Anyway, here goes.
RulesI track environmental damage using a made-up mechanic called "Exposure Levels". Every time your character gains an exposure level, I hand you a poker chip. When you get too many poker chips, you're dead.
Here's the key that makes this tense: you don't know how many poker chips you can take before the condition kills your character. Maybe two chips. Maybe five chips. Maybe thirteen chips. Who knows?
Before the game, I prepare a sheet like this. It tells me how many exposure levels you can take, and what kinds of bad things happen to you when I hand you a poker chip. It also tells me how these exposure levels can be removed. As you might have guessed, it's mostly just stuff copied off the internet.
Hypothermia in this example is a three-stage condition. When I give you one poker chip you're shivering and just can't get warm. When I give you a second poker chip, you're violently shivering and stumbling around like a drunkard. When I give you a third poker chip I'll probably roll to decide what other bad effects happen to you: maybe you'll feel warm and want to strip down for the beach, or maybe you'll go into a fetal position--who knows? And of course when I give you a fourth poker chip you're dead, although--fun fact!--your brain will live on a little bit longer due to decreased cellular blah blah blah.
Got it? Great! Now go out there and explore the great white north!
Who's influencing me at the moment: Wikipedia, Telecanter