Methyl Bromide Risk Characterization in California (2000) / Chapter Skim
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4 Risk Characterization
4 Risk Characterization
Pages 56-69
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From page 56... ...
used in the risk characterization, presents the exposure assessment in the form of tables of exposure measurements for different occupational and residential exposure categories, and presents margins of exposure for each of those categories based on the critical NOAELs and the exposure measurements. in Section V, "Risk Appraisal," DPR addresses the uncertainties in the toxicological and exposure databases, discusses the factors used for intraspecies and interspecies extrapolation, and discusses issues related to the Food Quality Protection Act.
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DPR has addressed each ofthese risk assessment components in its risk characterization document. In the sections below, the subcommittee reviews DPR's presentation of the information it gathered and analyzed in assessing the risk to agricultural workers and the general population from methyl bromide exposures.
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Because these were higher than the human equivalent NOAEL calculated from the rabbit developmental study, an RfC was not calculated from this study. The database for the subchronic studies appears to be quite extensive; there were numerous studies that DPR had an opportunity to evaluate to select a critical study for subchronic toxicity.
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Chronic Toxicity Database The existing database identified by DPR for derivation of an RfC for chronic toxicity includes two well-conducted chronic studies with different species, supported by subchronic studies in several species, a two-generation reproduction study, other data on developmental and reproductive effects, and pharmacokinetic studies employing inhalation exposure. The subcommittee considers the database available for derivation of a chronic RfC for methyl bromide good and neither of the key studies had major inadequacies.
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Developmental Neurotoxicity Methyl bromide is clearly a neurotoxicant in human adults; neurotoxic signs are prominent following high-level human exposures and one study suggests that mild neurotoxic effects might also occur at Tow levels (Anger et al.
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Although the 50°/O recovery adjustment used by DPR appears to be reasonable for many of the samples, the subcommittee considers it likely that the actual concentrations in air are underestimated rather than overestimated. The measured exposure data for any one occupational exposure category are variable and sparse and nonexistent for residents living near fumigated fields.
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The subcommittee agrees with DPR that it is appropriate to have a subchronic exposure category to describe worker exposures in preplan" soil fumigation and commodity fumigation, and that a subchronic category might also be appropriate for residents of fumigated houses or residents who live near fumigation facilities. As outlined in Section TV ofthe DPR report (Table 19, p.
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From page 63... ...
There does not appear to be chronic nonoccupational exposures for any populations associated with field agricultural applications, because the application of methyl bromide is on a seasonal basis, not year round. However, the subcommittee believes that chronic nonoccupational exposures could be possible for residents near commodity-fumigation faciTities or transport facilities, where fumigation of commodity storage warehouses or shipping containers might occur on a year-round basis.
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The level of concern for safe or dangerous exposure is an MOE of 100; when MOEs are greater than 100, the populations are assumed to have little risk of adverse effects, and when the MOEs are less than 100, there is a cause for concern for potential adverse effects. DPR appears to be asking, "Do the single-day exposure data that have been directly observed for particular groups, such as applicators, indicate that when methyl bromide is used at the maximum permissible application rate, these workers or residents will be exposed to concentrations that provide a less than a 100-fold margin below the projected humanequivalent NOAEL for acute exposures (21 to 45 ppm)
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Categories with More than Two Data Points DPR has summarized acute exposures for worker categories when there are more than two data points as the range of the data directly observed (after
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to assemble greater numbers of data points for basic statistical treatment within groups. UNCERTAINTY ISSUES In Section V, "Risk Appraisal," of the DPR report, DPR discusses the limitations of its risk characterization for methyl bromide and how it quantitatively and qualitatively dealt with the specific uncertainties The subcommittee comments upon these limitations and DPR's approach to them below.
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The two-generation rat reproduction study (American Biogenics Corporation 1986) and the rabbit and rat developmental toxicity studies (Breslin et al.
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Given that the NOAELs used for the various exposure scenarios are already quite conservative, the subcommittee felt that an additional safety factor for infants and children was not necessary. Multiple Exposures Although DPR acknowledges that workers might receive multiple exposures from methyl bromide, there is only a limited discussion on the potential exposure of residents who live in areas where multiple fields might be fumigated simultaneously or within a short period of time.
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The subcommittee found this approach to be reasonable for determining which workers or residents are likely to be exposed to potentially harmful methyl bromide concentrations. However the subcommittee had concerns about DPR's use of MOEs for risk characterizations and for protecting nonworkers, in particular, people living near fumigated fields.
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