Bioaerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 313

and decrease deposition in regions where diffusion dominates, for example, the alveolar region.
Although fungal spores are assumed to be hydrophobic, an almost 30% growth in the size of some
spores has been seen at relative humidities (RHs) >90%, and particle sizes from 0.5 to 2 μm have
been estimated to be most affected by hygroscopic growth. 274 An immediate increase in particle
diameter and separation of fungal and actinomycete spores was observed as they passed briely
through a test chamber of warm, humid air (38°C, 95% RH) compared with exposure to cooler,
dryer air (20°C, 40% RH). 275
12.4.1 a erodynaMic d iaMeters oF a irborne b iological a gents
An optical particle counter and ilter sampling have been used to characterize exhaled particles and
indirectly showed that most exhaled inluenza virus particles were in droplets <1 μm. 43 Air sam-
pling also has shown that >50% of inluenza A virus-containing particles are <4 μm. 276,277 These
indings conirm the existence of airborne virus in the ine particle size range. A fraction (∼10%)
of the pollen from angiosperms (lowering plants) and all of that from gymnosperms (conifers and
other plants that produce seeds within cones) are dispersed by wind, 278,279 but not all of these pollen
are allergenic. Pollen grains and plant spores that rely on wind transport (anemophilous species)
are generally smaller (∼50 μm) than those transported by other means, for example, by insects
(entomophilous species). However, there is considerable overlap, and some strictly wind-pollinated
plants (e.g., the pine family) produce larger pollen grains. 152 The texture or ornamentation of the
outer pollen wall differs between wind- and insect-borne grains. Anemophilous pollen are often
nearly spherical and have relatively smooth surfaces. Conversely, entomophilous pollen tend to have
more elaborate surface features, which help them adhere 152 but also affect their dynamic shape fac-
tor and thus their aerodynamic diameter (d a ) when airborne.
The density of common airborne fungal spores has been estimated to vary from 0.56 to 1.44 g/cm 3
and that for pollen grains from 0.39 to 1.1 g/cm 3 . 280 Size comparisons for fungal and actinomycete
spores by four size measurement methods have found disagreement. 281 The largest discrepancy was
seen for wet spores of Cladosporium cladosporioides for which the volume-equivalent physical
diameter by optical microscope was 4.0 μm, that is, d a = 4.0 μm if density = 1 g/cm 3 , whereas the d a
measured with an aerosol particle sizer (APS) was 1.8 μm. Thus, d a cannot be estimated accurately
without information on cell density, which may vary with hydration status.
For a given genus or species of microorganism or plant, the variability (geometric standard devi-
ation [GSD]) in spore or pollen grain diameter can be small and close to what is considered mono-
disperse (GSD < 1.2). 282 For example, using a six-stage impactor, the following GSDs were measured
for culturable fungi: Penicillium species 1.1-1.4; yeasts 1.1-1.5; Cladosporium species 1.1-1.9; and
Aspergillus species 1.4-1.9. 283 Even smaller size variations have been observed in the laboratory
with direct-reading particle counters for fungal spores aerosolized from pure cultures. 274,275,284
However, measurement of the dimensions of individual spores by microscope has shown much
higher variability. 285 Part of the variability in d a measurements may be due to the occurrence of cell
clusters or chains or the attachment of biological matter to other particles. Comparisons of measure-
ments made with a multi-stage viable impactor and an aerodynamic particle sizer have revealed that
fungal particles in chains behave aerodynamically like single fungal particles. 275,286 The effect of
several forms of particle aggregation on the average d a of a cluster of P. chrysogenum spores (physi-
cal size: 2.8 × 3 μm) has been considered for two particle densities (0.5 and 1 g/cm 3 ). 281 The average
d a of a unit-density, single spore was 45% larger than one of half that density (2.9 vs. 2.0 μm). For
unit-density eight-cell aggregates, a chain of spores had an 18% smaller d a than a compact aggregate
of spores (4.4 vs. 5.2 μm).
12.4.1.1 Allergens
Air sampling has provided some information on the size of indoor allergens of health signiicance,
for example, cat and dog: ∼5 μm; cockroach: ∼5 μm; and house dust mite: 10-35 μm. 234,287-291 Higher