Sampling By Filtration

Bioaersols can be collected on filters and analyzed by methods including direct microscopy, growth of propagules directly on the filter medium, elution of particles from the filter, and dilution plating.18 In addition, specialized methods are available for analysis of dust collected on filters for endotoxins, glucans, and mycotoxins, and by PCR.

Personal and area air sampling for hardy fungal and bacterial spores can be carried out using presterilized cassettes with cellulose ester or polycarbonate filters.56,57 Possible damage (loss of culturability) during sample collection may result from desiccation of sensitive cells and susceptible spores (e.g., Stachybotrys spores; vegetative bacterial cells), and this favors the recovery of hardy spores such as Aspergillus fumigatus. Personal sampling for culturable propagules by filtration in most appropriate for dusty environments where the concentration of bioaerosols is high (e.g., during mold abatement activities, wood processing, agricultural operations) enough to ensure that an adequate number of culturable propa-gules are present in the filter wash for dilution plating. Button (trademark) samplers using gelatin filters are reported to be efficient in recovering desiccation sensitive fungi and bacteria.18

Filtration is widely used for the collection of airborne endotoxins and glucans. Cassettes, filters, and instruments that touch the filter collection surfaces must be pyrogen-free (surfaces devoid of any environmental sources of endotoxins or glucans). Endotoxins are more readily extracted from polycarbonate, Teflon, and glass fiber than from cellulose ester filters.58 Sample air volumes adequate for analysis of endotoxins or glucans often exceed 1 m3 for nonindustrial indoor environments. Filtration sampling for airborne mycotoxins has been successful in dusty agricultural settings where airborne levels of total fungal spores were in the 105-107/m3 range. In one study, using a high-volume sampler (flowrate 0.83 m3/min; glass fiber filter; total air volume <50 m3), several Fusarium tri-chothecene mycotoxins were recovered.59 However, in nonindustrial indoor environments, detection of specific airborne mycotoxins has been limited to research studies for reasons including limited availability of mycotoxin standards for each of the many mycotoxins that may occur in spores and hyphae.18 High-volume air filtration sampling has also been used to detect cytotoxins in collected dusts in nonin-dustrial indoor environments.60

Interpretation of data from filtration sampling is hampered by absence of dose-response information not only for culturable and nonculturable fungi and bacteria but also for chemical agents such as glucans and mycotoxins. It is always important therefore, for purposes of data interpretation, to collect background control samples in the outdoor air and where possible in noncomplaint as well as complaint/problem zones.18 Table 3.3 provides an example where the indoor concentration of

3.13. SAMPLING BY FILTRATION 69

TABLE 3.12. Airborne Endotoxin Upwind and Downwind of Wastewater Treatment Plant"

Location

Endotoxin Concentration (EU/m3)

50 m upwind of plant

14

50 m downwind of plant at

11

entrance to office building

Above digester in plant

58

"Analysis by Limulus assay (see Ref. 61).

"Analysis by Limulus assay (see Ref. 61).

endotoxins collected by filter cassette in a complaint zone was at least two orders of magnitude greater than that present at outdoor and indoor control sites. As discussed in an earlier section, this suggested that the flulike illness in the complaint zone was associated with endotoxin exposure.

Table 3.12 provides an additional example of sampling strategy using filter cassettes where the objective was to determine whether airborne endotoxin from a wastewater treatment plant was degrading the air at a nearby office building. Air samples were collected outdoors in the air around the office building, in the air upwind from the wastewater treatment plant, and in the wastewater treatment plant. Endotoxin concentrations in the outdoor air around the office building were similar to those present upwind of the wastewater plant (Table 3.12). The endotoxin concentration in the wastewater plant was only slightly elevated relative to concentrations in the outdoor air at the office building and upwind of the plant. This sampling data suggested that Gram-negative bacterial aerosols from the wastewater treatment plant were not affecting the outdoor air entering the office facility at the time of sampling.

A sampling strategy for b-1,3-d-glucans using filter cassettes is illustrated in Table 3.13. The objective of sampling was to determine the extent of exposure to glucans under the unusual condition where moldy, moisture-impervious wall covering was physically removed from gypsum wallboard (concentration of Penicillium-Aspergillus spores >105/m3)34 as might occur during mold remediation activities.

TABLE 3.13. Air Sampling for b-1,3-D-Glucan When Moldy Moisture-Impervious Vinyl Wallcovering Is Removed from Envelope Wall"

b-1,3-D-Glucan Concentration Sample Location (ng/m3)

Indoor air; wallcovering 0.12 undisturbed

Indoor air; panel of wallcovering 13,500.0

removed from wall

Outdoor air 0.03

"Polycarbonate filter cassette; analysis according to Rylander et al.62

Sampling was carried out separately in empty rooms where one sheet of moldy vinyl wall covering was removed and in another room where the wall covering was undisturbed. b-1,3-d-Glucan levels increased by three to four orders of magnitude in rooms where moldy wall covering was peeled back from the wall as compared to indoor and outdoor control locations (Table 3.13; Morey and Rylander, unpublished data).

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