If something like oxygen were to be released into a room, it would diffuse much faster than something like a heavy smoke. Has someone ever measured the rate of diffusion of different gases and particles? A quick Google search turned up nothing. I'd like to know the distribution of how many particles of something are at each distance away from the release point after a certain amount of time. If the particles did not collide with the air molecules, then the rate of diffusion would be equal to if a vacuum were around them, so when you opened your door, almost all the air would be replaced by air from outside every second. But the air slows this process down, and it'd be interesting to know by how much. If something like this hasn't been tested, isotopes of elements could be used, and then the concentration could be tested. Another related question: how far, on average, does a particle of a certain volume traveling at a certain speed in a straight line get through air get before it hits something?