Article pubs.acs.org/JAFC
Determination of Methyl Isocyanate in Outdoor Residential Air near Metam-Sodium Soil Fumigations James E. Woodrow,† Jane T. LePage,§ Glenn C. Miller,† and Vincent R. Hebert*,§ †
Department of Natural Resources and Environmental Science, University of Nevada, Reno, 1664 North Virginia Street, Reno, Nevada 89557, United States § Department of Entomology, Washington State UniversityTri Cities, 2710 Crimson Way, Richland, Washington 99354, United States ABSTRACT: The soil fumigant metam-sodium (CH3NHCS2Na) produces the bioactive respiratory irritant methyl isothiocyanate (MITC). Recent laboratory gas-phase oxidative studies indicate that MITC rapidly transforms to the more toxic methyl isocyanate (MIC) in the lower atmosphere. Inhalation exposure risks from MITC plus MIC may therefore be an occupational worker and/or bystander health concern. To address this concern, MIC was monitored, along with MITC, in outdoor residential air in Washington state during the peak fall metam fumigation season. XAD-7 cartridges, coated with 1-(2pyridyl)piperazine, were developed to retain MIC as its stable substituted urea derivative. Of the 68 residential air measurements of MIC, 15 (22%) were observed to be above the California Environmental Protection Agency’s chronic inhalation reference level of 1 μg/m3, with an observed maximum MIC air concentration of 4.4 μg/m3. This study indicates MIC air concentrations can be anticipated along with MITC in residential air where seasonal metam soil fumigant applications occur. KEYWORDS: metam-sodium, methyl isocyanate, methyl isothiocyanate, air sampling, inhalation exposure
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INTRODUCTION Metam-sodium (CH3NHCS2Na) is a dithiocarbamate that is primarily used as an agricultural preplant soil fumigant with nationwide applications in the range 23−25 million kilograms in 2007.1 Potato growers in the Columbia Basin in Washington state rely on metam-sodium and, to a lesser degree, metampotassium (CH3NHCS2K) (trade names Vapam, Sectagon, Busan, and Metam CLR/KLR) hereafter collectively referred to as metam, to reduce the incidence of early die caused by Verticillium spp. Without the use of metam, this fungal organism can significantly reduce crop quality and yield.2 The effectiveness of metam is due to its rapid hydrolysis in moist soil shortly after application to yield methyl isothiocyanate (MITC), the bioactive agent responsible for controlling soilborne pathogens and pests.3 MITC is volatile (vapor pressure = 2.5−2.8 kPa at 20 °C; Henry’s constant ∼ 10 Pa·m3/ mol).3,4 Due to its high volatility, off-target movement of MITC from the land surface has resulted in losses of 10−60% shortly after metam application.5−9 Metam was recently classified by the U.S. EPA as a restricted-use soil fumigant, due in part to the acute and short-term inhalation risks of MITC to occupational workers, bystanders, and nearby residential communities.10 MITC vapor absorbs sunlight at wavelengths to 340 nm,11 and it degrades by photolytic decomposition and atmospheric oxidation, in part to the more volatile and more toxic methyl isocyanate (MIC; CH3NCO).12 MIC yields of 7% have been reported from photolysis in laboratory chamber studies.13 More recent atmospheric oxidation reaction chamber studies, involving the OH radical, indicate that MITC transforms primarily to MIC, with conversions >60%.14,15 Given that the estimated half-life for the MITC to MIC transformation is short (12−16 h),14,15 it is reasonable to anticipate that MIC can © 2014 American Chemical Society
coexist with MITC in breathing air during the metam soil fumigation season. The acute toxicity to humans of MIC at high air concentrations has been well investigated.16 However, there remains limited information regarding the short-term or chronic exposure effects of much lower MIC concentrations on human health near agricultural settings where seasonal soil fumigations occur. Although the U.S. EPA has not established a Reference Concentration (RfC) or Reference Dose (RfD) for MIC, the California Environmental Protection Agency (CalEPA) has calculated a chronic inhalation reference exposure level of about 1 μg/m3, based on lung and body weight effects in rats.17 This is a concentration level at or below which adverse health effects (e.g., eye and upper respiratory irritation) are not likely to occur. OSHA and NIOSH have established an 8 h time-weighted-average (TWA) recommended and permissible exposure level (REL and PEL) for MIC of 50 μg/m3.18 The California Air Resources Board (CARB), from field measurements in Kern county, California, USA, compiled the only agricultural fumigation air concentration data for MIC so far reported. CARB observed near-field MIC air concentrations in the range of 0.2−5.8 μg/m3 during 6 and 12 h. TWA sampling periods after metam soil incorporated shank applications to a 32 ha field over a 3 day air sampling period.19 Concurrent near-field air measurements of MITC concentrations were in the range of 0.24−250 μg/m3. Although this study indicates that bystanders will not be exposed to acutely toxic levels of MIC from a single soil application, it is Received: Revised: Accepted: Published: 8921
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still possible that lower short-term levels can occur in air downwind of treatment sites from multiple field sources over a fumigation season. This paper reports on a rugged and sensitive field sampling and analysis procedure for assessing MIC air concentrations in residential air. We also report for the first time on observed MIC air concentrations that were concurrently monitored with MITC during a period of metam soil fumigations near a residential area in southern Franklin county, Washington state, USA. Understanding the combined contributions of MITC with MIC will be important especially to residential communities existing in close proximity to where agricultural soil fumigations occur.
