Bioaerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 305

12.3.1.1 Seasonal, Diurnal, and Geographic Variability in
Ambient Bioaerosol Concentrations
ARIs follow regular seasonal patterns. For example, prior to the World Trade Center attack, deaths
in the United States from pneumonia and inluenza peaked predictably on February 17 ± 2 days.
However, following September 11, 2001 the season was delayed due to travel restrictions and
decreased air travel. 148 Domestic air volume in November (related to the Thanksgiving holiday in
the United States) predicted the rate of inluenza spread, and international travel inluenced the tim-
ing of lu mortality.
Fluctuations in bioaerosol concentrations are inluenced by climate and weather (resulting in
diurnal and seasonal cycles) and by local sources (resulting in regional variation). For example, hot,
dry conditions, length of drought, and mean annual rainfall have been associated with the incidence
of coccidioidomycosis caused by a soil fungus. 149,150 Weather also affects respiratory allergic symp-
toms directly, by acting on the airways, and indirectly, through episodic luctuations in aeroallergen
and air pollutant concentrations. 151
12.3.1.1.1 Time of Day
Diurnal patterns in bioaerosol concentrations are caused by changes in RH and air and surface
temperatures as well as luctuations in wind speed and turbulence, all of which affect the emis-
sion, suspension, and removal of pollen grains, fungal spores, and other bioaerosols from the atmo-
sphere. 145,152,153 Many plants (e.g., mugwort and sorrel) have their peak hour of pollen release just
before noon whereas other plant species (e.g., grasses) peak in the early afternoon. 153 Diurnal cycles
also have been observed for bacterial and fungal air concentrations (maxima in the early morning
and before nightfall), but patterns are species and microclimate speciic. 154 In a tropical rainforest
during the wet season, fungi as a percentage of coarse PM have been estimated at ∼25% during the
daytime and ∼45% at night. 141 Modeling estimations indicate that the highest fungal deposition dose
would occur between 11 p.m. and 5 a.m. and that the greatest number of spores would deposit in the
alveolar-interstitial region. 155
A northern California study compared the size characteristics of PM, protein, endotoxin, and
(1 → 3)-β-d-glucan indoors and outdoors simultaneously. 156 The largest mass fraction of protein was
associated with particles <2.5 μm, while the largest mass fractions of endotoxin and glucan were
associated with the coarse fraction. PM 10-2.5 concentrations of protein, endotoxin, and glucan tended
to be higher indoors than outdoors. Indoor pets were associated with elevated indoor PM and bio-
aerosols. However, there were no statistically signiicant associations between bioaerosols and dust
levels in homes, suggesting that dust loading may not serve as a satisfactory surrogate for human
inhalation exposure. Outdoors protein concentration, as a marker of primary biological particles,
was observed to follow the same temporal patterns as PM 2.5 concentration. 157
12.3.1.1.2 Time of Year
Seasonal patterns in outdoor bioaerosol concentrations are caused by temperature, moisture avail-
ability, and hours of daylight. Studies have found that culturable bacteria are more prevalent in
summer than winter in regions where dry, dusty summer conditions and associated agricultural or
human activities contrasted with wet winter conditions and snow cover. 158-162 A study in Colorado
observed higher concentrations of total airborne bacteria in summer than in winter both indoors and
outdoors, higher endotoxin concentrations outdoors in summer, but no seasonal endotoxin differ-
ence indoors. 163 Indoor bacterial concentrations typically are higher than outdoor levels throughout
the year and are associated primarily with human shedding, whereas fungal concentrations are
slightly lower indoors in buildings without interior sources. 164
Outdoor fungi are dominated by local sources and are often found year-round with maxima in
spring, summer, or fall. 165-167 Particularly in temperate northern regions, outdoor concentrations of
fungi decrease in winter due to subfreezing temperatures and snow cover, for example, a New York