It might require a mask attachment, analogous to 3-d glasses, which would dispense a small dose of essential gasses for each viewer rather than trying to fill and then, as @SirBailey notes, clear an entire room. It would be far easier to control the intensity of the sensation as well. Naturally, there would have to be a stock of a variety of substances, which would have to be refreshed every so often. Of course, a more efficient, if currently unfeasible, method is direct electric stimulation of the brain to induce the sensation of scent. This has been done in the laboratory, but my impression is that it is not yet sufficiently controllable and non-invasive for use in entertainment.