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MATERIALS AND METHODS
MIC Air Sampling. A batch (∼30 g) of XAD-7 medium-polarity polymeric adsorbent (XAD7HP; Sigma-Aldrich) was washed and
Figure 3. MIC air sampling mast. adsorbent (ca. 1.12 g by weight) were each packed into separate 5 cm3 plastic Bond Elut reservoirs containing a porous frit at the tip end (Agilent) and capped with a plug of glass wool. Using a syringe, 80 μL of 1-(2-pyridyl)piperazine (1-2PP, 98%; Sigma-Aldrich) was dissolved in 20 mL of pesticide grade methylene chloride (Fisher), yielding a concentration of ∼4.3 mg/mL. The 1-2PP solution was transferred to the reservoir of the cartridge-coating apparatus (Figure 1). The free end of the flexible tubing extending from the bottom of the reservoir was attached to the tip end of a cartridge containing XAD-7, and the cartridge was lowered to allow the 1-2PP solution to flood the cartridge up to the glass wool plug. The cartridge was then raised to drain the solution. This was repeated three times. The coated cartridge was then transferred to a vacuum source for air-drying (∼1 h for each cartridge). Each dry, coated cartridge was wrapped in aluminum foil and stored under refrigeration (1−7 °C) in an amber screw-cap jar. The cartridges were prepared no more than 5 days prior to use in the field. MIC Residential Air Monitoring. MIC receptor stations were set up at three separate sampling sites, two in yards of single-family homes and one at an outdoor commercial building location (site 1). All were bordered by agricultural land (Figure 2, sites 1, 3, and 5). These sampling locations have been routinely used for past MITC area-wide assessments in this residential area.20,21 The site 3 location consisted of
Figure 1. XAD-7 cartridge coating apparatus. drained several times by swirling with excess pesticide grade acetonitrile (Fisher). The final rinse was vacuum filtered until the adsorbent was dry and flowable. Three milliliter portions of the dry
Figure 2. 2012 MIC residential sampling site map (southern Franklin county, Washington state, USA). 8922
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momentarily deployed on a pump without flow and immediately removed to simulate sampling procedures for the other cartridges. The TB and STB samples were analyzed with the field samples. Fortification solutions of MIC (99%; ChemService) in methylene chloride (at 1, 10, and 500 μg/mL) were used for the laboratory and field cartridge fortifications. The concentrations of these solutions were verified against standards of the MIC-1-2PP-substituted urea derivative by spiking cartridges with the fortification solutions and then recovering the MIC-1-2PP derivative. The MIC-1-2PP derivative was prepared for use as an analytical standard according to the guidelines of OSHA method 54, section 3.3.1.22 One-tenth of a gram of MIC was dissolved in 25 mL of methylene chloride. This solution was slowly added to a dilute solution of 1-2PP in methylene chloride with stirring. The mixture was stirred for an hour and then evaporated to 90% of residential air samples. Of the 68 confirmed measurements, 15 (22%) were above CalEPA’s chronic inhalation reference level of 1 μg/m3, with some several times greater than this level, but were about 1/10 of the REL/PEL of 50 μg/m3 for an 8 h work shift. The CalEPA reference level is not a direct indicator of risk but rather a reference point to gauge potential effects. For short-term exposures greater than the reference exposure level, the potential for adverse health effects increases. South Franklin county, Washington, USA, continues to undergo rapid residential and commercial expansion onto large production rotational potato, corn, wheat, and alfalfa crop circle systems (each circle ca. 48−64 ha). This study was initiated to address public health agency concerns that earlier reported 2005−2008 MITC residential air concentrations could also result in significant MIC air contributions during periods of active soil fumigation.20,21 These earlier air monitoring studies indicated that MITC air concentrations were uniformly distributed throughout this basin and intermittently exceeded EPA regulatory acute [67 μg/m3 (1−8 h)] and short-term (15 μg/m3 over a 24 h period) inhalation exposure levels of concern (LOC). In 2008, stagnant air/inversion conditions just before the irrigation district’s water cutoff date contributed to MITC residential air levels exceeding the EPA acute LOC on 3 days, with a 4 h TWA observed maximum air concentration of 650 μg/m3. The EPA short-term MITC LOC threshold was exceeded on 20% of the days monitored.21 On the basis of the reported daytime high rate of oxidative conversion of MITC to MIC,14,15 2008 MITC levels could have potentially led to MIC air concentrations greater than the REL/PEL of 50 μg/m3, suggesting that monitoring of MIC is warranted. In 2012, windy conditions from early October through irrigation cutoff resulted in fewer soil fumigations occurring than in previous monitoring years (personal communications with Washington State Department of Agriculture). Although many soil fumigations were delayed by growers to spring 2013, the 2012 MITC emission profile does show consistency in seasonal trends with previous 2005, 2007, and 2008 monitoring seasons, with higher MITC concentrations during the peak period of fall soil fumigation 1 week before irrigation water cutoff (Figure 5). Traditionally, metam applications in Franklin county, WA, USA, have been performed mid-September to late October by center-pivot surface chemigation, which typically led to MITC
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AUTHOR INFORMATION
Corresponding Author
*(V.R.H.) Phone: (509) 372-7393. E-mail: vhebert@tricity. wsu.edu. Funding
We acknowledge funding provided by EPA Region 10. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the valuable assistance of Cynthia Lopez and Barbara Morrissey from the Washington State Department of Health.
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REFERENCES
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