Testing for airborne mold spore concentration is achieved by impacting a known volume of air onto a surface coated with sticky material. As the air hits the sticky surface the spores and any other particulates in the air are trapped. In the laboratory the spores are identified under a microscope, categorised into various groups and counted. This method is excellent for estimating how contaminated the air is but it does not tell us what proportion of the counted spores are still viable. If an estimate of the proportion of viable mold spores is needed, then the air has also to be impacted onto some growth agar media. Viable mold spores would then grow on the media and appear as mold colonies, usually referred to as colony forming units (CFU). CFU is not a very accurate way of measuring the viable proportion of airborne mold spores. This is because a single colony can develop from one spore or a group of spores. Secondly, fast growing colonies tend to overgrow slow growing colonies. Also, the agar media used may not support the growth of all categories of viable spores present in the air.
What are colony forming units? Colony forming units, usually abbreviated as CFU, refer to individual colonies of bacteria, yeast or mold. A colony of bacteria or yeast refers to a mass of individual cells of same organism, growing together. For moulds, a colony is a group of hyphae (filaments) of the same mould growing together. Colony forming units are used as a measure of the number of microorganisms present in or on surface of a sample. Colony forming units may be reported as CFU per unit weight, CFU per unit area, or CFU per unit volume depending on the type of sample tested. To determine the number of colony forming units, a sample is prepared and spread or poured uniformly on a surface of an agar plate and then incubated at some suitable temperature for a number of days. The colonies that form are counted. CFU is not a measure for individual cells or spores as a colony may be formed from a single or a mass of cells or spores.