I walked up to the flows on Fagradalsfjall when it was erupting a couple of years ago, and I found the cinereous, sulfurous air to be very medicinal and clearing. I'm not sure it'd have good for me for more than a few hours, but as it was, it was great. I occasionally wish I were able to just have a chamber with that air in it.

There are some saunas on Iceland that expose you to earth gasses, might be exactly the kind of chamber you are after. I've visited one, and it was unfortunately cold for a sauna because that's naturally varying too.