Bioaerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 314

resuspension rates were calculated for two dust mite allergens ( Der f 1: 1 × 10 −5 /m 3 ; Der p 1: 2 × 10 −6 /m 3 )
than for cat dander (8 × 10 −7 /m 3 ), dog dander (3 × 10 −7 /m 3 ), or endotoxin (4 × 10 −7 /m 3 ). 292 When sieved
house dust was reaerosolized by vacuuming the carpeted loor of a test chamber, 9% of airborne
cat allergen was detected on particles <5 μm, 30% on particles 5-10 μm, and >60% on particles
10-20 μm. 293 The allergenic content of the PM was calculated to be 0.03%-0.10% on a mass basis,
with a suggestion of somewhat higher cat allergen concentrations in submicrometer particles. Only
a small fraction of allergen particles may be <5 μm and able to enter the lungs. However, this vari-
able particle fraction is assumed to produce inlammatory responses in hypersensitive persons. 78
12.4.1.2 Culturable Bacteria and Fungi
Bacterial endo- and exospores resist environmental stress much better than vegetative cells and are
considerably smaller in size. 294 For example, in Polish homes the highest concentrations of typi-
cally single-spore, thermophilic, actinomycete spores were found in the 1.1-2.1 μm size range. 295
In contrast, the maximum concentrations of mesophilic Gram-positive and Gram-negative bacteria
were in the 3.3-7.0 μm size range, larger than the individual cell sizes, suggesting aggregation.
Cell aggregation has been shown to increase viability of airborne bacteria and may be a survival
strategy. 296
The size distributions of culturable airborne fungi have been studied in many environments.
The highest fungal air concentrations typically are found on the stages of size-segregating sam-
plers that correspond to the diameters of intact cells or cell agglomerates. 124,297,298 With a six-stage
impactor and malt extract agar (MEA), the highest concentrations of the major fungal genera were
observed in the 2.1-3.3 μm size range in residences with mold problems and control homes. 283 The
highest concentrations were found in the same size range for samples on dichloran glycerol 18 agar
(DG-18) from moisture-damaged and reference schools, but on MEA, the highest concentrations
were in the smaller size range. 298,299 These differences likely are caused by size differences in
the fungal species that grow on the two culture media as well as disparities in the fungal lora in
schools and homes.
12.4.1.3 Fragments
All particle sizes in Table 12.1 are for intact bioaerosol forms, but fragments of biological materi-
als may be found as particles much smaller than their original structures. For example, airborne
endotoxin has been found in particles smaller than intact Gram-negative cells. 300-303 The modes of
fungal-fragment size distributions have been observed to vary from 0.5 to 1.5 μm, depending on the
species and aerosolization method. 304,305 With an electrostatic, low pressure impactor, fungal frag-
ments were detected in particles as small as d a = 30 nm. 306 These indings have important implica-
tions for bioaerosol exposure measurement and dosimetry because pollen and microbial fragments
behave differently in air samplers, control devices, and the human respiratory tract than would be
expected given the size of their original pollen grains, cells, spores, or hyphae.
Antigen content has been measured in whole fungal spores and fragments entrained in air that
had passed over contaminated ceiling tiles or agar surfaces placed in a test chamber. 307 Common
antigens were identiied in both fragments and spores suggesting their shared origin and similar
potential importance to human health. In later studies, fungal fragments have been shown to contain
allergens, 209 mycotoxins, 210,223 and fungal glucans. 308-310 Therefore, both fungal spores and frag-
ments may play a role in mold-related adverse health effects. Currently it is not known if fungal
fragments originate primarily from hyphae or spores. However, a recent study suggested that broken
or fractured spores may be the main source. 304 The smaller size of fungal spores and fragments
allows them to reach the lower airways and trigger asthma, but a different exposure mechanism
must be involved for larger pollen grains. 232
High concentrations of birch pollen and antigen occasionally are found before and after the
pollination period. It has been suggested that allergy symptoms that occur outside the pollination
period may be explained by exposure to allergenic particles smaller than intact pollen grains. 311-314