Guidelines for Canadian drinking water quality – Malathion: Exposure considerations

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• Sources and uses
• Substance identity
• Exposure

Sources and uses

Malathion or diethyl[(dimethoxyphosphinothioyl)thio]butanedioate is a non-systemic, broad-spectrum organophosphate insecticide and acaricide used to control a broad range of insect and arachnid pests. It acts by inhibiting the acetylcholinesterase (AChE) enzyme, thereby disrupting nervous system function. In Canada, malathion is used on a wide variety of settings including agricultural and non-agricultural sites such as human habitats, recreational areas, and outdoor ornamentals (Health Canada, 2012). In 2018 (the most recent year for which data are available), over 25,000 kg of malathion was sold in Canada (Health Canada, 2020a).

Malathion may be released into surface water or soils as runoff from the application site (ATSDR, 2003; U.S. EPA, 2009; Health Canada, 2012). In natural waters, soil and sediment, breakdown of malathion occurs primarily through microbial degradation and hydrolysis (Laveglia and Dahm, 1977; ATSDR, 2003; Health Canada, 2010; Singh et al., 2014). Malathion hydrolyses readily under neutral to alkaline conditions but is increasingly stable under acidic conditions and at low temperatures. The major transformation products (as identified in biotransformation studies) are malathion's monocarboxylic acid (MCA), dicarboxylic acid (DCA), demethyl monocarboxylic acid and demethyl dicarboxylic acid, which are not expected to persist in the environment (Health Canada, 2010). Photolysis is not a significant breakdown pathway for malathion in water or soil, with reported half-lives ranging from 0.67 to 42 days in natural and distilled waters and 173 days in sandy loam soil (ATSDR, 2003; EFSA, 2009; U.S. EPA, 2009; Health Canada, 2010). However, in some natural waters containing photosensitizing agents, photolysis may contribute to the dissipation of malathion from the water layer in the photic zone (that is, upper layer penetrated by sunlight) (Health Canada, 2010).

In aquatic environments, malathion is non-persistent to slightly persistent under aerobic conditions (half-life of 0.3 to 19 days) and non-persistent in anaerobic systems (half-life of 2.5 days reported in flooded soil), with dissipation generally being fastest in alkaline systems, conditions that have been shown to favour hydrolysis (Health Canada, 2010).

As malathion is highly soluble in water (see Table 1) and does not adsorb strongly to soils, it is mobile in most soil types and its use may result in the contamination of groundwater, particularly in areas where soils are permeable (for example, sandy soil) and/or the depth to the water table is shallow (Gervais et al., 2009; Health Canada, 2012). However, malathion is unlikely to leach into groundwater, as it is rapidly degraded in soil by microbially mediated metabolism (half-life of 0.2 to 2 days) and hydrolysis under neutral to alkaline conditions (half-lives of 6.2, 1.5 and 0.5 days at pH 7, 8 and 9, respectively) (ATSDR, 2003; Health Canada, 2010). The degradation of malathion in soil is enhanced by increased moisture, pH levels, microbial activity, nitrogen content and carbon content (Laveglia and Dahm, 1977; ATSDR, 2003; EFSA, 2009; U.S. EPA, 2009; Health Canada, 2010; Kumar et al., 2019).

Based on its physical properties (vapour pressure and Henry's law constant), malathion is unlikely to volatilize appreciably from moist soils or water surfaces, or undergo long-range atmospheric transport (Health Canada, 2010). If present in air, malathion can be released to surface water or soils by rain or fog water, or be photo-oxidized (ATSDR, 2003; WHO, 2004).

Malaoxon, the oxidation transformation product that is responsible for some of the toxic effects of malathion, may form under certain environmental conditions but is expected to be non-persistent under aerobic soil conditions (half-lives of 6.5 days at pH 6.2 and 3.5 days at pH 8.2) (Gervais et al., 2009; Health Canada, 2010). Two monitoring studies investigating malaoxon formation in water, sand and soils reported a maximum of 10% malathion to malaoxon conversion (Health Canada, 2012). As with malathion, malaoxon is rapidly detoxified via hydrolysis under neutral to alkaline conditions, unlikely to leach into groundwater and more persistent under acidic conditions than alkaline conditions (half-lives of 32.5, 8.8 and 0.18 days at pH 5, 7 and 9, respectively) (ATSDR, 2003; Health Canada, 2010). Malaoxon may also be formed as a result of oxidation or advanced oxidation processes during the treatment of drinking water for the removal of pesticides (refer to the section on treatment considerations).

