Influence of Indoor Humidity on Physiological Strain in Older Adults During a Simulated Heat Wave

NCT06842953 | Recruiting

• 1 other identifier: HEPRU-2025-02-A
• Study Type: interventional
• Target: 10
• Locations: 1 country
• Sites: 1

Brief Summary

Laboratory-based studies show that exposure to high humidity can worsen the effects of heat stress in young and older adults by impeding sweat evaporation - the main mechanism by which the human body cools itself. At high levels of humidity, the efficiency of sweating decreases causing a greater rise core temperature and burden on the cardiovascular system. In this context, increasing temperatures and humidity with climate change thus pose a potential compound risk for human health. While humidity's role in heat-health outcomes could substantially alter projections of health burdens from climate change, the impact of humidity on physiological strain in vulnerable people in relation to the indoor environment has yet to be evaluated. In a recent study delineating the physiological effects of the proposed 26°C indoor upper limit (PMID: 38329752), relative humidity was set to 45% in all conditions based on indoor humidity standards by the American Society of Heating and Air-Conditioning Engineers. However, it is unknown whether a refinement of the recommended indoor temperature limit of 26°C is required in situations where humidity cannot be maintained at this level. On separate occasions, the investigators will assess the change in body temperature and cardiovascular strain in older adults (65-85 years) exposed for 10 hours at the recommended indoor temperature limit of 26°C and 45% relative humidity (equivalent humidex of 29 (considered comfortable)) (experimental condition A), to 26°C with a relative humidity of 15% (equivalent humidex of 23 (considered comfortable); humidex is used to measure the perceived temperature taking into account the humidity)) (experimental condition B), to 26°C with a relative humidity of 85% (equivalent humidex of 37 (considered somewhat uncomfortable)) (experimental condition C), and to 31°C and 45% relative humidity with an equivalent humidex of 37 (considered somewhat uncomfortable) that is similar to experimental condition C. With this experimental design, investigators will assess the effects of indoor humidity in driving human heat strain and identify whether refinements in the recommended 26°C indoor temperature limit may be required. Further, by evaluating changes in relation to ambient conditions with a similar humidex, the investigators can assess how individuals perceive and respond to both heat and humidity.

Conditions

Heat Stress; Physiological Stress

Keywords

Heat wave; Indoor overheating; Thermoregulation; Heat strain; Elderly; Heat vulnerability; Hyperthermia; Thermal comfort

Outcome Measures

Primary Outcomes (15)

• Core temperature (Peak) during daylong exposure to indoor overheating

  Peak rectal temperature (15 min average) during exposure. Rectal temperature is measured continuously throughout the 10 hours of the simulated indoor overheating.

  End of 10 hour daylong exposure

• Core temperature (AUC) during daylong exposure to indoor overheating

  Areas under the curve (AUC) of rectal temperature during the 10 hour simulate indoor overheating.

  End of 10 hour daylong exposure

• Heat rate (Peak) during daylong exposure to indoor overheating

  Peak heart rate (15 min average) during exposure. Heart rate is measured continuously throughout the 10 hours of the simulated indoor overheating.

  End of 10 hour daylong exposure

• Heart rate (AUC) during daylong exposure to indoor overheating

  Areas under the curve (AUC) of heart rate during the 10 hour simulate indoor overheating.

  End of 10 hour daylong exposure

• Cardiac response to standing from supine (30:15 Ratio) during daylong exposure to indoor overheating

  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 10 hour daylong exposure

• Systolic Response to Standing From Supine during daylong exposure to indoor overheating

  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 10 hour daylong exposure

• Heart rate variability: RMSSD during daylong exposure to indoor overheating

  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 10 hour daylong exposure

• Heart rate variability: SDNN during daylong exposure to indoor overheating

  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 10 hour daylong exposure

• Thermal comfort scale during daylong exposure to indoor overheating

  Thermal comfort assessed via a visual analog scale ranging from extremely uncomfortable to extremely comfortable (midpoint: neutral).

  At the start (hour 0) and end of 10 hour daylong exposure

• Thirst sensation scale during daylong exposure to indoor overheating

  Thermal sensation assessed via a visual analog scale ranging from extremely hot to neutral (midpoint: hot).

  At the start (hour 0) and end of 10 hour daylong exposure

• Arousal scale during daylong exposure to indoor overheating

  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 10 hour daylong exposure

• Fluid loss during daylong exposure to indoor overheating

  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).

