That’s a good consideration and something I haven’t seen brought up with regards to walipinis (didn’t think of it either, though I had read up on the risk awhile back for a postapoclyptic story I was working on about a guy who hunts for old bunkers and safe houses). Nature abhors a vacuum and it likes to fill them with water, mold, and heavier-than-air gasses. Luckily, greenhouses require a certain amount of ventilation, but I doubt most are configured properly for heavier-than-air stuff. Thanks!