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Methyl Bromide Risk Characterization in California (2000)

Chapter: Appendix C: Calculation of Air Exchange Rates

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Suggested Citation:"Appendix C: Calculation of Air Exchange Rates." National Research Council. 2000. Methyl Bromide Risk Characterization in California. Washington, DC: The National Academies Press. doi: 10.17226/9849.
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Page 90
Suggested Citation:"Appendix C: Calculation of Air Exchange Rates." National Research Council. 2000. Methyl Bromide Risk Characterization in California. Washington, DC: The National Academies Press. doi: 10.17226/9849.
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Page 91
Suggested Citation:"Appendix C: Calculation of Air Exchange Rates." National Research Council. 2000. Methyl Bromide Risk Characterization in California. Washington, DC: The National Academies Press. doi: 10.17226/9849.
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Page 92
Suggested Citation:"Appendix C: Calculation of Air Exchange Rates." National Research Council. 2000. Methyl Bromide Risk Characterization in California. Washington, DC: The National Academies Press. doi: 10.17226/9849.
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Page 93

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Appendix C Calculation of Air Exchange Rates Air exchange rates are defined in terms of a general one-compartment model of air exchange with immediate and perfect mixing of air inside residences. For any contaminant in the assumed well-mixed pool of air in the living spaces, this leads to an expectation of simple exponential decline of air con- centrations with time: C(t) = C(O)e~kt, where C(O) is the initial concentration of the contaminant inside the house, C(t) is the concentration of the contaminant at any specific time after t = 0, and k is a rate constant in units of reciprocal time (i.e., if time is expressed in hours, k is in reciprocal hours, or, by convention, "air changes per hourly. The relationship between the rate constant k and the half-life (the time re- quired to reduce the air concentration by haTi) is easily derived by setting C(t) to one-half of C(O): C ~ t ~ , 2 ~ = . 5 C ~ O ~ = C ~ O ~ e - ~ / 2 After the cancellation of the C(O)'s, and taking the natural logarithm of both sides of the equation: In(.5) = - kit, to= In(2~/k or k = In(2~/t~,2 90

PUBLIC ACCESSMATE~ALS 9] ILLUSTRATIVE LOGNORMAL TREATMENT OF DATA FOR SELECTED OCCUPATIONAL EXPOSURES Figure A- ~ shows Tognormal probability plots of the individual data points for several groups of workers in the shallow-shank tarp method application of methyl bromide. In this type of plot, correspondence of the points to the re- Lognormal plow of adjusted 24-hr exposure data for shallow shank-brped soil fumigation workers / 3.0 - o o 2.0 l 1.5 1.0 ,'~ / it/ _' I' . by/ / ~ Add/ o,,y/,: ~~ ~7 // a,./- ~ // ~',,~W y = 2.24 ~ 0.374x R2 = 0.886 C] Copilots y= 1.91 +0.426x R2=0.977 · Applicators y= 2.06 ~ 0.284x R2=0.880 ~ Shovelmen y = 2.93 ~ 0.226x R2 = 0.963 ~ Tarp removem , -2 -1 0 1 2 score FIGURE A-1

92 METHYL BROMIDE RISK CHARACTERIZA TIONIN CALIFORNIA gression line is a quick qualitative indicator of the degree to which the data points are well described by the chosen distribution. in these cases, the fits are far Mom perfect, suggesting some possible heterogeneity in the data, but the Tognormal plots in Figure AM are generally better than corresponding nor- mal distribution fits (Figure A-2~. For these same worker groups, Table A-! below compares the reported highest observed values with 95th percentile values calculated from the fitted nominal and lognormal distributions. In gen- eral, the lognor~nal fits project somewhat higher 95th percentiles than the nor- mal fits. TABLE A-1 Comparison of Observed Values with 95~ Percentile Values . Occupational Group Nu 7~7 Data Points Observed Acute Calculated Calculated from (Including (24 fir) Exposure from Normal Lognormal Fit Non-Detects) Fit 479 293 330 1820 Copilots Applicators Shovelmen Tan' Removers 7 8 10 s 518 303 515 _659 716 408 337 1990

PUBLIC ACCESS MA TERIALS 93 Dam from "MB exposure data" y = 223.71 ~ 155.22x R2 = 0.890 y = 102~49 ~ 11614x R2 = 0.933 y = 147.00 ~ 111.49x R2 = 0.605 y = 873.75 ~ 574.70x R2 = 0.911 ,' o = Q o ~ / 1000 - O - / /A Q 2 -1 0 1 2 Z-Score Figure A-2 O C - be , ... LL 8 ~ T"

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Methyl bromide is gaseous pesticide used to fumigate soil, crops, commodity warehouses, and commodity-shipping facilities. Up to 17 million pounds of methyl bromide are used annually in California to treat grapes, almonds, strawberries, and other crops. Methyl bromide is also a known stratospheric ozone depleter and, as such, is scheduled to be phased out of use in the United States by 2005 under the United Nations Montreal Protocol. In California, the use of methyl bromide is regulated by the Department of Pesticide Regulation (DPR), which is responsible for establishing the permit conditions that govern the application of methyl bromide for pest control. The actual permits for use are issued on a site-specific basis by the local county agricultural commissioners. Because of concern for potential adverse health effects, in 1999 DPR developed a draft risk characterization document for inhalation exposure to methyl bromide. The DPR document is intended to support new regulations regarding the agricultural use of this pesticide. The proposed regulations encompass changes to protect children in nearby schools, establish minimum buffer zones around application sites, require notification of nearby residents, and set new limits on hours that fumigation employees may work. The State of California requires that DPR arrange for an external peer review of the scientific basis for all regulations. To this end, the National Research Council (NRC) was asked to review independently the draft risk characterization document prepared by DPR for inhalation exposure to methyl bromide.

The task given to NRC's subcommittee on methyl bromide states the following: The subcommittee will perform an independent scientific review of the California Environmental Protection Agency's risk assessment document on methyl bromide. The subcommittee will (1) determine whether all relevant data were considered, (2) determine the appropriateness of the critical studies, (3) consider the mode of action of methyl bromide and its implications in risk assessment, and (4) determine the appropriateness of the exposure assessment and mathematical models used. The subcommittee will also identify data gaps and make recommendations for further research relevant to setting exposure limits for methyl bromide.

This report evaluates the toxicological and exposure data on methyl bromide that characterize risks at current exposure levels for field workers and nearby residents. The remainder of this report contains the subcommittee's analysis of DPR's risk characterization for methyl bromide. In Chapter 2, the critical toxicological studies and endpoints identified in the DPR document are evaluated. Chapter 3 summarizes DPR's exposure assessment, and the data quality and modeling techniques employed in its assessment are critiqued. Chapter 4 provides a review of DPR's risk assessment, including the adequacy of the toxicological database DPR used for hazard identification, an analysis of the margin-of-exposure data, and appropriateness of uncertainty factors used by DPR. Chapter 5 contains the subcommittee's conclusions about DPR's risk characterization, highlights data gaps, and makes recommendations for future research.

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