Identifying the Limits of Survivability in Heat-exposed Older Females

Summary

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.

Conditions

• Heat Stress
• Temperature Change, Body
• Aging
• Thermoregulation
• Heat Exposure

Interventions

OTHER

Humidity-Ramp Protocol

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.

OTHER

Above-inflection fixed-condition exposure (5%)

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.

OTHER

Below-inflection fixed-condition exposure (5%)

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.

OTHER

Below-inflection fixed-condition exposure (10%)

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.

OTHER

Control fixed-condition exposure

Participants are exposed for 9-hours to 28°C with 35% relative humidity. Participant will be allowed to drink tap water ad libitum.

Sponsors & Collaborators

• University of Ottawa

  lead OTHER

Principal Investigators

• Glen P Kenny, PhD · University of Ottawa

Study Design

Allocation

RANDOMIZED

Purpose

BASIC_SCIENCE

Masking

DOUBLE

Model

CROSSOVER

Eligibility

Min Age

65 Years

Max Age

85 Years

Sex

FEMALE

Healthy Volunteers

Yes

Timeline & Regulatory

Start

2025-07-16

Primary Completion

2026-01-31

Completion

2026-01-31

Countries

• Canada

Study Locations