Written by Jessica Mee
Passive heat interventions (PHIs), such as sauna bathing, hot water immersion, environmental chamber exposure and water-perfused suits, are increasingly used as non-exercise strategies to improve heat tolerance. They can reduce physiological strain, enhance cardiovascular function, and improve tolerance to hot environments when exercise-based acclimation is not feasible. But a key question remains largely unanswered:
Do the people studied in this literature represent the populations who will use these strategies?
Our recent review, published in Experimental Physiology, brings together a methodological audit of PHI research with a systematic review of physiological, perceptual and performance adaptations to passive heat exposure. The aim was twofold: to understand what PHIs do and who they have been studied in.
PHIs can improve heat tolerance—but evidence is uneven
Across 63 studies and more than 1,300 participants, PHIs were consistently associated with meaningful adaptations in heat-related physiology, including:
- Lower core temperature (up to ~0.5°C reductions in some protocols)
- Reduced heart rate (up to ~11 beats per minute in some interventions)
- Modest reductions in blood pressure
- Increases in plasma volume (ranging widely depending on modality)
- Changes in sweat rate and thermal strain
Collectively, these changes reflect improved cardiovascular efficiency and thermoregulatory function, key components of heat adaptation.
In simple terms: Passive heat interventions do appear to work, at least under certain conditions.
However, the strength of that conclusion depends heavily on who those data come from.
A striking imbalance in who is studied
One of the clearest findings from the audit was the imbalance in participant representation.
Across all studies:
- 31% of participants were female
- Only 6% of studies were female-only
- No study was designed specifically to compare male and female responses
This matters because heat regulation is not identical across the population, and sex-related physiological differences, particularly those linked to ovarian hormones, can meaningfully influence thermoregulatory responses.
Yet most current heat adaptation guidelines are still based largely on male-derived data.
Menstrual status reporting: a major blind spot
Perhaps the most important methodological gap identified was the lack of menstrual status reporting.
Among studies including female participants:
- Over half provided no usable menstrual status information
- No study achieved “gold standard” classification of menstrual status
- Only a small proportion included even basic categorisation
This limits interpretation, because hormonal fluctuations can influence core temperature, sweat responses, and cardiovascular function—changes that are often similar in magnitude to the adaptations being studied.
Without this information, it becomes difficult to determine whether observed “training effects” reflect heat adaptation, hormonal state, or both.
The bigger issue: we are often studying a narrow population
Beyond sex and hormonal status, the review highlighted broader gaps:
- Most participants were recreationally active or sedentary
- Very few highly trained or elite athletes were included
- Occupationally heat-exposed groups were largely absent
- Ethnicity and occupation were rarely reported
In other words, much of what we know about PHIs comes from relatively narrow and poorly characterised samples.
This raises an important limitation:
We may understand how PHIs work in controlled laboratory populations, but far less about how they translate to real-world, diverse populations exposed to heat stress.
Why this matters beyond sport science
PHIs are increasingly being discussed not only in sport, but also in:
- Clinical populations
- Occupational heat exposure settings
- Public health and climate resilience strategies
This makes representativeness critical.
If evidence is derived primarily from young, male, recreationally active participants, then applying it to:
- older adults,
- women across the reproductive lifespan,
- clinical populations,
- or outdoor workers
becomes scientifically uncertain.
What the evidence does support
Despite these limitations, one consistent message emerges: Passive heat interventions can induce meaningful physiological adaptations.
Across modalities such as sauna, hot water immersion, and environmental chambers, PHIs appear capable of improving key markers of heat tolerance.
However, these effects are best understood as: real but not yet fully generalisable
Where the field needs to go next
To strengthen both scientific understanding and real-world application, several priorities emerge:
1. Better participant reporting
At minimum, studies should consistently report:
- Sex
- Age
- Training status
- Health status
- Menstrual status (where relevant)
2. Sex-informed study design
When the aim is to understand differences, studies should be designed—not just analysed—to test them.
3. Improved inclusion of female participants
Not as a “subgroup”, but as a standard population within study design.
4. Broader and more applied populations
Including:
- Occupationally heat-exposed workers
- Highly trained athletes
- Older adults and perimenopausal populations
5. Standardisation where possible
Clearer protocols would help establish dose–response relationships across different PHI modalities.
Final thoughts
Passive heat interventions are a promising and flexible tool for improving heat tolerance. They are relatively accessible, adaptable across populations, and show consistent physiological effects.
But the field is still developing.
At present, the strongest conclusion we can draw is not simply that PHIs “work”, but that:
They appear to work under specific conditions in specific populations—but we still need to understand how broadly that evidence applies.
Improving inclusivity, reporting standards, and study design will be essential if PHIs are to move from promising laboratory findings to reliable tools for health, performance, and heat resilience in the real world.