I wore a gas mask for 4 hours last October. All I was doing was sitting on a couch watching TV (except for the first hour, when I tried to type out an e-mail, but got frustrated with the gloves). When done, I literally poured my sweat out of the mask. And it wasn't even hot, maybe 80 degrees and high humidity. Not 120-140 degrees.
I swear I won’t complain about our TX heat ever again!!
Wellllll, for this year at least... ;-)