I messed up °F/°C and it was "only" 250°F. I don't have the book "Space Medicine" available, but I found a different reference† explaining that it was Dr. Charles Blagden and other researchers in 1774 who went into a room heated above 250°F and cooked steak and eggs in there, proving the importance of perspiration in maintaining body temperature. It has to be dry heat; if there is humidity, the tolerable temperature is much lower. The original paper ‡ is also online; in order to read it, note that "ſ" is a "long s", not a "f".

† http://news.google.com/newspapers?nid=1499&dat=19560213&id=4...

‡ http://rstl.royalsocietypublishing.org/content/65/484.full.p...

For people of the world, that's about 121°C. BTW, I've been in a sauna in Sweden where the thermometer displayed 100°C. The women sauna, according to my mother, was only 96°C.

Out of interest, was that the ambient temperature or was the thermometer placed above the source of steam?

They normally keep the thermometer on the opposite wall to the source of heat.

That news.google-link appears to be broken?

I can't edit the expired link above, so search for "hot mutton cool magician" or try http://news.google.com/newspapers?id=4VQaAAAAIBAJ&sjid=ZSUEA...