Indoor air quality (IAQ) investigations rely on different methods to quantify levels of the airborne biota (bacteria, yeasts or fungi) in different rooms/areas. It is important that each method can be compared against other methods and that there is consistent interpretation. This paper reviews the literature regarding the use of the settling plate method for performing IAQ investigations. Inhalation of mould spores, hyphae and cell wall fragments is the dominant route of infection with dermal contact being reported far less frequently. Therefore, air sampling that addresses the airspace is preferred over streak/swab testing. Different air sampling methods are either (i) active or (ii) passive. Perhaps the best known active method is the Anderson-type sampler that takes a known volume of air across a stage and through a set of holes where air plus viable cells are deposited onto the agar surface of Petri plates. The other active testing method also uses a pump to take a known volume of air and deposit it onto a sticky microscope slide and then spores are counted and classified. Spore traps are a non-viable method. Both methods rely on a numerical value that indexes air quality. Disadvantages are that both methods require the use of expensive pumps and specialised equipment. In contrast, Petri plates are easily available and in different nutrient formulations, so IAQ testing can be made selective for different objectives. One criticism of viable settling plates is that the unit of measurement, the colony forming unit (CFU per plate) is not readily comparable to the CFU per m3 of air criterion used by spore traps. This review summarises multiple examples of passive settling plates being used in healthcare and industry to index air quality by exploiting the Ormeliansky formula on the CFU to convert the values into CFU per m3 following, N=5a x 104 (bt)-1 Where: N=CFU per m3, a=CFU counted per Petri plate, b=Petri plate surface area and t=exposure time in minutes.