Good topic...hopefully someone can add some solid research & info.
I've been dabbling with the steamroom after my workout. I take a couple spin-classes, run on the treadmill and finish-off with a mile in the pool. The chlorine odor permeates the skin, so I thought to try sweating it out in the steam room (or sauna).
Results & observations:
When in the steamroom, my HR is at about 125-130 and I don't quite understand why. After I'm done with my swimming (ending 3.5 hours of cardio) my HR will drop to about 95-100 (it lingers high on account of the prolonged cardio intensity)....but enter the steamroom or sauna, and up it goes to 125-130: why????
Also, in a book I read that 15 minutes in the steam room burns as many calories as running a mile: here again I don't quite follow why? We burn calories when we run because we are exerting ourselves and using energy to fuel our movement...so why would sitting motionless in a steamroom give rise to burning calories as such? We sweat in an effort to cool ourselves, but beyond that I'm under the impression we're just losing water, some toxins, relaxing the muscles, etc.
Btw...hot-tubs, if properly chlorinated, should be quite safe....about the only thing that and survive about 3 days in chlorinated water is crypto, which would just give you loose bowels if you somehow get that water in your mouth. As for a sauna or steamroom: tell me how warm & moist air isn't a welcome haven for bacteria/viruses and what-not?