Viable Sampling vs. Non-Viable Sampling
There seems to be a lot of confusion around viable sampling and when its appropriate to collect viables. The most common sampling method is the non-viable sampling method. I use Allergenco D sporetrap cassettes, on a buck pump calibrated to 15 liters per minute for 5 minutes. During the 5 minute sample, 75 liter of air pass through the cassette, where airborne particles are impacted on an internal slide that is coated with an adhesive substance. I send them off to a lab where a mycologist opens up the cassette and looks at the slide to see what stuck in the adhesive.
The mycologist uses a microscope to look at a portion of the slide and counts and identifies the mold spores present. They then use a mathematical formula to determine how many of each type(both viable (live) and non-viable (dead)) there are in a square meter of air. The inside results are compared to the outside results to determine if there is an elevation of one or more mold types. The two biggest drawbacks to the non-viable sampling method are:
- we are collecting only 75 liters of air from a room or area that may be several hundred thousand liters, then only a portion of the airborne particles actually stick to the slide, then the lab only reads a portion of the slide... so the non-viable sample results are from a small portion of a small portion of a small portion of the air in an area.
- Under a microscope, Aspergillus and Penicillium type mold spores look identical so they are lumped together as Penicillium/Aspergillus. The problem with this is that there are thousands of species included in that group and the non-viable method cannot differentiate between Aspergillus niger and Penicillium roqueforti... and there is a big difference in terms of potential health risks.
Thats where the Viable sampling comes in. The viable sampling method uses an Andersen N6 sampler with a high volume pump to impact the air into a perti dish. The petri's are incubated for 5 - 7 days and the mold that grows is identified. With viables, the mycologist is able to look at the spores, conida, hyphae and other growth structures to differentiate the Aspergillus and Penicillium species. The drawback to this type of sampling method is:
- It takes 5 - 7 days to get results where the non-viable sampling results come in the next day.
- Viable sample results include only the live spores that actually grew in the petri dish.
- Some mold types grow better on different media (the gelled nutrients in the dish).
For example; a general purpose media like potate dextrose agar (PDA) is great for the Penicillium and Aspergillus species but will not support the growth of Stachybotrys. So to wrap it up, each sampling method has its strengths and weaknesses. The non-viable method is great for most projects where the ultimate goal is to remove the mold growth and reduce the airborne mold spore concentrations. The viable method is usually used when speciating the Pen/Asp is important (like a legal case), but these samples are usually collected along with the non-viable samples. It would be rare for me to collect only viable samples, they just dont provide enough information.