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.
Indoor Air Testing For Mould
There are a number of methods that can be used to test air for airborne mould spores and fragments. The method chosen would depend on the objective of the investigation and subsequently the type of data required. The following methods can be used for testing air for airborne mould contamination.
Settle Plate Method For Detecting Viable Airborne Mould Spores And Fragments
With this method, a set of plates containing suitable growth media is exposed at table‑top level for half-hour to 4 hours. The mould spores and fragments suspended in the air settle by gravity onto the growth media. Light particles that remain suspended in the air for many hours are likely to be missed out. However, it is the cheapest way of sampling air since no other equipment is needed.
Volumetric Air Sampling For Detecting Viable Airborne Mould Spores And Fragments
The settle plate method is generally not recommended. A better method involves impacting a known amount of air on some suitable growth media. The media could be liquid or solid. With this method viable spores or vegetative material would form visible colonies (referred to colony forming units) in the media. These are counted in the lab and expressed as colony forming units (CFU) per cubic meter of air.
Volumetric Air Sampling For Detecting Total Airborne Mould Spores And Fragments
One disadvantage of impacting air on growth media is that only the viable propagules can grow on the media and hence detected. It is estimated that only about 10-15% of the mould spores may be viable. To enable us have a better estimation of how contaminated the air is, air is drawn on onto some inert sticky stuff. Depending on the efficiency of the sampling equipment used most of the spores and other airborne particulate matter including insect parts, pollen grain, and all sorts of fibres are trapped onto the sticky surface. Since these samples are directly examined under a microscope, the analyst is able to count the spores and vegetative fragments and express them as spores per cubic meter of air. It is also possible to identify some of the spores to genus level.
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