The first signs of mold in air vents is when you have cold air hitting warm air. That is the start to mold growth on your air vents and AC ducts. Mold really starts growing in the actual plenum. Before you know it, the ducts sweat and you end up having a situation where there’s mold growth.
You then start to see it manifesting on the air vents. A lot of people think it’s just dirt or dust when it is in fact a particular type of mold called cladosporium. (see mold library) Cladosporium is an allergenic mold.
Signs of mold in air vents can go from cladosporium, to then also having penicillium aspergillus on the vents and ducts. So you get a buildup starting to occur and now now goes airborne and starts contaminating others physical items and furniture throughout the home.
If your air conditioner isn’t working the way it should, which may be do to mold growth, you’re going to end up with humidity levels that aren’t ideal.
As soon as you start going over 60% humidity, and if that’s continuous and not drying out, you’re going to start seeing growth on shoes, on leather bags, on jackets etc. Once you are seeing growth physically manifesting, it is way past the point you want to be.