Dangerous Air

I was rereading yesterday's post on HF and issues of safety and was reminded that a lot of the time, the most dangerous situations occur when you let your guard down due to familiarity. Back when I was a graduate student, I taught several freshman chemistry labs. One of my students came by to ask some questions and found me next door in the mass spectrometry lab taking some measurements. At one point during the question and answer session, she asked if she could smoke and I said yes, since there weren't any chemicals in the room.

To make my measurements, I was using a styrofoam cup filled with liquid nitrogen to transfer gas samples between various locations within the mass spectrometer vacuum system. Turns out it was actually liquid air (apparently it's less expensive), as evidenced by the bluish color which the liquid would develop as the N₂ boiled off preferentially, leaving liquid O₂ at the bottom of the cup. I will never understand why that student decided to put out her cigarette into that cup without asking, but she did and the cigarette burst into flame. I push her back out of the way and then turned to watch as the cup itself proceeded to burn fiercely, sending a plume of ashes up to the ceiling where it spread to all corners of the room before settling -- on everything. I spent hours cleaning that lab, but I couldn't clean everything. A month or too later, my advisor noticed the ash, but I was too embarassed to say anything.

In retrospect, there hadn't really been much of a danger, but since that sequence of events had never occurred to me, how can I say that I assessed the hazards properly? How many dangerous situations have any of us been in that we didn't recognize at the time? Chemistry can be a dangerous business, people. Be careful out there!