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1
Introduction
BACKGROUND
Methyl bromide is a gaseous fumigant that kills insects, rodents, nematodes,
weeds, and organisms that cause plant diseases. It is used for pest control in
structures such as warehouses, ships, freight cars, and homes, in preplan"
treatment of soil, and in post-harvest treatment of commodities. Between
1993 and 1997, 14 tol7 million pounds of methyl bromide were used annually
in California. Methyl bromide is released into the air during and after its use,
and therefore, inhalation exposure to agricultural workers and the general
population is of considerable concern. The primary health effect of acute
methyl bromide exposure is neurotoxicity.
Methyl bromide is a Class T ozone depleter and, as such, it is regulated by
the Clean Air Act and the United Nations Montreal Protocol. It is scheduled
to be phased out of use in the United States by 2005. Because of its toxicity,
several federal agencies have established inhalation exposure levels for
methyl bromide. The U.S. Environmental Protection Agency (EPA) reference
concentration is 5 x 10-3 milligrams per cubic meter (mg/m3) (~.3 parts per
billion). The Agency for Toxic Substances and Disease Registry has minimum
risk levels of 50, 50 and 5 ppb for acute, intermediate, and chronic exposure
scenarios, respectively. The Occupational Safety and Health Administration
has an 8-fur time-weighted average permissible exposure limit (PEL) of 20
parts per million (ppm), whereas CaTifornia's PEL is 5 ppm, with a ceiling of
20 ppm (Title 8, California Code of Regulations 1998, Section 5155~. For
structural fumigation in California, the reentry level is 1 ppm within wall
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INTRODUCTION 9
voids (i.e., the cavity inside of walls). The American Conference of Govern-
mental Industrial Hygienists (ACGTH) has a Threshold Limit Value of ~ ppm
(3 .89 mg/m3) (ACGTH ~ 997), and the National Institute of Occupational Safe-
ty and Health has an immediately-dangerous-to-life-or-health level of 250
ppm.
CALIFORNIA REGULATIONS
In California, the use of methyl bromide is regulated by permit conditions
because it is classified as a restricted material. The California Department of
Pesticide Regulation (DPR) develops the permit conditions based on analyses
of exposure and toxicity data. The permit conditions specify the minimum
mitigation measures that must be used when applying methyl bromide. Per-
mits for methyl bromide use at a specific site and time are issued by local
county agricultural commissioners.
In ~ 992, DPR conducted a preliminary risk assessment on methyl bromide
to address acute inhalation exposures of residents reentering fumigated
homes. Based on that risk assessment, permit conditions were developed to
reduce acute exposures of workers and residents living near fumigated fields.
These changes included promulgation of emergency regulations by DPR to
require a longer aeration period following fumigation and lowering the reentry
level from 5 ppm to ~ ppm in the wall voids. DPR further required that Fact
Sheets explaining the potential human hazards of methyl bromide fumigation
be distributed to those potentially exposed.
In ~ 999, DPR conducted another risk assessment that reevaluated the ~ 992
acute exposure assessment and also considered subchronic and chronic inhala-
tion exposures to methyl bromide from all uses. This revised risk assessment,
which incorporates new health effects studies, additional air monitoring data,
and newly refined computer models for estimating methyl bromide emissions,
is intended to assist DPR in establishing new regulations for permitting the
use of methyl bromide (Title 3, California Code of Regulations).
The proposed regulations are designed to enhance protection for children in
schools, establish minimum buffer zones around application sites, and set new
limits on work hours for fumigation employees (Title 3, California Code of
Regulations). In addition to specifications for application rates and depths,
tarpaulin thickness, field size, application timing, and duration of fumigation,
the proposed regulations include the following specifications: (1) permit ap-
plicants must submit a work plan detailing the proposed fumigation to the
county agricultural commissioner before methyl bromide use will be a-
pproved; (2) neighbors living on sensitive sites (i.e., homes, schools, hospi-
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1 0 METHYL BROMIDE RISK CHAR 4 CTERIZA TION IN CALIFORNIA
tars, employee housing centers) that are within 300 feet ofthe outer boundary
of the buffer zone must be notified of fumigations, and they also have a right
to ask for a second notification 48 hr before the scheduled fumigation; (3) the
establishment of minimum buffer zones of 50 feet for workers and 60 feet for
residents to replace the suggested minimums of 30 feet (workers) and 100 feet
(residents) that are now advisory; and (4) a requirement that injection of
methyl bromide be completed 36 hr prior to the start of a school session.
The need for new regulations for permits has been driven by several fac-
tors. The primary factor is a California Superior Court decision ordering DPR
to adopt more specific regulations on the field fumigation use of methyl bro-
mide by June 2000. In addition, recently conducted toxicological and air-
monitoring studies (DPR 1999) have generated new data. The current DPR
risk characterization document incorporates these new data for the determina-
tion of the risks to workers and the general public from methyl bromide use.
However, for this risk characterization document to be used to support the
proposed regulations, DPR is required to "conduct an external scientific peer
review of the scientific basis of any new rule" (California Health and Safety
Code § 57004~. Consequently, DPR requested that the National Research
Council conduct a review of its draft risk characterization document and pro-
vide a critique addressing the issues identified in the assigned task. This task
was assigned to the Committee on Toxicology, which convened the Subcom-
mittee for the Review ofthe Risk Assessment of Methyl Bromide (see Appen-
dix A for biographical information). In addition, California DPR's risk char-
acterization document has undergone internal review by the California Office
of Environmental Health Hazard Assessment (OEHHA), part of the California
Environmental Protection Agency. Furthermore, DPR has also requested that
the EPA review the document.
THE SUBCOMMITTEE'S TASK
The task given to NRC's subcommittee on methyl bromide states the fol-
Towing: The subcommittee will perform an independent scientific review of
the California Environmental Protection Agency's risk assessment document
on methyl bromide. The subcommittee will (~) determine whether all relevant
data were considered, (2) determine the appropriateness ofthe 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 expo-
sure limits for methyl bromide.
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INTRODUCTION 1 1
DPR provided the subcommittee with the draft report to be reviewed
Methyl Bromide: Risk Characterization Document for Inhalation Exposure
(DPR 1999~. This report evaluates the toxicological and exposure data on
methyl bromide that characterize risks at current exposure levels for field]
workers and nearby residents. This document comprised the basis for the
subcommittee's review. The subcommittee also reviewed much of the pri-
mary toxicology literature cited in the risk characterization document, as well
as other supporting materials provided by DPR (OEHHA 1999; Seiber 1999~.
In addition to DPR's report and supplemental materials, the subcommittee
held a public meeting on October 4, 1999, to gather information from DPR
and other interested individuals and organizations. At this meeting, formal
presentations were made by Paul Gosselin, Lori Lim, and Thomas
Thongsinthusak of DPR; Vincent Piccirillo of NPC, Inc.; Bill Walter of the
Environmental Working Group; and Amy Kyle of the California Rural Legal
Assistance Foundation. A list of materials provided to the subcommittee may
be found in Appendix B.
ORGANIZATION OF TIIE REPORT
The remainder of this report contains the subcommittee's analysis of
DPR's risk characterization for methyl bromide. In Chapter 2, the critical tox-
icological studies and endpoints identified in the DPR document are evalu-
ated. 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.
Representative terms from entire chapter:
risk characterization
