Oh, don't fret, darlings. Sid won't feel any of this.
The pumps kicks on with a rumble and a roll. They were loud and their closeness made the glass box vibrate a little. More importantly, a gas started to seep in through the vents. It smelled thick and heavy, like smoke but much more chemical. It burned their eyes and nose and throat. More importantly, it would slowly start to constrict their throats.
Up between a pair of the vents, a button, formerly transparent, illuminated. If one of the characters pushed it there would be a delay. First the pumps would idle and then stop. The filling gas stopping. Then, the pumps would move in reverse, pulling the gas and their air back with them.
Don't wait too long before hitting that switch again, dear.