Dust Samples

A sampling strategy involving the analysis of settled dust especially from above-floor surfaces33,37 provides an indication of microbial agents that were likely once airborne. Settled dust is heterogeneous, consisting of particles from people (e.g., skin scales), pets (e.g., Fel d1 allergen), textiles, paper, cooking, and the outdoor air. The composition of settled dust can vary depending on the location in a room or building. Settled dust may be collected by filter cassette minivacuum,34 adapters fitted into the nozzle of a vacuum cleaner,38 or by a large precleaned vacuum cleaner.18,33 As with other kinds of microbial sampling, dust samples should be collected from representative areas (e.g., for floor dusts, collect samples from both high- and low-traffic areas). When collecting dust samples it is important to document both the surface area from which dust was extracted, as well as the time over which the sample was collected. Dust samples should be sieved prior to analysis in the laboratory (sterile sieve) to remove large inert particles (e.g., textile fibers and lint).

Analysis of settled dusts for fungal composition has been used forensically as an indication of a water or dampness problem.10,34, Table 3.6 provides an example where both air and dust sampling provided an indication of problematic mycological condition. More than 95% of the culturable molds found in the dust samples were Penicillium, Aspergillus, and Eurotium species (mostly xerotolerant taxa). These sampling data suggested a dampness condition or incomplete cleanup associated with a previous mold remediation (Table 3.6). The composition of fungal taxa in settled dust is typically heterogeneous with a significant percentage of phylloplane fungi (e.g., Cladosporium, Alternaria, Epicoccum etc.) being present.39,40 Dust

TABLE 3.6. Dust and Air Sampling for Molds in a Building that Had Undergone Incomplete Mold Remediation

Sample Description

Amalgamated floor dusts collected from many rooms

Air samples indoors by spore trap Air samples outdoors by spore trap

Sample Analysis

Dusts contain 108 CFU/g mostly (>95%) Penicillium, Aspergillus, and Eurotium species;, 1% phylloplane molds Penicillium/Aspergillus concentration 11,500 spores/m3; Cladosporium concentration 120 spores/m3 Penicillium/Aspergillus concentration 55 spores/m3; Cladosporium concentration 120 spores/m3

Source: Adapted from Morey.10

samples from an indoor environment heavily dominated by Penicillium/Aspergillus species (Table 3.6) or cellulose degrading fungi (e.g., Chaetomium species) would be considered atypical and forensically indicative of wet or damp conditions present in an indoor environment.37

In the example in Table 3.6, Penicillium/Aspergillus spores comprised more than 95% of the airborne molds collected by spore trap. However, the relationship between the fungal species in settled dust and the species in indoor air is often less clear than the example in Table 3.641,42 As with air sampling, guideline values relating total concentrations of fungi in dusts with health outcomes have not been established.

Settled dusts may be analyzed for culturable fungi by dilution or direct plating.18'39 In direct plating a small amount of dust (usually a few milligrams) is sprinkled on the surface of the culture medium. Direct plating is recommended for determination of the actively growing or ecologically important species in the sample.39,43 Dilution plating provides an indication of the total number of culturable fungi but can overemphasize the presence of those species with the longest half-lives or ability to withstand stress such as repeated mixing with sterile liquids. In addition, the error or deviation in quantitative dilution results increases as the dilution increases.18 Laboratory data generated by manipulating and combining media and dilutions are erroneous and misleading and should not be relied on.

Dust samples can be collected and analyzed for content of specific allergens such as those from mites, cats, dogs, and cockroaches.18,44,45 Table 3.7 provides an example where dusts from a complaint office (allergic symptoms), a control office, and soot in a HVAC system outdoor air inlet were analyzed for various allergens. In this example, cat allergen (Fel d1) concentrations were highest in the complaint office. The results in Table 3.7 suggested that pet owners were transporting Fel d1 on clothing into the workplace. The particles containing Fel d1 accumulated in settled dust, especially in the complaint office. A thorough dust removal program (HEPA filter vacuum cleaning), especially for fleecy office materials (upholstered furniture, modular partitions, and carpet), was carried out to lower the Fel d1 in the office environment.

Sampling strategies involving collection of settled dust should include consideration of the type of laboratory analysis most appropriate for evaluation of the microbial composition in the sample. Table 3.8 provides an example where very

TABLE 3.7. Concentration of Allergens in Settled Dust Samples

Allergens (mg/g)

TABLE 3.7. Concentration of Allergens in Settled Dust Samples

Allergens (mg/g)

Source of Dust


Der p1

Fel d1

Can f1

Complaint office





Control office





HVAC system soot





aND = less than the detection limit of 0.1 mg/g; increased risk of sensitization among atopic individuals occurs at a level of 0.5-2.0 mg/g of Fel d1 (see Ref. 18).

aND = less than the detection limit of 0.1 mg/g; increased risk of sensitization among atopic individuals occurs at a level of 0.5-2.0 mg/g of Fel d1 (see Ref. 18).


TABLE 3.8. Analysis of Carpet Dust for Stachybotrys Content by Culture and PCR Methodologies

Type of


% Stachybotrys among Identified



Fungal Taxa

Dilution plating on malt

1 x 103 CFU/g

Approximately 5% of culturable

extract agar

1 x 107 spore


PCR,a target list of 24

>95% of spore equivalent taxa



"Quantitative PCR assay; see Ref. 18.

"Quantitative PCR assay; see Ref. 18.

different results were obtained by dilution plating of dust for culturable fungi as compared to analysis of the same dust by PCR. The dust sample was obtained from an apartment occupant's vacuum cleaner that had been repeatedly used for housekeeping of carpeted flooring. Previous physical inspection of the building envelope showed that biodeterioration involving extensive Stachybotrys growth had occurred on building paper and sheathing in the envelope wall. Water had flowed into the apartment from the envelope, thereby soaking the carpet on numerous occasions. Dilution plating on malt extract agar (MEA) showed that Stachybotrys accounted for approximately 5% (or 1 x 103 CFU/g) of culturable molds found in the vacuum cleaner dust. By contrast, PCR analysis of the same dust (24 target fungi including Cladosporium, Aspergillus, and Penicillium species) showed that Stachybotrys accounted for more than 95% of the spore equivalents detected (Table 3.8). Thus, the analytical methods used in this example were important for data interpretation, namely, whether Stachybotrys dominated the molds present in the dust sample.

Some microbial investigative strategies include the collection of both air and dust samples (Table 3.6), and perhaps other kinds of surface samples. If the purpose of air sampling is to evaluate occupant exposure to bioaerosols the air samples should be collected first. Dust samples are collected last because disturbance of interior sur-faces12 increases bioaerosol levels in room air.

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