I once wrote a murder mystery game where the killer committed the act by filling a sealed room with bromine gas. I'm not entirely sure it would work, but once when I was in school the physics class removed itself to a chemistry lab to watch a demonstration on gas mechanics, and we used bromine.
“I have to warn you now,” the teacher said, “that bromine is toxic, although we have a very small quantity here, it can still be harmful, especially to the boys in the room, as it can cause male infertility.” A few teenaged boys shuffled uncomfortably. “So,” the teacher continued, “it is my responsibility to tell you that in case of accident, we must all evacuate the room, okay?”
We nodded, reassured that some safety measure was in place, and that it wasn't anything too drastic.
He then went on to outline the experiment. First, he would expose the bromine gas to an “empty” flask and we would time how long it took to diffuse through and look roughly mixed. We were using bromine, he explained, in spite of its hazards, because it was brown, and thus we could see it. This seemed sensible. In our second experiment, we would expose the bromine to a vacuum, whereupon we would see how fast a gas really travels, when it is unencumbered by collisions with other gas molecules!
We completed the first experiment. Obviously it was the less thrilling of the two, and we looked forward to the second, where the gas would shoot through the flask with the promised velocity and we could ooh and ahh appreciably before going back to the physics classroom and doing the maths.
He prepared the second experiment. He opened the valve between the bromine chamber and the vacuum...
…
“Ah,” he said.
Then, “So now, we shall all vacate the laboratory.”
And there was as sudden mad scramble as all the boys made for the door like bullets.