Healthy Indoors Magazine - USA Edition

Healthy Indoors | 35 have not been used by industry since the NIOSH 7400 method was formally published and then adopted into AHERA. For sam- pling, whether it is the AHERA TEM method or the NIOSH 7400 method, we use a 25-mm 3-piece cassette with 50-mm electrically conductive extension cowl cassettes. The two methods require the filter cassette to face downward (AHERA, 45 degrees downward) from the horizontal. The filter material used is mixed cellulose es- ter (MCE). The AHERA TEM method does allow for the use of polycar- bonate (PC) filters as well. The PC filters fell out of favor because post-sampling handling is more problematic than the MCE filters. With PC filters, if samples sent to the lab are not handled carefully, the sampled fibers can move significantly from the filter surface. This was widely described in the industry in the 1980s. Since then, only MCE filters are used outside of specialty applications. Filter cassette to face 45 degrees downward from the horizontal. Both methods require blanks, however, that is where the sim- ilarities end. The AHERA TEM method requires three blanks: two field blanks and one laboratory (sealed) blank. The NIOSH 7400 method requires a minimum of two blanks or 10 percent of sam- ples collected with a maximum of 10 blanks. How the blanks are handled is different as well. The AHERA TEM method laboratory (sealed) blank is not opened and kept sealed, while the field blanks are opened for 30 seconds at the entrance to the abatement area(s) and one at an ambient area. They are then closed. While the NIOSH 7400 method requires the blanks to be opened at the same time as the other cassettes (used for sampling) just prior to sampling and stored with the top covers of the cassettes that are running and remain open for the duration of sampling. These are kept in a bag or box. Many use the cassette box with the lid closed, some put field blanks zip-type bags while sampling. Another difference is that with the AHERA TEM method we are required to use a 0.45µm MCE filter, and the NIOSH 7400 method uses a 0.8µm MCE filter. This refers to the size of the air passages in the filter material. Filter manufacturers will color code or mark the label so that the type of filter within the cassette as- sembly is known to the user and the laboratory. A typical box of air sampling cassettes. TEM filter is 0.45µm. PCM filter (white label) is 0.8µm AHERA TEM Method Air Sampling Issues Let's get to some of the interesting items. The AHERA TEM meth- od is straightforward, it requires five samples taken inside the work area and 5 samples outside the work area that represent air enter- ing the abatement site plus the three blanks (described above) for a total of 13 samples. These samples should be set for a flow rate from 1 to less than 10 liters per minute (LPM) for a total volume of air, typically, from 1200 liters (L) to 1800 L (see "Table 1" below). It would be interesting to find out how many in industry sample at less than 10 LPM (i.e., 9.9 liters per minute) versus at 10 LPM. Likely most of the industry merely samples at 10 LPM. The statisti- cal difference between 10 LPM and the 9.9 some regulators insist upon is statistically insignificant and will affect method performance in no discernable manner. Either way, this means clearance samples will take a minimum of two hours to collect. At the time, this method was developed it became known through research involving flow rates higher than 10 LPM could cause fibers to impact the MCE filters vertically and not horizontal to the filter surface. This makes the sample analysis, counting and identification difficult and likely biased. This is also the reason the method specifies a second MCE filter under the 0.45µm sampling filter, the 5µm diffuser (see Figure I below). This additional filter is placed in this manner to attempt to create an even flow across the filter surface so that the fibers impact the filter uniformly. For the AHERA TEM method, the clearance test passes when the average concentration of the five samples from inside the work area does not exceed 70 structures per square millimeter squared (s/mm 2 ). See AHERA at §763.90(i)(3) for an optional clearance test based on the Z-test which compares the outside and inside air samples and the obtained blanks. This is rarely necessary. But

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