Abstract and Introduction
Abstract
Background: Methyl bromide, a fungicide often used in strawberry cultivation, is of concern for residents who live near agricultural applications because of its toxicity and potential for drift. Little is known about the effects of methyl bromide exposure during pregnancy.
Objective: We investigated the relationship between residential proximity to methyl bromide use and birth outcomes.
Methods: Participants were from the CHAMACOS (Center for the Health Assessment of Mothers and Children of Salinas) study (n = 442), a longitudinal cohort study examining the health effects of environmental exposures on pregnant women and their children in an agricultural community in northern California. Using data from the California Pesticide Use Reporting system, we employed a geographic information system to estimate the amount of methyl bromide applied within 5 km of a woman's residence during pregnancy. Multiple linear regression models were used to estimate associations between trimester-specific proximity to use and birth weight, length, head circumference, and gestational age.
Results: High methyl bromide use (vs. no use) within 5 km of the home during the second trimester was negatively associated with birth weight (β = –113.1 g; CI: –218.1, –8.1), birth length (β = –0.85 cm; CI: –1.44, –0.27), and head circumference (β = –0.33 cm; CI: –0.67, 0.01). These outcomes were also associated with moderate methyl bromide use during the second trimester. Negative associations with fetal growth parameters were stronger when larger (5 km and 8 km) versus smaller (1 km and 3 km) buffer zones were used to estimate exposure.
Conclusions: Residential proximity to methyl bromide use during the second trimester was associated with markers of restricted fetal growth in our study.
Introduction
Methyl bromide is a fumigant used for pre-planting soil preparation. Because of concerns about its effect on the ozone layer, methyl bromide was banned in 2005 under the Montreal Protocol (United Nations Environment Programme 2000). Before the ban, methyl bromide was one of the most heavily used pesticides in California, with nearly 5 million kg applied in 2000 [California Department of Pesticide Regulation (DPR) 2001b]. Although methyl bromide use has declined in recent years, > 1.75 million kg was applied throughout the state in 2010 due to critical use exemptions for strawberries and other agricultural commodities (California DPR 2012c). Critical use of methyl bromide in the United States has declined markedly since 2005, but authorizations for use are still granted by the U.S. Environmental Protection Agency (U.S. EPA 2012).
Animal studies of methyl bromide exposure suggest the potential for developmental toxicity (National Research Council 2000). In an inhalation study of New Zealand white rabbits, exposure to 80 ppm methyl bromide during gestational days 7–19 resulted in decreased fetal body weights and increased rates of fetal malformations including gall bladder agenesis and fused sternebrae (National Research Council 2000). In another inhalation study, pups born to dams exposed to 20 and 70 ppm during gestation had increased rates of skull ossification defects (National Research Council 2000). A similar study of rats found dose-dependent reductions in fetal body and brain weights, as well as reduced cerebral cortex widths (National Research Council 2000). Based on this limited evidence, the California DPR lists methyl bromide as a probable developmental toxicant (National Research Council 2000).
Several human poisoning case studies indicate that high levels of acute methyl bromide exposure are associated with dermal burns, neurological impairment, and death (Breeman 2009; Langård et al. 1996; Lifshitz and Gavrilov 2000). Much less is known about the effects of chronic, low-level exposure in human populations (National Research Council 2000). In a sample of 56 male workers, long-term occupational exposure to methyl bromide was associated with chronic symptoms of dizziness, numbness, nightmares, and fatigue (Kishi et al. 1991). Other studies have investigated the health effects of residential proximity to methyl bromide use as part of larger analyses of ambient pesticide exposure. To date, one study reported evidence of an association with prostate cancer (Cockburn et al. 2011), but studies of breast cancer (Reynolds et al. 2004), childhood cancers (Reynolds et al. 2005), and autism spectrum disorders (Roberts et al. 2007) did not report associations with proximity to methyl bromide use.
Unlike most pesticides, fumigants such as methyl bromide have a high vapor pressure and are more likely to drift off-site (Woodrow and Krieger 2007). It is estimated that 30–50% of agricultural applications of methyl bromide are released into the air even when protective measures such as plastic tarps are in place (Honaganahalli and Seiber 2000; Majewski et al. 1995; Yagi et al. 1993). Because inhalation is the primary route of exposure to methyl bromide, ambient air concentrations are important for assessing exposure to nearby residents (National Research Council 2000). In agricultural areas of California, measured outdoor concentrations of methyl bromide averaged 0.1–7.7 ppb over an 8-week monitoring period in 2000 (California Air Resources Board 2001), or up to one-tenth of the levels associated with adverse outcomes in some animal studies. In addition, methyl bromide has been detected in air > 70 km away from the nearest application site (Honaganahalli and Seiber 2000).
Chronic exposure to methyl bromide is difficult to assess because there are no exposure biomarkers (Minnesota Department of Health 1999). The few studies that have assessed chronic methyl bromide exposure have done so by residential proxy or occupational history reports (Cockburn et al. 2011; Kishi et al. 1991; Reynolds et al. 2004, 2005; Roberts et al. 2007). Studies in California can use extensive information on the timing, location, and amount of agricultural methyl bromide applications, which must be reported to the state's Pesticide Use Reporting (PUR) system (California DPR 2012b).
In the current study we examined whether residential proximity to methyl bromide applications is associated with fetal growth and gestational length in a cohort of pregnant women living in an agricultural community in the Salinas Valley of California. We linked California PUR data with birth outcome and demographic data for pregnant women participating in the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) study, a longitudinal cohort study examining the health effects of environmental exposures on pregnant women and their children living in an agricultural community in northern California.