How are you suffering equal heat stress from being submerged in moderately warm water and breathing very hot air? I could imagine quite different effects on airways and skin, for example. "Exactly the same effect" seems like the unexpected outcome here.
> intense heat exposure is a lot more agonising than 30 minutes of exercise for less benefit
Having to do absolutely nothing other than not leaving is quite different from pushing through a physical activity that can also easily be causing all kinds of discomfort.
It's all about raising your core temperature, water transfers heat to the body much more efficiently than air, so water at 104F ends up raising your body core temperature as much as a dry sauna at 170F. I did some experimentation on this, I have access to a dry sauna at my gym and I track my HR and exertion levels, I did the same with the hot tub at home making sure the water temperature doesn't go below 104F and im fully submerged to the neck, 30 mins session in both cases. The graphs look pretty much identical, same HR uptrends. So as far as cardio effects and heat shock proteins I do believe they are the same, not sure if there could be any benefit to breathing dry hot air for the lungs, but so far most benefits from sauna come from raising core temp
Too lazy to find it, but Dr Rhonda Patrick (a longtime advocate for saunas for their health benefits) reported that hot tubs can provide the same results as saunas -- and they are much more pleasant to use.
Not to beat my own dead horse but at the heat stress needed to cause an adaptation there’s nothing pleasant about the experience. If it’s not causing nausea and palpitations, it’s not hot enough.
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> How are you suffering equal heat stress from being submerged in moderately warm water
by the rules of this universe, you can't survive being submerged in 40C water for a prolonged period of time (even 37C would kill you as well), because humans produce heat and if you can't dispose of it you'll overheat and be dead soon enough
Have you tried submerging yourself in moderately hot water, I wonder? And have you spent some time pondering the difference in heat transfer between convection and conduction?