Evaluation of a Site-Specific Risk Assessment for the Department of Homeland Security's Planned National Bio- and Agro-Defense Facility in Manhattan, Kansas (2010) / Chapter Skim
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3 Evaluation of Methods
3 Evaluation of Methods
Pages 27-50
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From page 27... ...
that may result in an underestimation of actual risk. The estimated rate of 2.6 laboratory-scale spills per year is low, but no confidence intervals were given for the estimate, which would have allowed some assessment of a maximum-credible risk scenario.
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The hosing of waste materials (such as secretions and excretions) would create a protective bioburden matrix for virus particles, and their aerosolization would lead to a severe underestimation of the amount and duration of potentially infectious material generated.
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The annual risk estimates presented in the SSRA assume constant annual risks over the 50-year life span of the NBAF. However, annual risks will not necessarily remain constant over time because operating practices, experimental design and equipment, and staffing -- among many other aspects -- could result in either improvements or degradations that accordingly decrease or increase risks.
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risk of release that would result in 2 The 2009 National Research Council report Science and Decisions: Advancing Risk Assessment defines cumulative risk as "the combination of risks posed by aggregate exposure to multiple agents or stressors in which the aggregate exposure is by all routes and pathways and from all sources of each given agent or stressor." 3 According to the 1994 NRC report Science and Judgment in Risk Assessment: "An essential component of risk characterization is the aggregation of different measures and characteristics of risk: the risk assessor must communicate measures and characteristics of predicted risk in ways that are useful in risk management." 4 The committee notes that the term "risk dollars" may be useful as a way of ranking risks to identify which actions have the highest payoff to mitigate the impact, but the term can easily be misinterpreted as a measure of annual mitigation costs or total dollars at risk.
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From page 31... ...
The SSRA failed to provide an overall risk estimate for the accidental release of even a single agent such as FMDv. A lower-bound estimate of the cumulative risk of release that results in FMD infection over the NBAF's 50-year life expectancy can be obtained by using the sum of two of the SSRA's scenarios with the greatest risk of FMDv release from Tables 6-3 and 6-4 (FMDv fomite personal contamination and FMDv worker with no respiratory protection)
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. MODELING CRITICAL TORNADO AND AIR DISPERSION SCENARIOS Wind and Tornado Model Used by the Site-Specific Risk Assessment The SSRA analyzes the likelihood of tornadoes in Manhattan, Kansas, on the basis of tornado data on 1950-2009 archived at the Storm Prediction Center (SPC)
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. The use of a tornado hazard model would provide a more accurate assessment that correlates tornadic wind speed with the annual probability of occurrence (or the mean return period)
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Virus particles released over a short period are modeled as a "puff" of material that increases in size as it travels from the source to a receptor. The concentration of virus particles in the puff depends on meteorological variables (such as wind speed and turbulence)
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It did not go beyond suggesting that the dose at 10 km was less than the assumed minimum ID of 10 virus particles. The SSRA responds to the Government Accountability Office's valid criticism of the EIS by recognizing that the releases occur over a short period compared with the travel time from the source of material to the receptor.
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The assumption of a minimum dose could have been avoided by calculating the integrated dose over the group of animals exposed to a puff of virus particles. That approach would have required the dispersion calculations to explicitly account for livestock distribution in computing the integrated dose.
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The SSRA demonstrates substantial effort in modeling FMD spread, given the short period of time to conduct the assessment and the absence of critical data and well-documented assumptions for many parameter values. In particular, the SSRA team made a clear and constructive effort to collect data and estimate values for model parameters, including attempts to estimate locations of livestock herds throughout Kansas and nearby states.
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Evidence is presented (page 233, for instance) that epidemics are primarily limited by density and geographic characteristics rather than by mitigation measures, but this requires further elaboration by DHS inasmuch as many disease outbreaks are contained by mitigation strategies.
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The primary justifications for assuming sales barn transport as a surrogate were concerns about model complexity and identifiability. While adding sales barns to the NAADSM is an improvement because it takes some interstate animal movement into account, sales barns are not the sole source of long-distance animal movement 10 The SSRA did not fully consider modeling disease spread across state and country borders.
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Even if use of sales barns as surrogates for all interstate transport were sufficient for estimating total epidemic size, it introduces difficulties in assessing mitigation measures. There are differences in interests, regulation, and responsiveness between sales barns and large independent cattle operations that mitigation practices should consider.
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, diagnostic support for necessary investigation and mitigation of a suspected escape may be compromised. When an FMDv release occurred in 2007 from the Institute of Animal Health Pirbright Laboratory in the UK, some FMD work at that facility was halted, including work on vaccines that could have been necessary had the FMD outbreak not been controlled (Anderson, 2008)
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By not including a scenario of escape to another state, the model allows the disease to be diagnosed and controlled only in a very small area around Manhattan that presumably has been bolstered, because of the presence of the NBAF, with an unusually high degree of passive surveillance and education, which would be expected to provide earlier detection and control than would occur in other states. Consequently, model results will be biased toward a low estimate of magnitude and duration of an FMD epidemic in the seven states studied.
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To conduct quantitative studies of the risks posed by a release of RVFV, the SSRA team developed a new, custom epidemic model, using the VenSim software package. FMD has been the focus of numerous modeling studies, but Rift Valley Fever (RVF)
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Three economic models are used in concert with cost calculations external to the modeling. Methods Employed A partial equilibrium model of the U.S.
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The estimated impacts from the partial equilibrium model are generated from the shocks introduced into that model and its parameter values. Except for the livestock products price elasticities, the parameter values are documented in the report cited in the SSRA; the price elasticity estimates for livestock products are from more recent published research.
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The SSRA description of how the regional analysis is conducted is inadequate in the text and leaves the reader to assume that it was done correctly unless proven otherwise. The specific shock introduced into the regional input-output model is not reported.
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2000. Development of a probabilistic tornado wind hazard model for the continental United States, Volume 1: Main Report.
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2005. Disease contact tracing in random and clustered networks.
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OIE (World Organisation for Animal Health)
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2002. An Outbreak of Rift Valley Fever in Northeastern Kenya, 1997-98.
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