Methyl Isothiocyanate Residential Community Air Assessment for South Franklin County, Washington

Abstract

Metam sodium (metam; sodium methyl-dithiocarbamate) by weight is the most widely used soil fumigant, and the third most used pesticide in the United States (57–62 million lb/year) (USEPA, 2005a). Metam is applied preplant to a wide variety of crops, with potatoes, tomatoes, tobacco, and strawberries receiving more than 1 million pounds annually. Metam rapidly hydrolyzes upon contact with soil moisture to methyl isothiocyanate (MITC), the biologically active ingredient. The high vapor pressure of MITC (2.5–2.8 kPa at 20 C) can lead to substantial surface emission and subsequent off-target air mass movement if metam is not properly applied to the soil (Lee et al., 2002; Li et al., 2006; Sullivan et al., 2004). Thus, MITC may present a significant human inhalation exposure pathway in communities at the urban– agricultural interface (USEPA, 2005b). Methyl isothiocyanate is an acute respiratory irritant (Pruett et al., 2001). Many residential exposure incidences with reported nose irritation, sore throat, nausea, and dizziness from probable MITC exposure have been well documented in California (Thongsinthusak, 2003). Although not classified by the EPA as a hazardous air pollutant, MITC is considered a toxic air contaminant under the Code of California Regulations, Title 3, Section 6890(b). In Washington State, metam is the most widely used fumigant, with more than 20 million pounds applied in 2005 for control of soil-borne nematodes and diseases in large-scale potato production (NASS, 2006). It is typically applied preplant during the fall by center-pivot chemigation (PMSP, 2002). Recent expansion of suburban development into traditionally agricultural areas now appears to be associated with an increasing number of residential health complaints. Recently, 12 cases of possible or probable acute illnesses related to MITC inhalation exposure have been reported to the Washington State Department of Health (Burgess et al., 2000; Morrissey, 2006). Data on methyl isothiocyanate concentration in residential air are lacking for estimation of possible inhalation exposure near potatogrowing regions in Eastern Washington. A few agricultural air monitoring studies have evaluated off-target movement of MITC to residential communities. The majority of these evaluations have been conducted in California near row crop production operations (California Air Resources Board, 1994; California Department of Pesticide Regulations, 2003; Seiber et al., 1999). In particular, Seiber et al. (1999) studied regional off-site movement of MITC to residential areas in Bakersfield, California during the summer fumigation season in this region. The 1-h timeweighted average (TWA) MITC residential outdoor ambient concentrations ranged from 0.3 parts per billion (ppb) to a maximum of 14 ppb. Thongsinthusak (2003) has comprehensively summarized the MITC concentrations found in the California residential studies. Fumigation practices for Pacific Northwest field crops differ appreciably from those used for California row crops. Whereas metam is typically shanked directly into the soil during the spring or summer in California, it is generally applied by center-pivot chemigations in eastern Washington during the cooler fall months of September and October before irrigation districts shut off water supplies. The intensity of J. H. Merriman V. R. Hebert (&) Washington State University Tri-Cities, 2710 University Drive, Richland, WA 99354-1671, USA e-mail: vhebert@tricity.wsu.edu