  At end of 10 hour daylong exposure

• Changes in plasma volume during daylong exposure to indoor overheating

  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

  At end of 10 hour daylong exposur

• Systolic blood pressure during daylong exposure to indoor overheating

  Systolic blood pressure measured in triplicate via automated oscillometry (\~60 seconds between measures)

  At the start (hour 0) and end of 10 hour daylong exposure

• Rate pressure product during daylong exposure to indoor overheating

  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 10 hour daylong exposure

Secondary Outcomes (8)

• Impulse control (cognitive function) during daylong exposure to indoor overheating

  At the start (hour 0) and end of 10 hour daylong exposure

• Memory recall (cognitive function) during daylong exposure to indoor overheating

  At the start (hour 0) and end of 10 hour daylong exposure

• CDC 4-Stage Balance Test (Postural stability) during daylong exposure to indoor overheating

  At the start (hour 0) and end of 10 hour daylong exposure

• BTrackS Balance Assessment (Postural stability) during daylong exposure to indoor overheating

  At the start (hour 0) and end of 10 hour daylong exposure

• Profiles of Mood States (POMS) during daylong exposure to indoor overheating

  At the start (hour 0) and end of 10 hour daylong exposure

Study Arms (4)

Exposure to recommended indoor temperature limit with normal indoor humidity

EXPERIMENTAL

Participants exposed daylong (10 hours) to an indoor temperature maintained at 26°C and 45% relative humidity (humidex equivalent of 29).

Other: Simulated indoor overheating

Exposure to recommended indoor temperature limit with low indoor humidity

EXPERIMENTAL

Participants exposed daylong (10 hours) to an indoor temperature maintained at 26°C and 15% relative humidity (humidex equivalent of 23).

Other: Simulated indoor overheating

Exposure to recommended indoor temperature limit with high indoor humidity

EXPERIMENTAL

Participants exposed daylong (10 hours) to an indoor temperature maintained at 26°C and 85% relative humidity (humidex equivalent of 37).

Other: Simulated indoor overheating

Exposure to high indoor temperature with normal indoor humidity

EXPERIMENTAL

Participants exposed daylong (10 hours) to an indoor temperature maintained at 31°C and 45% relative humidity (humidex equivalent of 37).

Other: Simulated indoor overheating

Interventions

Simulated indoor overheating OTHER

Older adults are exposed to a 10-hour simulated exposure

Exposure to high indoor temperature with normal indoor humidity; Exposure to recommended indoor temperature limit with high indoor humidity; Exposure to recommended indoor temperature limit with low indoor humidity; Exposure to recommended indoor temperature limit with normal indoor humidity

Eligibility Criteria

• Age: 60 Years - 85 Years
• Sex: all
• Healthy Volunteers: Yes
• Age Groups: Adult (18-64), Older Adult (65+)

You may qualify if:

• Non-smoking.
• English or French speaking.
• Ability to provide informed consent.
• with or without a) chronic hypertension (elevated resting blood pressure; as defined by Heart and Stroke Canada and Hypertension Canada), b) type 2 diabetes as defined by Diabetes Canada, with at least 5 years having elapsed since time of diagnosis

You may not qualify if:

• Episode(s) of severe hypoglycemia (requiring the assistance of another person) within the previous year, or inability to sense hypoglycemia (hypoglycemia unawareness).
• Serious complications related to diabetes (gastroparesis, renal disease, uncontrolled hypertension, severe autonomic neuropathy).
• Uncontrolled hypertension - BP \>150 mmHg systolic or \>95 mmHg diastolic in a sitting position.
• Restrictions in physical activity due to disease (e.g. intermittent claudication, renal impairment, active proliferative retinopathy, unstable cardiac or pulmonary disease, disabling stroke, severe arthritis, etc.).
• Use of or changes in medication judged by the patient or investigators to make participation in this study inadvisable.
• Cardiac abnormalities identified during screening

Sponsors & Collaborators

• University of Ottawa: lead

Study Sites (1)

University of Ottawa

Ottawa, Ontario, K1N6N5, 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

Central Study Contacts

Glen P Kenny, PhD

CONTACT

6135625800; gkenny@uottawa.ca

Study Design

• Study Type: interventional
• Phase: not applicable
• Allocation: RANDOMIZED
• Masking: SINGLE
• Who Masked: OUTCOMES ASSESSOR
• Purpose: BASIC SCIENCE
• Intervention Model: CROSSOVER
• Sponsor Type: OTHER
• Responsible Party: PRINCIPAL INVESTIGATOR
• PI Title: Full Professor

Study Record Dates

• First Submitted: February 17, 2025
• First Posted: February 24, 2025
• Study Start: March 14, 2025
• Primary Completion: March 31, 2026
• Study Completion (Estimated): September 1, 2026
• Last Updated: June 15, 2025

Record last verified: 2025-06

Locations

CA (1)