Bioaerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 303

12.2.4 P otential P rotective e FFects oF M icrobial a gents on a llergies and a stHMa
The “hygiene hypothesis” postulates that growing up in a more hygienic environment may increase
the risk of developing allergies. 11 This hypothesis was developed to explain epidemiological indings
that overcrowding and unhygienic conditions were associated with a lower prevalence of allergies,
eczema, hay fever, and asthma.
Conirming indings from cross-sectional studies of signiicant inverse associations between
indoor endotoxin concentrations and atopic sensitization, hay fever, and atopic asthma, a prospec-
tive birth cohort study found an inverse association between endotoxin and fungi in dust at 3 months
of age and doctor diagnosed asthma and persistent wheeze at age 4. 10 For infants born to atopic
parents in another study, indoor exposure to high β-(1 → 3)-d-glucan concentrations (>60 μg/g) was
associated with a decreased risk for recurrent wheeze. 12,118
However, several large studies have not found a protective effect or even have seen a positive
association between less hygienic environments and respiratory health. For instance, the U.S.
National Survey of Allergens and Endotoxin in Housing found an endotoxin exposure-dependent
increase in diagnosed asthma, wheeze, and asthma medication use in adults. 13 Animal studies also
have shown mixed evidence regarding the possible protective effect of endotoxin for atopy and
airways hyperreactivity. 68 Part of these inconsistencies may be related to the timing of exposure,
with early life exposures protective and exposures later in life a risk factor for these conditions.
Alternatively, it may be that endotoxin exposure prevents allergic asthma but at higher exposures
causes nonallergic asthma. 119
There are other limitations of the current hygiene hypothesis. In particular, although housing
conditions in the United States have not become more hygienic for inner city populations, their
asthma prevalence has increased signiicantly. The rise is greatest among African Americans living
in poverty, which is in contrast to previous indings showing a positive association between aflu-
ence and asthma prevalence. Also in the past few years, asthma prevalence appears to have dropped
in many high income countries with no indication that hygiene has deteriorated. 71 Further, epide-
miological studies have assessed exposure to a limited suite of bioaerosols and the causal agents
responsible for immunomodulation may remain unknown.

12.3 SOURCES AND EXPOSURES

12.3.1 b ioaerosols FroM o utdoor s ources
A variety of environments, including soil, surface water, and plants, are natural reservoirs of bio-
logical agents. In number, bacteria are the most abundant organisms in surface soils, where their
concentration can exceed 10 10 cells/g, of which typically 10 7 -10 8 cells are culturable on standard
media. 120,121 Total emissions of bacteria to the atmosphere have been estimated to be 1400 Gg/year
with an upper bound of 4600 Gg/year, originating mainly from grasslands, shrubs, and crops. 122
Estimates of 590 and 100,000 bacterial species/g of soil have been obtained through empirical mod-
els of species frequency distribution. 123 Protozoa and algae are also abundant in soil (10 3 -10 6 cells/g)
along with fungi (typically 10 5 -10 6 cells/g), but the overall biomass of fungi is greatest because of
their larger cell sizes.
Soil microorganisms can be aerosolized during mechanical disturbances, such as the tilling
of agricultural ields, soil excavation, the mechanical turning of compost, landill site operation,
and dust storms. 124,125 Many plant pathogens are transmitted by air, for example, during crop har-
vesting, but these agents do not appear to pose a hazard for people. However, some human infec-
tions result from exposure to soil dust contaminated with infectious bacteria or fungi, for example,
Burkholderia pseudomallei : melioidosis; Aspergillus species: aspergillosis, Coccidioides immitis :
coccidioidomycosis; Cryptococcus neoformans : cryptococcosis; and Histoplasma capsulatum : his-
toplasmosis (Table 12.1).