Lab fires don’t happen as often as you might think, at least to hear the way organic chemists talk. We all have alarming stories of alarming reactions (often set up by some rather alarming labmates), but these things are harvested over a fairly broad range of experience. It’s a familiar enough topic that I can remember someone sitting down at lunch while we were swapping lab stories and saying “Oh, this conversation. . .”
But happen they do, and it’s always worth taking a couple of minutes to think about what you do in such a situation. That depends on the fire, of course. For starters, a small one burning out of the neck of a flask can be put out quickly just by slapping a beaker over the top of it. Never neglect that possibility, because it’s fast, effective, and (truth be told) if no one saw you do it, no one necessarily has to know that your (minor!) fire even happened.
Larger ones aren’t going to be so easy, but there are some potential ways out of those, too. My wife had a labmate in her molecular biology department who was always setting off blazes with the ethanol she used to wipe things down with. (This person neglected to turn off the Fisher burner used for sterilizing wire loops, etc., before she started sloshing the alcohol around). A fire like that will just burn itself out if you close the hood sash and let it rip for a few seconds, as long as you’re sure that there’s no fuel source (like the wash bottle of ethanol you might have chucked in there in a moment of panic, for example).
Most chemistry hoods, though, have all too many sources of fuel in them, so you probably won’t be able to put out a blaze through benign neglect. If it comes to a fire extinguisher, make sure you already know where the nearest one is, for starters. You'd be surprised how hard it is to find one of the darn things when you really need it. And once you've found it, make sure that you know which kind you’re using. The carbon dioxide ones don’t make the horrible mess that the dry-chem ones do, which is one thing in their favor, although I think in general they’re a bit less effective. You can tell the difference immediately – the carbon dioxide ones have the big nozzle on them, while dry-chem is a short, plain hose. My lab is outfitted with the latter, which makes me wish more than ever that we never have to use them.
And if you happen to have halon extinguishers (are those still around?), make a note of that, because the technique you may have learned for using the other ones won’t work. Instead of coming in and aiming at the base of the fire, with halon you have to stand further back and let the stuff shower down on it. A colleague of mine once blew the contents of a flaming oil bath all over the lab because he hadn’t been trained in that distinction.
The safety people always tell you that if you’ve used up one extinguisher and the fire still isn’t out, to head for the door rather than reach for a second one. That’s probably good advice (although I’ve seen it disregarded), and I’d advise you to take it. Actually, I’d advise you never to have that decision to make at all, but that’s not always up to you. You may be doing nothing but adding sodium sulfate to a bunch of dichloromethane today, but who knows? The guys next door might be gearing up for Trimethylaluminum Fiesta Days. You never can tell.