mould. won't make your ceiling cave in but might contribute a little towards it. what it /will/ do though, is irritate your lungs and sinuses and any allergies you might have. I can't have mould in the house at all or my hayfever goes into overdrive.

smoke alarm. tell me more. this sounds like a market that needs filling. I'm thinking a smoke alarm that triggers not just the noise alarm, but a flashing light too, or a siren in another room. This is very feasible and not particularly expensive or technically difficult. $500 is seriously over priced. $100 is much more reasonable, and that would be for a one off, not a production run.