Department of Pesticide Regulation
Environmental Monitoring and Pest Management Branch
1020 N Street, Room 161
Sacramento, California 95814-5624

Environmental Hazards Assessment Program Study #164


August 21, 1997


Methyl bromide is a widely used pesticide registered for use as a preplant soil fumigant, a structural fumigant, and a fumigant for control of stored products pests. The Department of Pesticide Regulation (DPR) has reviewed the health-related studies on methyl bromide and determined that a 24-hour time-weighted average concentration of 0.21 parts per million (ppm) provides a 100-fold margin of safety to protect the public health. This 0.21 ppm target level has been used to develop permit conditions for field, commodity, and other types of fumigation.

The permit conditions include buffer zones. These are areas that must be maintained between the application site and places where people conduct certain activities or practices. The buffer zones are set so that concentrations at this distance do not exceed 0.21 parts per million. The buffer zone is not an exclusion zone. People can walk or drive through a buffer zone and still not be at risk because they are spending only a short time near the application site. The size of the buffer zone varies based on the amount of time a person would normally spend doing a certain activity. The 0.21 ppm target level for methyl bromide is based on a 24-hour continuous exposure. For example, a person could be exposed to 0.42 ppm for 12 hours or 0.63 ppm for eight hours without exceeding the 0.21 ppm, 24-hour time-weighted average. Accordingly, buffer zones must be larger if the activity can be conducted for 24 hours, such as spending time at home or in a hospital. The buffer zone can be reduced when the activity is conducted for less than 12 hours, such as a normal work shift or a round of golf.

While numerous fumigations have been monitored, additional data are needed to verify the effectiveness of the buffer zones. Several application methods have not been monitored; the buffer zones for these methods are extrapolated from available data. Additional monitoring will determine if these extrapolations result in appropriate buffer zones. Also, monitoring conducted last winter showed a wider range of air concentrations than expected. Additional monitoring may help characterize the effect on air concentrations of potential seasonal weather effects or differences between geographic regions.


A. Determine the effectiveness of buffer zones for application methods for which there are no current off-site air monitoring data.

B. Determine if buffer zone size varies with season or geographic region, if sufficient resources are available.


This study will be conducted by personnel from the Environmental Hazards Assessment Program under the overall supervision of Randy Segawa, Senior Environmental Research Scientist.

Key personnel include:

Project LeaderPam Wofford
Senior Staff ScientistBruce Johnson
Field CoordinatorsKevin Bennett and Dave Kim
Statistician/ModelerTerrell Barry
Laboratory LiaisonCarissa Ganapathy
Analyzing LaboratoryCalifornia Department of Food and Agriculture,
Center for Analytical Chemistry
Agency and Public ContactMadeline Brattesani

All questions concerning this project should be directed to Madeline Brattesani at (916) 324-4100.


A. Application Methods With No Data

DPR will monitor two to three fumigations for the following application methods: (1) Winged Chisels [permit conditions method 1.1]; (2) Colby Bed Shaper [permit conditions method 9.1]; and (3) Raised Tarp [outdoor nursery method]

B. Seasonal/Regional Variability

Depending on the number of fumigations monitored for the first objective, DPR may monitor additional fumigations to estimate seasonal and regional variability. DPR will select one to three of the following application methods for study based on the availablity of fumigations during the dates and in the regions of interest: (1) Nontarp-Shallow [permit conditions method 1]; (2) Nontarp-Deep [permit conditions method 3]; (3) Tarp-Shallow [permit conditions method 4/5]; (4) Very High Barrier [permit conditions method 8.1]; and (5) Hot-Gas [permit conditions method 12]. For each of the application methods selected, DPR will monitor two to five fumigations during different seasons and/or different geographic areas.

C. Sampling Plan

For each application monitored, DPR will establish 8 - 16 monitoring stations. Four to eight stations will be established 30 feet from the edge of the treated area--one station at the center of each side and/or one station at each corner. Four to eight stations will be established at the resident buffer zone distance specified by the permit conditions--one to two stations on each side. DPR will monitor each application for 24 - 48 hours--two 6-hour periods and one to three 12-hour periods beginning with the start of application.

Number of samples for each application:

8-16 stations x 3-5 sample intervals = 24-80 samples

Total number of samples:

10-24 fumigations x 24-80 samples = 240-1920 samples


Air monitoring will be conducted using SKC personal air sampling pumps. Each air sampler will be positioned approximately four feet above ground level and will be fitted with activated charcoal vapor collection tubes, stacked two in a series, consisting of a 400 milligram primary tube and a 200 milligram secondary tube. Flow rates will be set at 15 milliliters per minute. Once samples are collected, each tube opening will be tightly capped and samples will be placed on dry ice and remain frozen until analysis.

A weather station will be set up next to the treatment area to measure wind speed, wind direction, ambient air temperature, and relative humidity.


Chemical analysis will be performed by the California Department of Food and Agriculture Center for Analytical Chemistry. Methyl bromide will be extracted with ethyl acetate and then analyzed by gas chromatography with an electron capture detector. Matrix blanks and spikes will be analyzed as a quality control measure. Concentrations for primary and secondary tubes will be reported separately to document any breakthrough in the primary tube.


Results will be reviewed to determine if the buffer zones were adequate to ensure that methyl bromide concentrations do not exceed the 24-hour target level for residential areas. In addition, the weather data and measured concentrations will be entered into the Industrial Source Complex-Short Term 3 (ISCST3) computer model. The ISCST3 model uses the flux rate, field size, weather, and terrain to simulate air concentrations. These studies will provide data on the field size, weather, terrain, and air concentrations, but not the flux rate. DPR will use the ISCST3 model to "back-calculate" an estimate of methyl bromide flux rates.

DPR will back-calculate flux rates by inputting the specific field dimensions, weather, and terrain data into the ISCST3 model, as well as an assumed flux value. The air concentrations simulated by the ISCST3 model (using the assumed flux value) will be regressed on the measured air concentrations. The slope of the regression line yields an adjustment or calibration factor for the assumed flux value used in the ISCST3 model. Using the calibrated flux value in the ISCST3 model gives the best match to the measured air concentrations. This flux calibration factor represents the flux rate for modeling purposes.

DPR will compare the flux calibration factors determined from the monitoring data to those currently assumed in the permit conditions and to flux calibration factors determined from previous monitoring studies.


Field Sampling8/97 - 2/98
Laboratory Analysis8/97 - 3/98
Data Analysis3/98 - 5/98
Report Preparation6/98 - 7/98