Bioaerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 306

study found 26-fold higher concentrations of indoor culturable fungi in summer than in winter. 166
Water vapor can condense on cold indoor surfaces and this, along with releases of liquid water, may
create conditions that allow microbial growth with both qualitative and quantitative changes in the
bioaerosol composition inside buildings. 168-170
The fungal fraction of OC mass concentration has been shown to be lower in spring than summer
(2%-5% vs. 8%-14%). 140 The relative frequency of occurrence of individual fungal species, that is,
the proportion of samples in which the species were detected, has allowed comparisons of species
richness and seasonal variability for Ascomycota and Basidiomycota. 171 For example, in central
Europe the prominent allergenic ascospore groups, Cladosporium and Alternaria species, occurred
most frequently in summer and fall, whereas Penicillium species and the plant pathogen Blumeria
graminis were found most often in winter and spring. The fall maximum for basidiospores primar-
ily was due to species that were found only once during the year and can be attributed to mushroom
fruiting and enhanced plant decomposition during that season. In the extratropics, where vegetation
cover varies signiicantly throughout the year, simulated fungal emissions and air concentrations
exhibited a peak in late local summer and early fall. 142
Airborne pollen are found predominantly during their respective pollination periods from early
spring to fall when plants are producing and releasing whole grains. Spring typically is the main
pollen season for those trees and shrubs that release airborne pollen grains. Pollen dehiscence
(opening of mature anthers) occurs before foliage is fully grown, which otherwise could impair
the release and distribution of pollen grains. Normally, the primary pollen season for trees is no
longer than 2-3 weeks. Herbs and grasses can lower several times during each growing season;
thus, airborne pollen grains from these plants can be found throughout the year in snow-free parts
of the world.
Over a 27 year period in western Liguria, Italy, a progressive increase in pollen-season dura-
tion has been noted for Parietaria species (pellitory) (+85 days), olive (+18 days), and cypress
(+18 days). 172 The authors attributed this increase to an overall advance of the seasons' start dates. In
addition to a longer season, the total pollen load throughout the season has progressively increased
as well (∼25% on average) for the groups mentioned earlier but not for grasses. Percentages of
patients sensitized to these outdoor allergens increased over this 27 year period while the rate for
house dust mite sensitivity remained stable. These behaviors paralleled the constant increase in
direct radiation, temperature, and number of days >30°C. Climate changes may affect the quality
and amount of airborne allergenic pollen and the prevalence and severity of hypersensitivity and
inlammatory responses. 151,173 However, the direct assessment of such an effect requires long obser-
vation periods for restricted geographic areas. 172 Dispersion of microorganisms in dust may play
a signiicant role in the biogeographical distribution of pathogenic and nonpathogenic species as
long-range atmospheric transport routes and concentrations shift through time due to climatic and
geologic change. 174
12.3.1.1.3 Geographic Region
Bioaerosol concentrations, in particular bacterial concentrations, are generally higher in urban than
rural atmospheres in the absence of local sources such as animal houses, agricultural operations,
waste treatment plants, or composting facilities. In contrast, fungal contributions to total OC, PM 10 ,
and the coarse fraction were approximately half at a trafic-dominated urban versus a suburban
sampling site. 140 A study in Boulder, Colorado, observed more phylogenetic similarity between
bacteria collected from geographically distant sites (Colorado, Texas, Antarctica, and France) than
between bacteria collected in Boulder over 8 days. 147 From this inding the authors concluded that
outdoor air may harbor similar types of bacteria regardless of location and that short-term tem-
poral variability can be large and must be considered when designing a sampling plan. Another
study found that bacterial diversity was higher in a rural than in urban and alpine locations. 146 The
distribution of pollen-producing plants is a result of natural loristic patterns, but landscaping has
signiicantly changed the air biota in many parts of the world.