An interventional study of Humidity-Ramp Protocol and Above-inflection fixed-condition exposure (5%) in Heat Stress, Temperature Change, Body and Aging, sponsored by University of Ottawa. Completed at 2 sites in Canada. Open to female participants aged 65 Years to 85 Years, including healthy volunteers. Per ClinicalTrials.gov, last updated 2026-06-11.
Sponsored by University of Ottawa · Not applicable, Interventional, and Basic science
Climate change increases extreme heat events, elevating global heat-illness risk. Females have reduced heat loss capacity (\~5%) compared to males, driven by differences in skin blood flow and sweating responses. While findings on sex-mediated mortality are mixed, some studies suggest older females (≥65 years), face higher heat-related mortality/morbidity risks, evidenced by disproportionate female deaths in the 2021 Western Heat Dome. The effects of extreme uncompensable heat on older females remain understudied.
Heat exposure initially causes net heat gain, raising core/skin temperatures and triggering heat-loss responses. Under compensable heat stress, heat loss balances gain, stabilizing core temperature. Uncompensable heat stress (exceeding maximal dissipation capacity) causes continuous core temperature rise, posing severe health risks. The specific temperature and relative humidity (RH) limits where compensability is lost are critical survival determinants, influenced by age and sex.
Ramping protocols identify these limits: participants face progressively increasing heat stress (e.g., staged humidity rises) while core temperature is monitored. Core temperature typically stabilizes initially, then exhibits an abrupt rapid increase at an inflection point, operationally defined as the limit of compensability. Despite increasing use, ramping protocol validity for accurately identifying this threshold remains unverified.
This project assesses ramping protocol validity for determining uncompensable conditions in older females and evaluates cumulative thermal and cardiovascular strain, as well as psychological and cognitive responses to both uncompensable and compensable heat. Participants will complete five trials. Trial 1 (Ramping): Rest at 42°C, 28% RH for 70min, then incremental RH increases (3% every 10min) to 70% RH. Individual core temperature (rectal) inflection points are identified from the ramping trial. Trials 2-5 (Fixed Conditions, Randomized): i) \~10% below inflection; ii) \~5% below inflection; iii) \~5% above inflection; iv) Thermo-neutral control (26°C, 45% RH). Comparing the rate of rectal temperature change and cumulative strain during prolonged fixed exposures (especially below vs. above inflection) will validate if the ramping inflection point represents the true limit of compensability for older females.
118 studies on the registry are indexed under Heat Stress Disorders; 45 are open to participants now.
This study's enrollment of 12 is below the median of 24 across 104 interventional studies indexed under Heat Stress Disorders.
Browse Heat Stress Disorders studies →University of Ottawa is the lead sponsor of 125 studies on the registry; 34 are open to participants now.
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Exclusion Criteria:
Underlined terms in the criteria open these too. Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
Informed consent
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.
Other: Humidity-Ramp Protocol
Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.
Other: Below-inflection fixed-condition exposure (5%)
Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.
Other: Control fixed-condition exposure
Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.
Other: Below-inflection fixed-condition exposure (10%)
Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.
Other: Above-inflection fixed-condition exposure (5%)
Participants are exposed to 42°C and 28% relative humidity for 70 minutes. Thereafter, humidity is increased 3% until an ambient humidity of 70% is achieved. The humidity at which rectal temperature inflect is subsequently determined.
After a 1 hour equilibrium at 42°C and 28% relative humidity, humidity will be increased 3% every 10 min until it is \~5% higher than the participants' rectal esophageal temperature inflection point identified in the humidity ramp protocol. These conditions will be held constant for the remainder of the 9-hour exposure period (starting from the beginning of equilibrium). Tap water will be provided at regular intervals to limit dehydration.
After a 1 hour equilibrium at 42°C and 28% relative humidity, humidity will be increased 3% every 10 min until it is \~5% lower than the participants' individual rectal temperature inflection point identified in the humidity ramp protocol. These conditions will be held constant for the remainder of the 9-hour exposure period (starting from the beginning of equilibrium). Tap water will be provided at regular intervals to limit dehydration.
After a 1 hour equilibrium at 42°C and 28% relative humidity, humidity will be increased 3% every 10 min until it is \~10% lower than the participants' individual rectal temperature inflection point identified in the humidity ramp protocol. These conditions will be held constant for the remainder of the 9-hour exposure period (starting from the beginning of equilibrium). Tap water will be provided at regular intervals to limit dehydration.
Participants are exposed for 9-hours to 28°C with 35% relative humidity. Participant will be allowed to drink tap water ad libitum.
Rectal temperature rate of change
Rate of change of rectal temperature measured over the final 2-hours of heat exposure
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Number of participants unable to finish fixed-condition heat exposure
Number of participants who could not complete each fixed-condition heat exposure due to participant withdrawal or achieving a core temperature of 39.5°C
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Mean skin temperature rate of change
Rate of change of skin temperature measured over the final 2-hours of heat exposure
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Core temperature (AUC)
Area under the curve (AUC) of rectal temperature (in degree-hours) measured over the fixed-condition heat exposure
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Predicted time until 40.2°C esophageal temperature
Estimated time for rectal temperature to reach 40.2°C, a commonly-cited criteria for severe heat-illness (e.g., heat stroke), calculated from the end-exposure rectal temperature and its rate of change (assuming this rate of change is sustained)
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Heart rate rate of change
Rate of change of heart rate measured over the final 2-hours of heat exposure
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Heart rate (AUC)
Area under the curve (AUC) of heart rate measured over the fixed-condition heat exposure
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Thermal sensation
Self-reported thermal sensation at the end of the fixed-condition exposures using and 8-point scale ranging from neutral (0) to extremely hot (8).
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Thermal comfort
Thermal comfort assessed via a visual analog scale ranging from extremely uncomfortable to extremely comfortable (midpoint: neutral).
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Sweat rate
Sweat rate at the end of the fixed-condition exposures (calculated via change in body weight)
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Net fluid loss
Net fluid loss estimated as the percentage change in body mass over the fixed-condition exposures.
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Profiles of Mood States (POMS)
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 (reference). 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.
Time frame: Prior to (hour 0) and at the end of fixed-condition heat exposure (hour 9 or termination)
Environmental Symptoms Questionnaire (ESQ)
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").
Time frame: Prior to (hour 0) and at the end of fixed-condition heat exposure (hour 9 or termination)
Rectal temperature inflection point
Inflection point for rectal temperature during the humidity-ramp protocol
Time frame: During the ramp protocol (up to 2.5 hours)
Heart rate inflection point
Inflection point for heart rate during the humidity-ramp protocol
Time frame: During the ramp protocol (up to 2.5 hours)
Plan to share: Yes — De-identified participant data will be made available upon reasonable request and signed access agreement.
Supporting information: Study protocol, Statistical analysis plan, Analytic code
Time frame: Data will be made available following publication of the primary study report.
Access criteria: Reasonable request and signed access agreement
No publications or documents are linked to this record.
Ran from Jul 2025 to primary completion in May 2026, 9 months.
Of 75 non-industry interventional studies with no phase under Heat Stress Disorders, half reached primary completion within 16 months of starting (the median), counting those still running; a quarter within 4 and three quarters within 27. This study's 9 months is below that.
Counting every sponsor, half of 75 studies reached it within 16 months; a quarter within 4 and three quarters within 27.
How these figures are worked out, and which studies are left out of them.
This study is completed, as verified in Jun 2026. You cannot join it, but the record below documents what was studied.
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