Scarey! I've only had hives once in my life and it was triggered by heat. I'd been in a sauna for I think the second time in my life and later that night I started itching around my knees, elbows and on the tops of my feet. I didn't really think anything about it at the time, but the next morning my face! was swollen - enough that it was extremely noticable, but not affecting my breathing. Plus all the areas that had been itchy the night before had nice itchy red hives on them..Needless to say I have not gone back in a sauna since then.