Portable steam sauna - are they safe?

I have found one online that is made of a waterproof cotton as the inside material and polyester as the outside material. I am thinking about getting one, however, my concern is: won't the heat release chemical substances from the cotton and polyester? (nowadays a lot of chemicals are used in a textile production)
Or maybe the surface of material used in a portable sauna is so small that there is nothing to worry about?

I have asked today at the univeristy's textile institute in my city and they were very nice and answered my questions. :) The response was that cotton and polyester should not be a problem. However, they could not comment the safety of the substance used to waterproof the cotton since I didn't provide the info what kind of the substance it is. As you say, Garden Peace.
I don't want FIR sauna, because I don't know if regular exposition to infrared used in these saunas is safe.