Bioaerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 299

and exacerbation. 67,68 In the last few decades, Western countries have experienced an epidemic of
asthma prevalence and incidence that appears to have commenced after the Second World War and
has only recently peaked and begun to decline for reasons not fully understood. 69 WHO estimates
that some 300 million persons currently suffer from asthma, which is the most common chronic
disease among children. 70 In 2010, the annual economic cost of asthma in the United States was esti-
mated to be >$15 billion in direct costs and >$5 billion in indirect costs such as lost productivity. 71
Viral infections are a common cause of asthma exacerbation. 72 In fact, viral respiratory infec-
tions are detected in the majority of asthma exacerbations (80%-85% in children and 75%-80%
in adults); of these, ∼60% are from rhinoviruses. 73 There also is a strong association between viral
infections and hospital admission for asthma in both children and adults. In addition, viral infec-
tions may be involved in asthma development, but the evidence is less clear. 74-76 The mechanisms
of viral-induced asthma are poorly understood but it has been speculated that impairment of innate
immune responses may play a crucial role. 73
The environmental exposures most commonly associated with asthma in the general population
include a wide range of bioaerosols such as nonallergenic microbial agents including bacterial endo-
toxin; 68,70 house dust mite, pet, and cockroach allergens; 77 and allergenic and nonallergenic fungal
agents (Section 12.3.2). 78,79 Many of these exposures are elevated in damp indoor environments.
Several structured expert panels have reviewed the literature on dampness and asthma and have
concluded that there is suficient evidence of an association between exposure to damp environ-
ments and asthma symptoms and exacerbation in sensitized persons but that for asthma develop-
ment there is only limited or suggestive evidence. 70,80 Another expert panel, investigating housing
interventions concluded that there is suficient evidence that mold and moisture remediation reduces
exposure to asthma triggers. 81
It is commonly assumed that fungal exposure plays a major role in explaining the association
between damp homes and asthma, but the evidence is based primarily either on self-reported or
investigator-assessed visible mold and water damage. Therefore, it is unclear whether fungi and
other microbial agents are causally related to asthma or simply are markers of dampness and
that nonmicrobial dampness-related agents are responsible. Of those studies that have included
quantitative exposure measurements, few have found signiicant associations, 15,82,83 which may be
due to problems related to exposure assessment (Section 12.6.1). 70,78 However, a recent publica-
tion was the irst study showing an association between quantitatively measured mold and asthma
development. 84 Early exposure to molds at one but not 7 years of age signiicantly increased the risk
for asthma at 7 years of age. If these associations are causal and fungi play a major role, then mold
exposure has been estimated to be associated with an approximately 30%-50% increase in a variety
of respiratory and asthma-related health outcomes. 80 The same authors estimated that, if reported
associations were causal, 21% of current asthma cases in the United States would be attributable to
dampness and mold in housing, for an annual cost of $3.5 billion. 85 Thus, if the underlying asso-
ciations identiied in epidemiological studies truly are causal, dampness and mold have enormous
health and social costs worldwide.
Bioaerosol exposure is also a cause of occupational asthma, for example, agricultural and related
industries, waste recycling, wood processing and furniture making, biotechnology and primary
enzyme production, and the bakery industry (Section 12.3.3). 86 More than 250 agents have been
identiied as causes of occupational asthma, 87 divided broadly into those that may cause allergic
asthma (e.g., enzymes, latex, and other allergens from microorganisms, animals, and plants) and
inlammatory agents that may cause nonallergic asthma (e.g., bacterial endotoxin, bacterial DNA,
and possibly (1 → 3)-β-d-glucans).
In the occupational literature the nonallergic phenotype is sometimes referred to as “irritant-
induced asthma” or “asthma-like syndrome.” Occupational asthma is the most common work-related
respiratory disease in developed countries, with approximately 10%-15% of cases of adult asthma
attributable to occupational factors. 88 Estimates of the total proportion of adult asthma thought to be
occupational in origin range from 2% to 15% in the United States, 15% in Japan, 89 5% in Spain, 90