Bioaerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 298

Some of the information on infectious doses for inhalation exposure has been modeled from
outbreak situations or calculated from experimental exposures and surveillance data. 52 For example,
the dynamics of measles and tuberculosis transmission have been analyzed using the concept of
infectious quanta in mass-balance equations similar to those applied to the study of other environ-
mental contaminants. 53-57 Investigations of outbreaks in which the number of infected persons, their
duration of exposure, and the indoor ventilation rate were known have allowed estimation of the
average air concentrations of infectious agents and their generation rates. Infectious doses may be as
low as ten adenoviruses or Coxiella burnetii , Franciscella tularensis , or M. tuberculosis cells, 10 2
for the SARS coronavirus, and as high as 10 3 -10 4 for Bacillus anthracis spores. 41,58,59 In animals,
the LD 50 for B. anthracis was found to increase with increasing aerosolized particle diameter. 60
Increasingly sophisticated mathematical models and data from animal experiments provide a prom-
ising approach for the assessment of human risk. 61
Modeling allows examination of the effects of environmental factors such as ventilation rate, air
cleaning, room occupancy, and respiratory protection as well as public health interventions such as
vaccination, isolation, and contact tracing. 62,63 A review of publications on the association between
building ventilation and the transmission of airborne infections concluded that there is strong and
suficient evidence to demonstrate an association between ventilation, air movement, and trans-
mission but that there is insuficient data to specify minimum ventilation requirements to prevent
aerosol spread. 64
Model-based cost-effectiveness analyses can provide mathematical projections to better under-
stand the key factors that affect outcomes, for example, dependence on the interplay among
exposure rate, transmissibility (secondary attack rate), case-fatality rate, and risk of transmission
from atypical cases. 65 A predictive model integrating inluenza viral kinetics (target cell-limited
model), indoor aerosol transmission potential (Wells-Riley mathematical equation), and a popula-
tion dynamic model (susceptible-exposed-infected-recovery model) was applied to a proposed
susceptible population to estimate inluenza infection risk. 42 Another group applied a generaliz-
able environmental infection transmission system (EITS) framework to provide a theoretical basis
for understanding and modeling intervention eficacy in realistically detailed situations involving
diverse venues where transmission may take place. 66 EITS provides a way to conceptualize the
extent to which transmission is a density- or frequency-dependent contact process, for example,
transmission through thoroughly mixed air or infrequently touched fomites generates density-
dependent patterns while transmission through frequently touched fomites, such as doorknobs, gen-
erates frequency-dependent patterns.
12.2.2 r esPiratory d iseases otHer tHan a erosol t ransMissible i nFections
Respiratory symptoms are the most common health effects associated with noninfectious bioaero-
sol exposure and generally result from airway inlammation caused by toxins, pro-inlammatory
agents, or allergens (Section 12.2.3). Many of the symptoms or conditions described in detail in the
following are not exclusively associated with bioaerosols but also can be caused by exposure to a
number of nonbiological agents, for example, cigarette smoke, ambient air pollutants, and chemi-
cals. The current section describes the most important of these respiratory diseases and symptoms
and briely discusses causal factors. The following sections discuss the health effects of speciic
biological agents in more detail.
12.2.2.1 Asthma
Asthma is a heterogeneous chronic inlammatory disorder of the airways involving airlow limita-
tion that is at least partly reversible and results in recurrent episodes of symptoms such as wheez-
ing, breathlessness, chest tightness, and cough. There is increasing evidence that the allergic
phenotype of asthma may account for only half of all asthma; therefore, exposures that induce
a nonallergic inlammatory response may play equally important roles in asthma development