The effect of age and mitigation strategies during hot water immersion on orthostatic intolerance and thermal stress

<p> Hot water immersion improves cardiovascular health and sporting performance, yet its adverse responses are understudied. Thirteen young and 17 middle-aged adults (<em>n</em> = 30) were exposed to 2 × 30 min bouts of whole-body 39°C water immersion. Young adults also completed cooling mitigation strategies in a randomized cross-over design. Orthostatic intolerance and selected physiological, perceptual, postural and cognitive responses were assessed. Orthostatic hypotension occurred in 94% of middle-aged adults and 77% of young adults. Young adults exhibited greater dizziness upon standing (young subjects, 3 out of 10 arbitrary units (AU) vs. middle-aged subjects, 2 out of 10 AU), with four terminating the protocol early owing to dizziness or discomfort. Despite middle-aged adults being largely asymptomatic, both age groups had transient impairments in postural sway after immersion (<em>P</em> < 0.05), but no change in cognitive function (<em>P</em> = 0.58). Middle-aged adults reported lower thermal sensation, higher thermal comfort, and higher basic affect than young adults (all <em>P</em> < 0.01). Cooling mitigation trials had 100% completion rates, with improvements in sit-to-stand dizziness (<em>P</em> < 0.01, arms in, 3 out of 10 AU vs. arms out, 2 out of 10 AU vs. fan, 4 out 10 AU), lower thermal sensation (<em>P</em> = 0.04), higher thermal comfort (<em>P</em> < 0.01) and higher basic affect (<em>P</em> = 0.02). Middle-aged adults were predominantly asymptomatic, and cooling strategies prevented severe dizziness and thermal intolerance in younger adults. </p>