The concern isn't contamination of the medium -- you pasteurize the medium and use enough spawn so that your preferred mushroom mycelium outcompetes other fungi

The concern isn't contamination of the medium -- you pasteurize the medium and use enough spawn so that your preferred mushroom mycelium outcompetes other fungi and bacteria. This intentionally happens fairly rapidly (because if it doesn't, you're very likely to get some form of bacterial or fungal contamination, which usually means tossing that chunk of medium). Once the mycelium have colonized the medium, it's fairly resistant to contamination in the same way that a healthy sourdough starter is resistant to contamination. The main health concern is over spore production and inhalation of spores. If you're growing in a household environment you'll want to use a growing chamber with filtered air exchange.