Mold spores and their mycotoxins (secondary metabolite) are very small and difficult to detect except thru testing until a colony has grown to a significant size. Mold and mycotoxins, in fact, are so small and lightweight they can subsequently become aerosolized with normal air velocity and circulation found in the indoor environment (ie. ceiling fans, central HVAC system) and become airborne. Due to their small size, once aerosolized are easily respirable and can deeply penetrate into the respiratory tract.
Aleksic, B., Draghi, M., Ritoux, S., Bailly, S., Lacroix, M., Oswald, I.P., Bailly, J.D., Robine, E., Aerosolization of Mycotoxins after growth of Toxinogenic Fungi on Wallpaper. AEM 207 Jun 23.
Brasel, T.L., Douglas, D.R., Wilson, S.C., Straus, D.C., Detection of Airborne Stachybotrys chartarum Macrocyclic Tricothecene Mycotoxins on Particulate Smaller than Conidia. AEM Jan 2005