There are two different ways mold inspectors sample airborne particulates for mold testing. Both are called impact sampling, because in this type of testing, spores collide with a medium and stick to it.
Culturable Air Sampling
In culturable air sampling, mold spores are sucked by a vacuum pump into the impactor and trapped directly on the nutrient surface in a petri dish. The covered dish is then placed in an incubator at the lab and allowed to sit for a week to ten days. Only the viable mold spores germinate within and grow, often into identifiable colonies.
One drawback to culturable sampling is that even though there may be many spores of one particular type in an air sample, if they are dead they will not produce any colonies at all, yet the dead spores can still be allergenic. Also, even though they can cause allergy symptoms, most hyphae do not grow into colonies on a petri dish.
Another problem with culturable sampling is that aerosolized spores often are often part of large clusters. But when a cluster lands in a petri dish, no matter how many spores it contains, the resulting growth will appear to be only a single colony. As a result the sample may underestimate the actual concentration of spores and the exposure to allergens.
Culturable sampling also will not tell you whether the spores contain mycotoxins, because in the petri dish, the fungus may not produce toxins. If you suspect that you have been exposed to mold toxins and you really want to know, hire a professional to collect a bulk sample and send it to a lab for mycotoxin analysis, because petri-dish culturing of mold spore samples alone will not suffice.
Many of the do-it-yourself mold testing kits use nutrient petri dishes. This is called settle-plate testing. A petri dish is left open in a room for an hour or so and is then sent off to a lab for analysis. Unfortunately, there are a number of uncertainties with this method. For example, it’s difficult to connect the growth that may occur in the petri dish with the sources of the spores, either indoors or outdoors. Also, the settle-plate test is biased toward larger spores, because they settle out of the air faster than smaller spores do. Yet it is the smaller spores that may be more problematic for sensitized individuals. And unlike impact samplers, which use pumps that can be calibrated for airflow, settle-plate testing cannot provide any information regarding the concentration of spores, so this type of sampling is useless in determining exposure levels.
In spore-trap sampling, air is drawn through a device by a pump and suspended particulates, including both viable and dead spores, as well as hyphae, are trapped on a sticky surface such as tape. The samples can then be stained and examined under a microscope. Spore-trap sampling is usually adequate to determine if there is a mold problem in a building, though any air sampling done indoors, whatever the type, should also include one or two samples of outdoor air for comparison — as controls, unless it is winter and the ground is frozen in which case there are few spores outdoors.