Methyl Bromide Risk Characterization in California (2000) / Chapter Skim
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3 Exposure Assessment
3 Exposure Assessment
Pages 35-55
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From page 35... ...
risk characterization document provides exposure estimates for a wide variety of worker and resident exposure scenarios in Sections IV.B, Risk Assessment, and V.C, Risk Characterization, as well as in Appendices F-K. Summary estimates of exposure to methyl bromide, listed in Tables ~ 6-20 of the DPR report, correspond to occupational (Section {V.B.~)
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From page 36... ...
Although a variety of analytical techniques were used to determine methyl bromide concentrations in the air samples, these were not reliably tested. In addition, data were collected under different sampling protocols and field conditions (e.g., temperature, relative humidity)
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From page 37... ...
"Residential, school, and other exposures" refers to people who are exposed to methyl bromide due to its atmospheric transport from the site of direct application. This category specifically includes residents in houses, students in schools, and occupants of buildings near fumigated fields, structures, or near fixed commodity fumigation facilities.
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From page 38... ...
For example, DPR never describes or provides data on exposures to children and the elderly, who might be more sensitive to methyl bromide than the worker or general adult populations. Furthermore, DPR never addresses exposure scenarios for residents living near fumigated fields, because these homes were considered to be outside of the permit buffer zone.
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From page 39... ...
The subcommittee is not confident that under these conditions, exposures of children and adults to methyl bromide concentrations above the 6-week reference concentrations of and 2 ppb, respectively, do not, or are unlikely to, occur. Finally, DPR does not address less common exposure scenarios that might occur under unique weather and terrain conditions, such as when a low-level temperature inversion or other similar low-wind condition prevents the dilution of methyl bromide that would normally be expected to occur.
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From page 40... ...
Although the analytical method for extracting methyl bromide from the samples had been used previously, it appears that a rigorous testing of the method has not been conducted. The primary uncertainty with the analytical method centers on the procedure for recovering methyl bromide from the charcoal tubes that are used to collect ambient air samples.
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From page 41... ...
limit of the California Depa~l~ent of Food and Agriculture laboratory that did the analysis. The subcommittee is concerned about the lack of reliable recovery estimates at low methyl bromide concentrations, because the reference concentrations (RfCs)
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From page 42... ...
The location, temperature, and relative humidity for each house appears to be subject to the same variability and uncertainty as for the outdoor air-sampling studies discussed previously. The analytical data from these studies are clearly compromised by the lack of a robust analytical method for measuring methyl bromide concentrations in air.
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From page 43... ...
For air samples taken at higher temperatures, the methyl bromide concentrations are probably underestimated, potentially by a factor of 2. If, for example, the outdoor recovery values of 21% to 26% were to prove typical, then the average methyl bromide concentrations would be expected to be about double those estimated by DPR.
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From page 44... ...
Therefore, the subcommittee believes that there is considerable uncertainty about how accurately the observed measurements represent the real distributions of exposure concentrations and durations in the occupational groups that were studied. Appropriateness of Normalizing Assumptions for Different Application Rates To estimate occupational exposure levels from soil fumigation, DPR made a simple linear adjustment from the application rates used to the maximum permitted application rates.
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From page 45... ...
In the second case, if the goal of the exposure analysis is to represent exposures under the worst-case conditions permitted by the pesticide labels, then the subcommittee agrees that some adjustment for application rates should certainly be made. However, if the goal of the exposure analysis is to represent the distributions of exposure levels that actually exist for the workers, then DPR's goal should be to assure that the exposure data collected appropriately reflect the actual distribution of application rates that are used in practice.
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From page 46... ...
It allows us to use our mechanistic understanding of a system to draw inferences about exposure levels and associated risks, even in cases in which we do not have an extensive set of direct observational data. As discussed in more detail below, the subcommittee concludes that in general the basic structure of the residential indoor air dilution and outdoor air dispersion models used in the DPR exposure assessment are appropriate.
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From page 47... ...
lit also permitted the subcommittee to determine 24-hr estimates of methyl bromide concentrations to compare with the regulatory target level of 210 ppb that is assumed to apply for the 24 hr immediately following the reentry of residents into their homes. These data are presented in Table 3-1, in which the estimated average methyl bromide concentrations for 1 day and 7 days after the 24-hr ventilation period are shown, along with the standard errors.
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From page 48... ...
and the rates of decline. Because the average methyl bromide concentrations are already relatively high in relation to the regulated target level of 2 ~ 0 ppb (Table 3-1)
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From page 49... ...
This means that the data do not account for differences in varying external temperatures, wind conditions, and humidity, and possibly, house structural characteristics in different areas of California and at different times of the year. Because of the uncertainties surrounding the current data set on exposures of residents returning to fumigated homes, the subcommittee finds that DPR's conclusion that current fumigation practices result in methyl bromide concentrations that do not exceed the regulatory exposure level of 210 ppb does not seem warranted.
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From page 50... ...
, is used to calculate the size of buffer zones that are required to prevent methyl bromide concentrations at the boundary from exceeding 210 ppb. DPR usecl the 210ppb value to represent the acute exposures of residents near fumigated fields and commodity fumigation facilities in Table 19-d (DPRI999,p.
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From page 51... ...
. Additional calculations have been made by the subcommittee, including Column e, the absolute distance between sampling distance
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From page 52... ...
notes that: of the 39 applications monitored, seven exceeded the 0.21 ppm target level at the buffer zone distance...TarpauTin-bedded applications and applications using "very high barrier" tarpaulins appeared to have higher air concentrations than originally assumed in the permit conditions. Of the seven tarpaulin-bedded applications monitored, four exceeded the 0.21 ppm target level at the original buffer zone distance.
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From page 54... ...
Accordingly, the subcommittee is unable to fully evaluate the accuracy of the modeling used for estimating off-site residential exposures in the DPR report, nor can the subcommittee determine if the proposed, or even current, buffer zones actually protect nearby residents from exposures to methyl bromide concentrations greater than 210 ppb. SUMMARY The DPR report contains a large compilation of exposure data, particularly on worker exposures.
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From page 55... ...
The subcommittee believes that it is extremely important for DPR to address such exposures, considering that 95% of methyl bromide is used in soil fumigation. Furthermore, there is considerable uncertainty surrounding the analytical recovery methods used in the exposure-assessment studies.
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