Constant sweat and heat can lead to severe dehydration inside of a steam room or even a regular sauna.
You should keep a ceramic double-wall water bottle by your side whenever you stay inside of a steam sauna, and drink regularly.
Staying inside a steam room for 30 minutes can make you lose as much as 5 lbs worth of water weight. You should immediately replace this by drinking water inside and as soon you step out of the sauna.