Bioaerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 315

In Japan, plant antigens were found in two particle size ranges (0.5-1.4 and 29-40 μm, the latter the
size of intact pollen grains) 315 as have been ragweed and grass pollen. 316-318 The antigenic activity of
the smaller particle size fraction was eight times greater than that of the larger fraction, 315 suggest-
ing a greater allergenicity for the smaller particles (Section 12.3.1).
There is evidence that during thunderstorms, pollen is concentrated by changes in airlow, the
grains are ruptured by osmotic shock, and each grain releases hundreds of allergen-containing
starch granules that are small enough to be respired. 173,319-322 This hypothesis has been conirmed in
laboratory studies. 323 A signiicant interaction has been seen between the effects of grass pollen and
weather conditions on emergency room visits for asthma in England, with the increase most marked
on days of light rainfall. 324 A relationship also has been noted between thunderstorms and asthma
admissions in Canada. 325 However, while fungal air concentrations increased twofold, changes in
pollen and air pollutants on thunderstorm days were relatively small.
12.5 RESPIRATORY DOSIMETRY
Exposure refers to the amount of a biological, chemical, or physical agent to which persons come
into contact in such a way that they develop the relevant outcome, which may be adverse (e.g., a
respiratory disease) or beneicial (e.g., immunity). 27 For bioaerosols, exposure usually is measured
as the air concentration of an agent and the time the person spends in the contaminated environ-
ment; and respiratory dose refers to the amount of the biological agent that deposits in the airways.
In some situations measurements of external exposure and internal dose may be strongly correlated,
for example, in some occupational environments. In other situations, air concentration can vary
widely due to an individual's lifestyle and activities, and dose may be correlated only weakly with
measured air concentration, for example, house dust mite allergen.
Bioaerosol exposures may be chronic if they occur repeatedly over extended time periods, for
example, as experienced by the occupants of microbially contaminated buildings. Acute exposures,
on the other hand, take place over relatively short time intervals, often during periods of elevated
air concentrations, for example, in agricultural and manufacturing workplaces and outdoors during
peak pollen seasons (Section 12.3.1.1). The risk of becoming sensitized to an allergen may be much
greater if the exposure is prolonged or intense, therefore, measurement of total cumulative exposure
may be important to understand a dose-response relationship. On the other hand, once a person has
become sensitized, the intensity of an allergen exposure may be crucial in provoking a response
even if the encounter is brief.
More accurate estimations of bioaerosol exposures can be made if both particle size and
breathing-zone air concentration are known so that the dose of an agent that deposits in a particular
area of the respiratory tract can be determined (Section 12.5.1). Indoor air quality data have been
coupled with a compartmental lung model to estimate indoor/outdoor/personal exposure relation-
ships for airborne fungi. 155
12.5.1 r esPiratory d ePosition and c learance
Generally the same physical principles govern the respiratory deposition of biological and nonbio-
logical particles. A semiempirical model has been applied to size distribution data from homes to
predict that for nasal breathing 30%-50% of fungal spores would deposit in the nose and 30%-40%
in the alveoli, whereas for oral breathing 70% would deposit in the alveoli. 283 However, some aspects
of bioaerosols may cause inaccuracies in modeling respiratory deposition. For example, the size of a
hygroscopic particle can increase after it has been inhaled (Section 12.4.1). A 30% increase in spore
size due to hygroscopic growth has been calculated to cause a 20% increase in respiratory deposi-
tion, primarily in the bronchi. 274 Likewise, uncertainty in the measurement of biological particle
size can introduce inaccuracy in dose estimation. For example, errors in size measurement were
found to result in overestimation of respiratory deposition of C. cladosporioides spores by a factor