Substance identity

Malathion (C10H19O6PS2) is a colourless to amber liquid belonging to the organophosphate class of chemicals (U.S. EPA, 2009; Health Canada, 2010). Formulations of malathion can contain a number of impurities at very low levels, notably malaoxon and isomalathion. In the past, manufacturing processes and improper product storage led to the presence of isomalathion, a toxic metabolite that potentiates the toxicity of malathion; however, regulatory standards have since been put in place to limit its presence and formation (Buratti and Testai, 2005; U.S. EPA, 2009; Jensen and Whatling, 2010; Health Canada, 2010, 2012).

Exposure

The general Canadian population can be exposed to malathion primarily through food and drinking water (Health Canada, 2010, 2012). Based on dietary risk estimates using conservative assumptions and combining exposure estimates for both malathion and malaoxon (Table 2), Health Canada's Pest Management Regulatory Agency (PMRA) did not consider exposure to malathion and malaoxon from both food and drinking water to be of concern for Canadians (Health Canada, 2010). In its exposure assessment based on surveillance data, PMRA combined both malathion and malaoxon residues by converting the malaoxon residues (by multiplying them by a toxicity adjustment factor [TAF] of 24) into malathion equivalents. The use of a TAF accounts for the greater potency of malaoxon compared to malathion in regard to cholinesterase (ChE) inhibition and represents a very conservative estimate of exposure to malathion and malaoxon (refer to the section on derivation of the health-based value for more information) (Health Canada, 2010).

Water monitoring data on malathion were available from the provinces and territories (municipal and non-municipal supplies), PMRA and Environment and Climate Change Canada (Environment Canada, 2011) (See Canadian Water Quality Data).

The exposure data provided reflect different method detection limits (MDL) of accredited laboratories used within and amongst the jurisdictions, as well as their respective monitoring programs. The data provided by the provinces and territories do not indicate the timing of monitoring in relation to pesticide application and runoff events. As a result, the exposure data and statistical analysis provide only a limited picture. Data provided by the provinces and territories indicate that malathion levels are below the method reporting limit (MRL) or MDL in most samples collected from a variety of water supplies in Canada, including surface water and groundwater, as well as treated and distributed water (British Columbia Ministry of Health, 2019; Indigenous Services Canada, 2019; Manitoba Sustainable Development, 2019; Ministère de l'Environnement et de la Lutte contre les changements climatiques, 2019; Nova Scotia Environment, 2019; Saskatchewan Water Security Agency, 2019; Ontario Ministry of the Environment, Conservation and Parks, 2020). Table 3 summarizes the monitoring data for all jurisdictions. The maximum concentration reported was 5 μg/L for treated surface water in Ontario, which is well below the proposed MAC. There were no monitoring data available in New Brunswick, Newfoundland and Labrador, Prince Edward Island or Yukon (New Brunswick Department of Environment and Local Government, 2019; Newfoundland and Labrador Department of Municipal Affairs and Environment, 2019; PEI Department of Communities, Land and Environment, 2019; Yukon Environmental Health Services, 2019).

As part of its assessment, PMRA (Health Canada, 2010) summarized Canadian water monitoring data on malathion up to 2005. Malathion was detected in 10 samples (n = 4 274) from Canadian municipal drinking water sources with a maximum concentration of 0.08 µg/L recorded in Quebec (1991-1993) and in > 79 samples (n = > 6 716) from Canadian ambient water that may serve as a drinking water source with a maximum concentration of 1.54 µg/L recorded in Ontario (2003). The maximum malathion concentration in water sources unlikely to be used for drinking water was 2.1 µg/L (11 samples with detections; n = 150).

Canadian water monitoring data were available from the published literature and indicated that malathion is not frequently detected in drinking water sources. Sampling in corn and soybean crop sectors in Quebec from 2015 to 2017 indicated an average malathion detection frequency of 2% and a maximum malathion concentration of 5.5 µg/L in the Chibouet, Saint-Régis, des Hurons and Saint-Zéphirin Rivers (limit of detection (LOD) = 0.02 µg/L) (Giroux, 2019). The maximum detection frequency and maximum malathion concentration for four streams from orchard and vegetable crop zones in Quebec were 33.3% and 2.7 µg/L, respectively, for the 2013-2014 period (LOD = 0.02 µg/L) (Giroux, 2017). No malathion was detected in Quebec from sampling performed in individual wells in proximity to corn, soybean, vine, orchard, vegetable and small fruit crop sectors (LOD = 0.02 µg/L) (Giroux, 2016, 2019).

