at the start and end of each exposure using the technique by Dill and Costill.	End of 8 hour daylong exposure
Orthostatic intolerance symptoms assessment during daylong exposure to indoor temperature limit.	Cumulative sum of scores on 6 questions asking participant to rank symptoms associated with orthostatic intolerance during the lying to standing tests. All symptoms scored on a scale from 0 (none) to 10 (worst possible) and include feelings of: (1) "dizziness, lightheadedness, feeling faint, or feeling like you might black out"; (2) "Problems with vision (blurring, seeing spots, tunnel vision, etc.)"; (3) "Weakness"; (4) "Fatigue"; (5) "Trouble concentrating"; and (6) "Head and neck discomfort".	At end of 8 hour daylong exposure
Feel good scale during daylong exposure to indoor temperature limit.	Feel good assessed via a visual analog scale ("How good do you feel?") ranging from "very good" to "very bad"(midpoint: neutral).	At the start (hour 0) and end of 8 hour daylong exposure
Perceived exertion scale during daylong exposure to indoor temperature limit.	Perceived exertional assessed via a visual analog scale ("How hard are you working?") ranging from "no exertional at all" to "maximal exertion".	At the start (hour 0) and end of 8 hour daylong exposure

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Profiles of Mood States (POMS) during daylong exposure to indoor temperature limit.	Potential changes in mood (7 subscales of mood: tension, anger, depression, fatigue, confusion, vigor and esteem related affect). The POMS-40 is a validated, self-administered questionnaire that examines seven distinct aspects of mood state across two positive subscales (Esteem-Related Affect, and Vigor) and five negative subscales (Fatigue, Tension, Confusion, Anger, and Depression), which are described across 40 distinct adjectives. For each individual item, participants were asked to describe "how you feel right now" by responding using a 5-point Likert scale (0 = "Not at all", 1 = "A little", 2 = "Moderately", 3 = "Quite a lot", or 4 = "Extremely"). The values of items associated with a specific subscale (e.g., Fatigue) were summed to calculate its score.	At the start (hour 0) and end of 8 hour daylong exposure
Environmental Symptoms Questionnaire (ESQ) during daylong exposure to indoor temperature limit.	Self-reported environmental reactions and medical symptomatology associated with prolonged heat exposure. The ESQ-IV is a validated 68-item, self-administered questionnaire that has been used successfully in identifying symptomatology during exposure to a wide variety of environmental conditions, including heat exposure [24, 25]. Participants are asked to assess and described "how you have been feeling today" by responding to each item using a 6-point Likert scale (0 = "Not at all", 1 = "Slight", 2 = "Somewhat", 3 = "Moderate", 4 = "Quite a bit", or 5 = "Extreme"). Total Symptom Score was calculated from this data by taking the sum of the intensity ratings from all 68 individual items using reverse scores for the three positive items from the list ("I Felt Good", "I Felt Alert", and "I Felt Wide Awake").	At the start (hour 0) and end of 8 hour daylong exposure
Sleep quality and quantify assessment during daylong exposure to indoor temperature limit.	The Pittsburgh Sleep Quality Index (PSQI) is a self-report questionnaire that works by asking 19 questions about a person's sleep habits over the past month. It is scored by evaluating seven components: subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleeping medication, and daytime dysfunction. Each component is scored from 0 to 3, and these are summed to create a global score from 0 to 21, where a higher score indicates poorer sleep quality.	At the start of 8 hour daylong exposure.

Collaborators and Investigators

• Sponsor: University of Ottawa
• Investigators: Principal Investigator: Glen P Kenny, PhD, University of Ottawa

Study record dates

Study Major Dates

• Study Start (Actual): November 7, 2025
• Primary Completion (Estimated): March 30, 2026
• Study Completion (Estimated): June 13, 2026

Study Registration Dates

• First Submitted: November 21, 2025
• First Submitted That Met QC Criteria: November 21, 2025
• First Posted (Estimated): December 5, 2025

Study Record Updates

• Last Update Posted (Actual): December 11, 2025
• Last Update Submitted That Met QC Criteria: December 4, 2025
• Last Verified: December 1, 2025

More Information

Terms related to this study

• Keywords: Activities of daily living; Thermoregulation; Hyperthermia; Heat wave; Heat strain; Thermal comfort; Indoor temperatures; Indoor overheating; Heat vulnerability; Clothing insulation
• Additional Relevant MeSH Terms: Wounds and Injuries; Body Temperature Changes; Pathological Conditions, Signs and Symptoms; Signs and Symptoms; Hyperthermia; Heat Stress Disorders
• Other Study ID Numbers: HEPRU-2025-10B

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No