Spraying difluoroethane into an enclosed space

I'm working on developing physics lab activities that my students can do at home during the covid quarantines. For an activity involving waves and thermodynamics, I came up with an experiment at home in which I blow over the top of a bottle (beer bottle or 2-liter soda bottle), measure the frequency, and then spray a can of "dust-off" ([difluoroethane](https://en.wikipedia.org/wiki/1,1-Difluoroethane)) into the bottle and measure the frequency again. The frequency gets lower because difluoroethane is much more dense than air.


Environmentally, it looks like difluoroethane is fairly benign. It's used as an alternative to substances that are worse for our planet's atmosphere. People can huff it, and if you do, it's dangerous. It's flammable, so you want to stay away from flames and sparks. All of those things seem like they're not huge problems for my purposes.


But the label also says, "**Never spray into an enclosed space, such as a trash can or paper shredder.**" What's up with this? That's exactly what I need to do for this experiment. Why is this dangerous?


Would there be any safety or environmental advantage to using [2,3,3,3-tetrafluoropropene](https://en.wikipedia.org/wiki/2,3,3,3-Tetrafluoropropene) ("Opteon" or R-1234yf)? It doesn't seem as readily available in small packages.


[EDIT] 


Following up a comment by @MaxW: Interesting, thanks. I found [a video](https://www.youtube.com/watch?v=qrKfaX1lCUM) of the liquid exploding when it's poured into water. Also [a demonstration](https://www.youtube.com/watch?v=nZGBbUgL5Vs) that it's flammable in gas form if you collect it in an open-topped container. Burning produces toxic $\ce{HF}$ and $\ce{COF2}$. So I'm hypothesizing (tell me if I'm wrong) that it's safe for my application if you (1) stay away from any source of ignition, (2) do it in a well ventilated area, and (3) in case of ignition, get away and let the fumes dissipate thoroughly. I'd be interested in answers as to whether this seems sufficient.


