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3 Committee's Review of the DoD-O'Flaherty Model
Pages 13-29

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From page 13...
... In its review, the committee focused on the appropriateness of the compartmental structure of the model and processes to represent various aspects of occupational exposure and dynamic background lead concentrations in the context of relating worker lead exposure to BLLs. Prior use of the model for contextually similar analyses and prior review of models was considered evidence of model appropriateness.
From page 14...
... The committee considered whether changes might affect the representation of physiological compartments or processes that either alone or together impact lead biokinetics. The committee also considered changes made for supporting model operations, such as Monte Carlo analysis.
From page 15...
... modified elements of the model structure and several model parameters to add Monte Carlo analysis functionality and incorporate findings from additional studies, but these changes necessitated minimal calibration of the model. The Bayesian approach is an alternative to the model calibration strategy used by O'Flaherty (1993)
From page 16...
... Though a comprehensive check of the correspondence between the code implementation and the external documentation of the DoD-O'Flaherty was not conducted by the committee, it performed several checks when questions arose about the code, particularly regarding the Monte Carlo analyses, and the committee found that the documentation faithfully reflected the implementation in the code. However a comprehensive error check of the model code is an important aspect of developing a biokinetic model for regulatory application.
From page 17...
... . BOX 3-1 Threshold Limit Values (TLVs)
From page 18...
... For particles that deposit in the upper airway -- a function of particle size -- and are later swallowed, conditions in the GI tract also influence the ITC. Particle size is, therefore, an important determinant of the ITC in lead biokinetic modeling and particle sizes can vary significantly in the occupational environment.
From page 19...
... The use of a reliable method to sample the inhalable particle size fraction of airborne lead is an important aspect of estimating BLLs from airborne lead concentrations. The 37-mm plastic cassette is the typical sampling method used in the United States and many other countries for measuring airborne lead concentrations.
From page 20...
... Because dietary intake of lead tends to be the largest source of background lead exposure, estimates of the magnitude of the dietary component can have a substantial effect on model estimates of non-occupational lead concentrations. Previous versions of EPA's Air Quality Criteria for Lead (EPA 1977, 1986)
From page 21...
... In addition, DoD should consider reviewing the 1977 and 1986 EPA Air Quality Criteria for Lead to determine if using a lower dietary lead concentration for the pre-1980 background exposures would be more appropriate than those currently used in the DoD-O'Flaherty model. Inhalation Rates A key challenge for modeling DoD occupational lead exposure scenarios is to estimate long-term average daily lead intake via inhalation by using inhalation rates that adequately represent an expected range of activity patterns across the TriServices.
From page 22...
... The data sources and general approach for developing the probability distributions of inhalation rates are reasonable. However, DoD should consider conducting additional Monte Carlo simulations at the candidate OELs using a distribution of inhalation rates (and cardiac outputs)
From page 23...
... ; it is unclear if even the upper percentiles of a distribution derived from these data are sufficiently representative of a distribution of inhalation rates for a receptor group that routinely engages in more strenuous activities (e.g., military at a firing range)
From page 24...
... Gender Distribution of Exposed Worker Population The distribution of men and women is a critical input into the Monte Carlo simulations because biokinetic differences between the genders produce lower BLLs for women for a given air lead exposure 24
From page 25...
... The gender distribution selected by DoD based on 2016 information on lead exposed DoD workers documented in a DoD database is sufficiently representative of the worker population for developing candidate OELs. Randomness in Birth Year Within the DoD-O'Flaherty model, an individual's year of birth has a strong influence on BLLs through several factors: • Amount of accumulated lead in the body owing to historical changes in exposures to background concentrations of airborne lead and the duration of potential occupational and background expo sure; • Body mass, and hence the associated mass of individual tissues; • Rate of change of body mass; and • Number of factors associated with the bone (a reservoir for lead)
From page 26...
... Changing inhalation rate, without corresponding physiologically accurate changes in cardiac output and GFRs, could establish unrealistic scenarios in which a lead dose rate increases but lead elimination through a correlated process decreases, instead of increasing. A main question is whether either issue would change the final distributions of BLLs for a given airborne lead concentration used to produce the final BLL distribution.
From page 27...
... Characterization of DoD Worker Populations Exposed to Lead at the Candidate OELs To inform occupational health managers, Sweeney (2019, Section 4.4) presented simulations to illustrate the predicted time series of BLLs in DoD workers resulting from exposures to various airborne lead concentrations.
From page 28...
... The information topics included: the DoD-O'Flaherty model structure, changes DoD made to the 2000 version of the O'Flaherty model, the basis for DoD's estimated average removal duration for DoD workers, who exhibited elevated BLLs; DoD job activities that have the potential to result in lead exposure; modeled exposure scenarios; and approaches for selecting model parameters for the Monte Carlo analyses.
From page 29...
... The committee recognizes DoD's leadership in applying an innovative approach for establishing an OEL for lead using modern biokinetic modeling to develop quantitative relationships between occupational exposure and BLLs. Overall, the committee found that the DoD-O'Flaherty modeling approach and application to support the development of an OEL for lead are appropriate.


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