However, there are thousands of mold species, far too many to be included in standard allergy testing. Mutants cannot be tested for. Less common molds are often not tested for, and can be highly allergenic. All molds have a need for something to grow on, a nutrient. The dust in your airconditioning system will work fine.
Mold needs dark.
You never see mold growing on bright, sun-lit surfaces. Dusty or musty papers, cardboard, carpet, upholstered furniture and bedding. Damp wood, such as house framing wet from rain during construction, or for that matter, before the contruction even began. Use kiln-dried lumber if you are building. Air-conditioner insulation, coils, and drain pans, even in furnaces and automobiles.