On the link provided by Piff above, this is what they have also to say about the sweat lodge:
What is a Sweat Lodge made up?
The lodge in former times was covered with the hides of buffalo, bear or moose. In this day, the animal skins have been replaced with blankets, plastic sheeting, old carpet, heavy gauge canvas sheets and tarps to retain the heat and the steam.
There are several styles of structures used in different cultures; these include a domed or oblong hut similar to a wickiup, a permanent structure made of wood or stone, or even a simple hole dug into the ground and covered with planks or tree trunks. Stones are typically heated and then water poured over them to create steam. In ceremonial usage, these ritual actions are accompanied by traditional prayers and songs.
(Bolding Mine)
http://msi-healing.com/blog/what-is-a-sweat-lodge-ceremonyPlastic sheeting, old carpet??? These are not natural materials. Most carpet is made from polyester or nylon, I think, and there is latex backing many times.
BTW, that second paragraph came directly from Wiki:
There are several styles of structures used in different cultures; these include a domed or oblong hut similar to a wickiup, a permanent structure made of wood and earth, or even a simple hole dug into the ground and covered with planks or tree trunks. Stones are typically heated and then water poured over them to create steam. In ceremonial usage, these ritual actions are accompanied by traditional prayers and songs.
http://en.wikipedia.org/wiki/Sweat_lodge
