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

NCT07267598 | Recruiting

• 1 other identifier: HEPRU-2025-10B
• Study Type: interventional
• Target: 10
• Locations: 1 country
• Sites: 1

Brief Summary

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.

Conditions

Heat Stress; Physiological Stress; Cognitive Change

Keywords

Indoor overheating; Indoor temperatures; Heat wave; Thermoregulation; Heat strain; Heat vulnerability; Hyperthermia; Thermal comfort; Activities of daily living; Clothing insulation

Outcome Measures

Primary Outcomes (33)

• 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

• 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

• 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 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

Secondary Outcomes (3)

• Profiles of Mood States (POMS) during daylong exposure to indoor temperature limit.

  At the start (hour 0) and end of 8 hour daylong exposure

• Environmental Symptoms Questionnaire (ESQ) during daylong exposure to indoor temperature limit.

  At the start (hour 0) and end of 8 hour daylong exposure

• Sleep quality and quantify assessment during daylong exposure to indoor temperature limit.

  At the start of 8 hour daylong exposure.

Study Arms (2)

Exposure to indoor temperature upper limit without physical activity and added clothing.

EXPERIMENTAL

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

Exposure to indoor temperature upper limit with physical activity and added clothing.

EXPERIMENTAL

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

Interventions

Simulated indoor temperature upper limit OTHER

Young adults exposed to an 8-hour simulated exposure.

Exposure to indoor temperature upper limit with physical activity and added clothing.; Exposure to indoor temperature upper limit without physical activity and added clothing.

Eligibility Criteria

• Age: 18 Years - 29 Years
• Sex: all
• Healthy Volunteers: No
• Age Groups: Adult (18-64)

You may qualify if:

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

You may not qualify if:

• 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

Sponsors & Collaborators

• University of Ottawa: lead

Study Sites (1)

University of Ottawa

Ottawa, Ontario, K1N 6N5, Canada

RECRUITING

MeSH Terms

Conditions

Heat Stress Disorders; Hyperthermia

Condition Hierarchy (Ancestors)

Wounds and Injuries; Body Temperature Changes; Signs and Symptoms; Pathological Conditions, Signs and Symptoms

Study Officials

• Glen P Kenny, PhD

  University of Ottawa

  PRINCIPAL INVESTIGATOR

Central Study Contacts

Glen P Kenny, PhD

CONTACT

6133029267; gkenny@uottawa.ca

Study Design

• Study Type: interventional
• Phase: not applicable
• Allocation: RANDOMIZED
• Masking: SINGLE
• Who Masked: PARTICIPANT
• Purpose: BASIC SCIENCE
• Intervention Model: CROSSOVER
• Sponsor Type: OTHER
• Responsible Party: PRINCIPAL INVESTIGATOR
• PI Title: Professor and University Research Chair

Study Record Dates

• First Submitted: November 21, 2025
• First Posted: December 5, 2025
• Study Start: November 7, 2025
• Primary Completion: March 30, 2026
• Study Completion (Estimated): June 13, 2026
• Last Updated: December 11, 2025

Record last verified: 2025-12

Locations

CA (1)