Back when I mixed in the clubs I grew to detest fog machines. They don't use "fog" they use esters that particulate at low temperatures. More accurately, they're "smoggers" (that smell like bad suntan lotion because the cheapest "flavor" is "coconut"). I did some work on using liquid nitrogen because (A) it makes a nice clean fog that (B) dissipates quickly and also (C) cools down the club quite effectively, which is handy because those places are generally the Devil's armpit but at the time, nitrogen toxicity was tough to map, calculating airchanges per hour in a club environment is a nightmare and as you said, there's no warning so I decided there was no real mechanism to prevent a Lake Nyos situation. Not two years later? Some outfit in Florida started doing installs. They made all the magazines. I wrote 'em to see how they'd gotten around the toxicity problem. They never wrote back. They did uninstall all their systems a month later.