In British Columbia, malathion was not detected in a study (2003-2005) of surface water and groundwater from the Lower Fraser Valley region (reporting limit = 2.22 ng/L, n = 40 samples) (Woudneh et al., 2009a, 2009b).

Thirty-four wells from the Nova Scotia Groundwater Observation Well Network were tested for pesticides between 2004 and 2011. Malathion (LOD = 0.5-5 µg/L) and malaoxon (LOD = 1 µg/L) were not detected (Nova Scotia Environment, 2015).

Other Canadian water exposure data were not available for malaoxon. Owing to the extensive monitoring programs that exist in the United States (U.S.) along with comparable runoff events, local use patterns, site-specific hydrogeology as well as testing and reporting methods, U.S. monitoring data were considered relevant to the Canadian context. U.S. temperatures are generally warmer (leading to quicker degradation of malathion) with longer growing seasons and more pesticide applications. Additionally, annual usage data (up to 13 million pounds in 2000 and ~15 million pounds in 2009) indicate malathion is used in the U.S. at much higher quantities than in Canada (up to 100,000 kg [~200 000 pounds]) (U.S. EPA, 1999; Health Canada, 2010, 2020a). The use of American monitoring data would therefore generate a conservative estimate of Canadian exposure.

In its assessment, PMRA (Health Canada, 2010) summarized U.S. water monitoring data for malaoxon. Out of 6,297 samples collected between 1999 and 2000 from surface water and groundwater sources, malaoxon was only detected in 7 samples. Its maximum concentration was 0.18 µg/L (LOD = 0.005-0.15 µg/L). In 11 samples (n = 538) of raw and finished water (collected up to 2008), the maximum concentration was 0.556 µg/L (LOD = 0.016 µg/L). Water monitoring data for malaoxon collected from 2008 to 2013 was also available from the Pesticide Data Program, a national pesticide residue monitoring program conducted by the United States Department of Agriculture. Malaoxon was not detected in 1,221 samples (LOD = 0.37-600 ng/L) from groundwater sources (for example, private wells, daycare/school wells). In raw and finished water, malaoxon was detected in 2 samples (n = 1 283) at a maximum concentration of 1.8 ng/L (finished water, LOD = 0.37-600 ng/L) (USDA, 2020).

Based on surveillance and field trial data, malathion residues in food are expected to be low and to not pose a dietary risk to Canadians (Health Canada, 2010, 2012). In Canada, the established maximum residue limits for malathion range from 0.5 to 8 ppm for various food commodities (for example, fruits, vegetables, grains and beans/legumes) (Health Canada, 2020b). The Canadian Food Inspection Agency sampled and tested domestic and imported food products (namely, fresh fruits and vegetables, meat, nuts and seeds) between April 1, 2015, and March 31, 2016. Malathion residues were detected in 43 samples (n = 998) at a maximum level of 0.64000 ppm (CFIA, 2019a). Among infant foods and formulas monitored by the Canadian Food Inspection Agency, 2 samples (n = 221) tested positive for malathion contamination below the maximum residue limit of 2 ppm, with levels of 0.0195 ppm and 0.0322 ppm recorded (CFIA, 2019b).

Based on its physical properties, airborne exposure to malathion is not expected to be a concern for the Canadian population, with air monitoring data indicating that malathion is only present at low levels in areas where it is used (Health Canada, 2010).

Most Canadians have very low urine levels of DCA, as measured in Cycle 3 (2012-2013) and Cycle 4 (2014-2015) of the Canadian Health Measures Survey (CHMS) (Health Canada, 2019b). In the CHMS, group geometric means of urinary DCA were not calculated if more than 40% of the samples were below the detection limit of 0.19 μg/L. Geometric means were not calculated for any sex or age group due to low detection. The 95th percentile for the total age group (3 to 79 years) was 1.2 µg/L (95% confidence interval (CI): 0.70-1.6 µg/L) in Cycle 3 and 0.95 µg/L (95% CI: 0.46 to 1.4 µg/L) in Cycle 4.