I didn’t say ‘everything molds.’ I said that every house has mold in it. Under your carpet, behind your drywall, in the confines of your furniture. Small (and large) amounts of mold are everywhere. It is extremely hard to prevent it from happening in climates that experience humidity.
Certain foods have preservatives in them specifically to prevent mold from developing. Food isn’t really a large concern - food can become hotbeds of mold but the real concern is water leakage into materials that are not meant to get wet and have a hard time drying out.
Carpet, drapes, upholstery, leather, wood products, clothing, paper, cardboard, books, rags, drywall, cloth, ceiling tiles, ductwork, paint, wallpaper, and household dust - all of these can and will support mold colonies given the right environmental conditions, and houses are absolutely full of these things. Carpet is a big one; if a house leaks water then the carpets can become hotbeds of mold that you can’t see, and when you walk around on it, the mold gives off spores into the air. It’s not uncommon for carpets to be taken out and huge colonies of mold found beneath. Same goes for drywall. And this is in houses people have been living in for decades not realizing they had a mold problem. Mold will grow practically anywhere. They’re everywhere, you’re breathing their spores right now, they are on your food, your body is fighting them right now; they are constantly waiting for the right time to bloom into a full colony. It’s disgusting. D=
I think the way I’d handle warnings and stuff, is on the bottom right you could have a constant air quality alert; If an area could have mold (but doesn’t necessarily) then the air is musty. If it’s clean then the air is fresh. If it’s very bad then it just flat out smells moldy. You can smell mold if there’s a lot of it. It’s a very distinctive odor and it leaves no doubt. No need to be coy about it.
Musty air could impart no exposure at all (the air is just stale) or it might be kinda bad. How the player would get a sense for that is how often their character is sneezing, coughing, and getting itchy eyes. These messages would pop up with a frequency depending on how much exposure they are receiving (so modified by environmental protection), and would give the player a sense of how much exposure they are getting, and give them a sense of whether the house they’ve chosen to live in is safe or not. In genuinely moldy areas these messages might be supplemented by periodic hacking and burning eyes. These things don’t actually happen with mold acute exposure, but for the sake of gameplay I think it makes sense as a warning mechanism.