Identification of critical thermal environments for aged adults
Validating the liveability thresholds for a range of ambient temperatures and activity levels in older adults
The University of Sydney
36 participants
Nov 20, 2025
Interventional
Conditions
Summary
The human body releases heat by sweating and increasing blood flow to the skin. However, when the temperature gets too hot, or the humidity too great, these processes become impaired. This causes heat to be stored in the body, and the body temperature to increase. Understanding the specific environmental conditions when this occurs is very important to prescribe advice regarding upper environmental limits for older adults engaging in physical activity in the heat. The objective of this study is to determine the ambient temperature and humidity at which age-related impairments can result in greater risk of heat illness. To determine these limits, participants will perform light intensity exercise in various environmental conditions and body temperature will be monitored while the humidity is increased throughout the trial. These data will be used to derive upper environmental limits for older men and women exercising in the heat.
Eligibility
Inclusion Criteria6
- -85 years old
- Generally healthy
- Body mass index less than 30 kg/m2
- Non-smokers or ex-smokers (at least 2 years since quitting)
- Able to read and understand English in order to provide informed consent to participate in the study
- women must not be taking hormone replacement therapy
Exclusion Criteria10
- Any contraindications to low intensity physical activity based on the Physical Activity Questionnaire
- Mobility restrictions
- Have a prior diagnosis of cancer, cardiovascular disease, diabetes, or metabolic syndrome
- Are currently participating in another study involving physical activity training or weight loss
- Are planning to have surgery within the next year
- Are taking any medications that could alter cardiovascular or thermoregulatory responses to exercise
- Tobacco use
- Illegal/recreational drug use
- Women who are takin hormone replacement therapy
- Contraindications to maximal exercise test or abnormal resting or exercise electrocardiogram.
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Interventions
Participants will complete 5–9 experimental sessions (~3 hours each) at the Thermal Ergonomics Laboratory, University of Sydney. During each session, participants will perform light-intensity cycling exercise corresponding to 2, 3, or 4 metabolic equivalents (METs). Exercise intensity (METs) and ambient temperature (set between 34°C and 43°C) will remain constant within each session, according to the assigned experimental condition. Relative humidity (RH) will be progressively increased at a rate of approximately 1% per minute. This increase will continue until an inflection point in core temperature is detected or until participants complete 2 hours of cycling in the heat, whichever occurs first. Each session will last approximately 3 hours, consisting of 2 hours of cycling exercise and additional time dedicated to preparation, instrumentation, baseline measurements (10 min), and post-exercise recovery. Exposure levels (combinations of MET and temperature) will be predefined, and participants will complete a series of five or nine preset environmental conditions in a randomized order. The preset environmental conditions are as follows: 1. 34°C and 2 METs (control condition) 2. 37°C and 2 METs 3. 40°C and 2 METs 4. 43°C and 2 METs 5. 34°C and 3 METs 6. 37°C and 3 METs 7. 40°C and 3 METs 8. 34°C and 4 METs 9. 37°C and 7 METs Each experimental session will be separated by at least 72 hours. To support adherence, participants will be instructed to abstain from alcohol for at least 24 hours and from caffeine for 12 hours before each session, and to consume a light meal and approximately 0.5 L of water beforehand. Adherence will be assessed via self-report at the start of each session.
Locations(1)
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ACTRN12626000095369