I froze a wart with regular dry ice once.
You can go out with your cheeks exposed at -30 celsius. It’s not especially fun but your skin doesn’t fall off in 20 minutes.
Sigh. Winter is approaching, I’ll get to find this out firsthand once again.
I’m actually giggling as I read Cheese Monster’s post. I’ve gone out in -30 (-40 or less with windchill) with a bare face and hands and not gotten frostbite! 
That’s because air doesn’t transmit temperature very effectively. I can be in a sauna that’s 100 degrees warm for 20 minutes without getting any burns or problems, but if I dip my hand in boiling water for 20 minutes I won’t be so lucky.
You should be able to get frost bite as soon as it’s down to freezing if you stay out long enough. Most people don’t. That’s the reason you can wander around in the winter; in fact I just got back in from -13C (-21C with wind chill). My ears feel a little sore, that’s it. Had I stayed out for 6 hours in the wind I wouldn’t be bragging. Of course it takes longer to get frostbite at -5 than -26…
And if you recall, we are talking about canned air.
Actually, it’s the liquid propellant in the canned air that we’re talking about. Or did I miss hearing a big whoosh as you sprayed some in my direction?
That must have been some serious shrinkage.
Many years ago I used liquid propylene (boils at minus 45, available where I worked) to freeze off a large wart on my thumb. I made a little container from poly styrene to contain the propylene around the wart so not do freeze area outside of the wart. Worked great, the wart came off in few days!
A few years later I heard that doctors were using liquid N2 to do the same thing. N2 is much colder and safer for it is inert.
Umm… nope. I feel really dumb.
This thread makes a lot more sense now. And I’m reminded why I rarely post in GQ.
Sorry for hijacking on an incorrect assumption, guys.