A Framework for Assessing the Health Hazard Posed by Bioaerosols (2008) / Chapter Skim
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2 Exploring the Complexity of Health Risk
2 Exploring the Complexity of Health Risk
Pages 15-28
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From page 15... ...
are currently grouped into three categories: bacteria, viruses, and toxins. 2.1 BIOLOGICAL WARFARE AGENTS 2.1.1 Bacteria Bacteria are single-celled microorganisms that are capable of replication independent of other living cells.
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Knowing whether the aerosol particle can establish an infection in a host cell culture or animal model is critical to determining the health hazard. Methods that are specific to a number of types of virus have been developed to characterize biological activity.
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These assays involve surrogate host cells and animal models that are meant to simulate or predict infectivity, morbidity, and mortality in humans. The above brief description of viruses, methods by which they can be characterized, and the challenges in predicting their impact on human health illustrates the difficulties associated with developing a unit of measure for virus-containing aerosol particles.
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Toxin-containing aerosol particles present a relatively simpler case for characterization than that of bacteria or viruses. Nonetheless, complexity exists in host response depending deposition site in the respiratory tract, so aerosol particle size remains important.
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For aerosol measurements, recent developments in air-to-air particle concentrators, particle charging and manipulation, and selective particle collection may also augment current BW detection system capabilities. For the purposes of this report, it is important to remember that whichever collection method is used and whichever physical characteristics of the agent are measured, it is necessary to be able to relate the measurements made by a detector to the health hazard posed by the aerosolized agent.
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The longer a particle remains airborne, the greater the potential for human respiration, so the aerodynamic diameter is directly related to risk of bioaerosol exposure. Diameter also greatly influences where a particle will be deposited in the 2 Birch and other tree pollens have been detected in Finland weeks before the trees there begin to flower, but after flowering has occurred farther south in Europe.
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segregate or measure particles in a number of size fractions from which more detailed assessments of respiratory tract deposition patterns can be determined. 2.2.2 Deposition in the Human Respiratory Tract Particle size is a major factor determining the probability that an inhaled particle will be deposited in a specific region of the human respiratory tract.
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In the battlefield, oral breathing is likely during light to heavy workload, increasing the risk of lung deposition for agent-containing particles of all sizes. 2.3 BIOLOGICAL EFFECTS OF INHALED PARTICLES Because particle size determines where aerosol material deposits in the respiratory tract, it is an important factor in predicting the health consequences of exposure to aerosolized bacteria, viruses, or toxins.
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region increases as particle size increases up to 8 µm. ET deposition also increases when particle size decreases from 0.02 µm.
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Even though deposition patterns in humans and various animal species differ because of variations in respiratory tract anatomy and breathing dynamics, experimental studies are in agreement that the infectivity of an aerosolized pathogen is greatest when it includes significant numbers of 1-5 µm particles that can reach the bronchioles and alveolar spaces.(Druett et al. 1953; Druett et al.
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A 12 µm MMAD aerosol actually produced septicemia more rapidly, but without causing pulmonary disease.(Druett, Henderson, and Peacock 1956) Experiments using aerosolized Brucella suis ranging in size from single organisms through 12 µm particles showed that the smallest particles were 600-fold more likely to initiate disease, presumably because they were able to replicate more readily after depositing on the vast exposed surface of the pulmonary alveolar epithelium.(Druett et al.
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1953) The investigators found that different combinations of particle size and the number of spores per particle could produce similar health effects, and concluded that " to achieve the same mortality, the total number of spores presented to the animal, in particles of a given size, must be the same, irrespective of the number of particles carrying them." This result suggests many different factors contribute to the nature of the disease produced by an aerosolized microbe or toxin, including the identity of the agent, the vulnerability of the host, and the size of the aerosol particles.
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The collection system can be conceptualized mathematically as a transfer function that blurs or integrates over a range of particle sizes. Therefore, performance of a detection system depends on the distribution of particle sizes, and this system bias should be determined as part of test and evaluation.
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For aerosolized particles with biological content, the aerodynamic diameter is a critical parameter in determining the final site of deposition in the respiratory tract and must be considered. In light of these limitations, the committee developed a new, robust framework for evaluating the health hazard posed by biological aerosols.
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