Heat therapy: strategies to enhance thermal comfort without compromising benefits

<p dir="ltr">Regular passive exposure to heat, using methods like sauna bathing or hot water immersion, has been linked to a reduced risk of cardiovascular disease (Laukkanen et al., <a href="https://physoc.onlinelibrary.wiley.com/doi/10.1113/EP093030#eph70008-bib-0005" target="_blank">2015</a>; Ukai et al., <a href="https://physoc.onlinelibrary.wiley.com/doi/10.1113/EP093030#eph70008-bib-0012" target="_blank">2020</a>). Whilst the epidemiological evidence is less robust than for physical activity interventions, both heat therapy and exercise hence represent non-pharmacological tools with the capacity to benefit health. Especially older laboratory studies investigating the effects of passive exposure to heat often employed comparably high temperatures (resulting in a ≥1.5°C core temperature increase) and/or prolonged exposure times (≥60 min) (to cite two particularly taxing ones: Laing et al., <a href="https://physoc.onlinelibrary.wiley.com/doi/10.1113/EP093030#eph70008-bib-0004" target="_blank">2008</a>; Oehler et al., <a href="https://physoc.onlinelibrary.wiley.com/doi/10.1113/EP093030#eph70008-bib-0007" target="_blank">2001</a>). Such intense protocols can lead to changes in health-related physiological outcomes. Acute increases in pro- and anti-inflammatory cytokine concentration, blood flow and shear stress have been observed, alongside long-term improvements in the resting inflammatory profile, glycaemic control, vascular function and blood pressure (Hoekstra et al., <a href="https://physoc.onlinelibrary.wiley.com/doi/10.1113/EP093030#eph70008-bib-0002" target="_blank">2020</a>; Pizzey et al., <a href="https://physoc.onlinelibrary.wiley.com/doi/10.1113/EP093030#eph70008-bib-0008" target="_blank">2021</a>). Subjective participant feedback regarding intervention tolerability is not available for most older investigations, and one might think that the idea of relaxing in a hot bath, rather than sweating it out in the gym, may be appealing to many. However, the experience of whole-body passive heating can in fact be taxing and uncomfortable. Indeed, more recent investigations, assessing the associated thermal comfort for the historically often-used ∼60-min protocol to raise core temperature by ∼1–2°C, report participant feedback such as ‘it was intensely hot’, or worse – ‘too hot’, ‘too uncomfortable’ and ‘dreadful’ (Mansfield et al., <a href="https://physoc.onlinelibrary.wiley.com/doi/10.1113/EP093030#eph70008-bib-0006" target="_blank">2021</a>; Su, Hoekstra, Leicht, <a href="https://physoc.onlinelibrary.wiley.com/doi/10.1113/EP093030#eph70008-bib-0010" target="_blank">2024</a>). (Cont.)</p>