Healthy Indoors Magazine
Issue link: https://hi.healthyindoors.com/i/1095622
Healthy Indoors | 27 the smaller particles, such as mold spores, will pass by and not get collected. Not really an ideal situation when you are trying to sample for mold spores. If the flow rate is too fast, then larger particles will strike the surface of the collection media too hard and bounce off. Face velocity and outdoor sampling The face velocity of a sampling cassette is essentially how fast the air is moving, at the face of the cassette, as it gets pulled in by the pump. In industrial hygiene, know- ing the face velocity of the sampler is critical when doing isokinetic sampling in an airstream because of the inertial impaction thing. The face velocity of the standard Air-O- Cell spore trap cassette, when run at the recommended flow rate of 15 liters per minute, is slightly less than three- and-a-half miles per hour. What this means is, right at the face of the cassette the air being pulled into the cassette is moving at three and a half miles per hour. As you move away from the face of the cassette, the air velocity drops off precipitously, so much so that the velocity a mere two inches away from the cassette face is virtually non-exis- tent. So why does this matter? Well, if you are sampling in a nice quiet room where there are no real air flows or currents, then we don't have any real issues. However, if you are collecting an outdoor air sample, and there is a wind blowing across the face of the cassette at a fast- er speed than a light breeze, then many of those small particles get blown right by and are never captured. The faster the wind speed, the more particles within that air stream that the device will fail to capture. So much for trying to compare the results from your outdoor samples to the indoor samples. Cut size and Aspergillus/Penicillum results The 50% cut-size (D50) of a standard Air-O-Cell cassette, at room temperature and when run at the recommended flow rate, is about 2.6 microns (um). What this means is that half of the particles with an aerodynamic diameter of 2.6um will impact the collection media in the sampler, and half of them will pass right by. Mind you, I said that half the parti- cles will impact the collection media. This does not mean, however, that all of them will actually stick and stay there. The more particles that are already stuck on the media, the more likely that other particles will hit them and bounce off. In addition, the cut-size curve for spore trap samplers tends to be extremely steep. For instance, the D50 for the Via- Cell sampler is 1.56 um. However, when you get down to an aerodynamic diameter of about 1.51 um the collection efficiency drops down to about 26%, and at an aerodynamic diameter of 1.4 um it is only around 7%. So, you may ask, why is this important? Well the aerodynamic diameter for Penicillium and Aspergillus spores is generally in the 1 to 3 um range, so if you are sampling in an environment which has been contaminated by these types of mold then chanc- es are you're likely missing more than half of the spores that are actually present when you collect your sample. The trouble with laboratory results Another thing that many newly-minted mold assessors need to be aware of is just how inexact the laboratory anal- ysis of spore trap samples really is. Back in 2011, Larry