of Report EH 94-02 -- Entitled
"Air Monitoring for Methyl Isothiocyanate
During a Sprinkler Application of Metam-Sodium

Environmental Monitoring and Pest Management
Department of Pesticide Regulation

In response to complaints of odor and irritation from metam-sodium applications, this study was conducted to determine possible air concentrations of methyl isothiocyanate (MITC), the major breakdown product and active fumigant of metam-sodium, during a "worst case" application scenario. Metam-sodium was applied by a fixed-set sprinkler system to a 20-acre fallow field in Kern County in August, 1993.

The Environmental Hazards Assessment Program of the Department of Pesticide Regulation (DPR) collected air samples during application and the following 68 hours at 10 locations around the field, ranging from 5 m to 150 m from the edge of the application area. Hydrogen sulfide and carbon disulfide levels, minor breakdown products of metam-sodium, were also monitored during several periods following application.

Maximum MITC concentrations occurred during the application period where downwind levels measured 2450 ppb at 5 m from the field edge and 1320 ppb at 150 m from the field edge. The next highest levels occurred during the interval following the watering-in period. Hydrogen sulfide readings were highest during application, ranging from below the detection limit to 76 ppb at a distance of 75 m from the field. All samples analyzed for carbon disulfide were below the laboratory quantification limit of 4 ppb.

Executive Summary


The Department of Pesticide Regulation, Environmental Hazards Assessment Program, monitored air surrounding a field in Kern County treated with metam-sodium in August 1993. This information will be used to determine whether it is necessary to reduce off-site movement of metam-sodium breakdown products in air.


Metam-sodium is a soil fumigant used to control fungi, bacteria, nematodes, weeds, and soil insects. It is applied before planting, either by direct injection into soil or by metering into an irrigation system. Applied to moist soil, metam-sodium degrades rapidly to methylisothiocyanate (MITC), which is the pesticidal active agent, and several minor breakdown products, including hydrogen sulfide (H2S) and carbon disulfide (CS2).

Over the last several years, there have been reports of odor, eye and nose irritation, headache, nausea, and sore throats by residents living near fields treated with metam-sodium. These incidences have been reported under a variety of conditions and in various locations, including in Contra Costa, Kern, Kings, Riverside, San Joaquin, and Santa Barbara Counties. To minimize off-site movement, guidelines were developed for all metam-sodium applications. These guidelines specify soil conditions for applications; field monitoring, especially of odors; soil sealing procedures for soil injection applications; and conditions to minimize off-site movement during and after sprinkler irrigation applications.

Before 1993, there was little information on the concentrations of metam-sodium breakdown products in air adjacent to metam-sodium-treated fields. In March 1993, the Air Resources Board (ARB) conducted monitoring in Contra Costa County to obtain air data under conditions typical for application in that county where MITC emissions were expected to be relatively low: cool air, cool soil temperatures, and a soil injection application. The current study was conducted to gather air monitoring data under conditions that would favor maximum emissions.


Monitoring was conducted in August 1993 in Kern County under worst-case conditions for off-site movement of metam-sodium breakdown products in air. In combination with the ARB Contra Costa County data, these monitoring results would provide the full range of emission conditions that DPR could consider if mitigation of off-site movement is necessary. Metam-sodium was applied at the highest label rate by sprinkler application and during conditions of high air temperatures, low humidity and warm soil temperatures.

Air samplers were placed at ten sites off the perimeter of the treated field at three approximate distances: 5, 75, and 150 meters. Air samples for MITC were taken during the 6-hour application period, the following 1.5 hour watering-in period, and for the next 66 hours.

Instantaneous H2S air concentrations were measured with a portable H2S analyzer during application and watering-in and approximately 22 hours after application began at the ten monitoring sites. Air samples were also analyzed for CS2. These samples were taken at the 5-meter locations during application, watering-in, and for the following 18 hours. Data on wind direction, wind speed, ambient air temperature, relative humidity, and soil characteristics were also collected.



Maximum concentrations of MITC were detected downwind during the application: 2450 ppb at 5 meters, 2110 ppb at 75 meters, and 1320 ppb at 150 meters. Concentrations decreased during the watering-in period (145-539 ppb at 5 meters), but peaked again at 1050 ppb at 5 meters downwind during the next 6-hour sampling interval. Following a significant decline in the following 6-hour interval (none detected to 106 ppb), levels increased slightly and then declined so that most samples were below the detection limit of 2 ppb 60 hours after the application began. The half life for the breakdown of MITC was estimated to be 7.3 to 7.6 hours.


H2S was detected at 3-76 ppb during application and at 3-8 ppb approximately 22 hours after application began. Measurable H2S residues were not detected above the detection limit of 3 ppb during the watering-in period or during subsequent monitoring 28 and 34 hours after application began.


No CS2 was detected above the detection limit of 4 ppb.


The Office of Environmental Health Hazard Assessment has estimated a 1-hour action level for eye irritation for MITC of 0.4 ppb. (An action level is the concentration of the contaminant in air, in this case, for a time period of one hour, that provides an advisory level for taking mitigation measures, such as avoidance of contact.) Under these worst-case application conditions, air concentrations of MITC exceeded this action level at all sampling locations until 13.5 hours after the application began. Levels of MITC exceeded the action level at one 150 meter sampler location until 36.5 hours after application, at one 75 meter site until 60.5 hours after application, and at two 5 meter sites until 72.5 hours after application began.

The California Occupational Safety and Health Administration has established a permissible exposure limit (the maximum permitted 8-hour time-weighted average concentration of an airborne contaminant) of 10,000 ppb for H2S. Maximum air concentrations of H2S (76 ppb) were well below this level.

Current metam-sodium use guidelines require a Notice of Intent to be filed with the County Agricultural Commissioner for applications made within a half mile of inhabited areas. Commissioners may also impose restrictions, such as buffer zones or specific warning requirements, at their discretion.

The Department of Pesticide Regulation is evaluating new use restrictions to reduce off-site movement of MITC under these worst-case conditions.