Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Trichloroethylene
4.0 Exposure
Canadians can be exposed to TCE through its presence in drinking water, air and food. In addition, certain segments of the population can be exposed via contaminated soil, through the use of specific consumer products or in occupational settings. Since TCE has been detected in human milk, nursing infants could potentially be exposed (U.S. EPA, 2001b). Although some exposure data are available, they are considered insufficient to justify modifying the default allocation factor for drinking water of 20%.
4.1 Water
TCE has been detected frequently in natural water and drinking water in Canada and other countries. Due to its high volatility, TCE concentrations are normally low in surface water (≤1 µg/L). However, in groundwater systems where volatilization and biodegradation are limited, concentrations may be higher if contamination has occurred in the vicinity and leaching has taken place.
Because analytical methods have improved over the years since TCE was first assayed, concentrations that were once considered "non-detectable" are now quantifiable. This confounds the use of historical TCE data, as the values for "non-detectable" have changed over time.
TCE was detected in raw and treated water at 10 potable water supply facilities in Ontario in 1983 at levels ranging from ≤0.1 to 0.8 µg/L (Mann Testing Laboratories Ltd., 1983). In 1979, TCE was found in over half of potable water samples taken at 30 treatment facilities across Canada; mean concentrations were 1 µg/L or less, and the maximum level was 9 µg/L (Otson et al., 1982).
Monitoring data from eight Canadian provinces for the period 1985-1990 indicated that 95% of 7902 samples from drinking water supplies (raw, treated or distributed water) had TCE concentrations below 1 µg/L. The maximum concentration was 23.9 µg/L (groundwater sample). Most (75%) of the samples in which TCE was detected were from groundwater sources (Department of National Health and Welfare, 1993). More recent data from New Brunswick (1994-2001), Alberta (1998-2001), the Yukon (2002), Ontario (1996-2001) and Quebec (1985-2002) for raw (surface water and groundwater), treated and distributed water indicated that more than 99% of samples contained TCE at concentrations less than or equal to 1.0 µg/L. The maximum concentration was 81 µg/L. Of those samples with detectable TCE concentrations, most were from groundwater (Alberta Department of Environmental Protection, 2002; New Brunswick Department of Health and Wellness, 2002; Ontario Ministry of Environment and Energy, 2002; Yukon Department of Health and Social Services, 2002; Ministère de l'Environnement du Québec, 2003).
A 2000 survey of 68 First Nations community water supplies (groundwater and surface water) in Manitoba found that TCE concentrations were non-detectable (<0.5 µg/L) (Yuen and Zimmer, 2001).
Groundwater is the sole source of water for an estimated 25-30% of the Canadian population (Statistics Canada, 1994). In 1995, a national review of TCE occurrence data was carried out to determine the extent of groundwater contamination by TCE and the number of people potentially exposed to contaminated drinking water. The majority of sites were from Ontario and New Brunswick. The review was based on urban groundwater supplies. Of the 481 municipal/communal and 215 private/domestic groundwater supplies (raw water), 8.3% and 3.3%, respectively, contained TCE, at average maximum concentrations of 25 µg/L and 1680 µg/L, respectively. This review involved a compilation of data from a variety of sources over different periods of time. Consequently, interpretation of the data is made more difficult by the range of detection limits. A majority of all sites (93%) had non-detectable levels (<0.01-10 µg/L), 3.6% had a maximum concentration of <1 µg/L, 1.4% had a maximum of 1-10 µg/L, 0.43% had a maximum of 10-100 µg/L and 1.3%Footnote 1 had a maximum of >100 µg/L (Raven and Beck Environmental Ltd., 1995).
It was estimated that approximately 1.67 million of the 7.1 million Canadians who relied on groundwater for household use in 1995 were covered by this study. Of the 1.67 million surveyed, the water supplies of 49% had non-detectable levels of TCE (<0.01-10 µg/L), 48.1% had a maximum of 1-10 µg/L, 2.1% had a maximum of 10-100 µg/L and 0.8% had a maximum of >100 µg/L. Despite the problems associated with the wide range of detection limits reported in this study, the results of the survey suggested that more than 95% of Canadians who rely on groundwater are exposed to less than 10 µg TCE/L in their drinking water. In fact, this probably represents a worst-case scenario, since the sampled data were for raw water and may not be representative of water received at households (Raven and Beck Environmental Ltd., 1995).
4.2 Multi-route Exposure through Drinking Water
Due to TCE's volatility and lipid solubility, exposure can also occur dermally and through inhalation, especially through bathing and showering. For the purposes of assessing overall TCE exposure, the relative contribution of each exposure route needs to be assessed. These contributions are expressed in litre equivalents per day (Leq/day). For example, an inhalation exposure of 1.7 Leq/day means that the daily exposure to TCE via inhalation is equivalent to a person drinking an extra 1.7 L of water per day.
Bogen et al. (1988) accounted for oral, dermal and inhalation routes of exposure to TCE from household uses of tap water. They proposed lifetime Leq/day values for 70-kg adults of 2.2 (ingestion), 2.9 (inhalation) and 2 (dermal). The ingestion value was based on the consideration of U.S. age-specific consumption rates, and the dermal number was derived using a generic dermal absorption coefficient value for VOCs, rather than a TCE-specific value. In addition to the shower scenario, these authors quantified exposure via household air when determining the Leq/day value for the inhalation route.
Weisel and Jo (1996) concluded that the dermal and inhalation routes contribute internal doses similar to that from ingestion of tap water and that their total contribution is greater than that from ingestion. However, in the absence of data for route-specific doses and the TCE concentration in air, a verification of their conclusions and the determination of Leq/day values for the various routes are not easily achieved.
Lindstrom and Pleil (1996) outlined simple methodological approaches for the calculation of potential doses received by the ingestion, dermal and inhalation routes. Using a water concentration of 4.4 µg/L, these authors calculated that the ingested dose was more important than the inhaled dose for a 10-minute shower, which in turn was greater than the dermal dose.
Krishnan (2003) determined Leq/day values for dermal and inhalation exposures of adults and children (6-, 10- and 14-year-olds) to TCE (5 µg/L) in drinking water for a 10-minute shower and a 30-minute bath on the basis of the methodological approach of Lindstrom and Pleil (1996), the use of physiologically based pharmacokinetic (PBPK) models and consideration of the fraction absorbed (Laparé et al., 1995; Lindstrom and Pleil, 1996; Poet et al., 2000). The "fraction absorbed" for the dermal and inhalation exposures took into consideration the TCE dose that was absorbed following exposure as well as that portion that was excreted in the following 24 hours. It was assumed that 100% of the skin is exposed in both the shower and bath scenarios, and a dermal absorption coefficient specific to TCE was used (Nakai et al., 1999). Complete (100%) absorption of ingested drinking water was assumed for all subpopulations; this was supported by the extent of hepatic extraction of TCE (Laparé et al., 1995).
Leq/day values for the inhalation and dermal routes were higher for the 30-minute bath scenario than for the 10-minute shower for all subpopulations based on the longer exposure time. The highest value was 3.9 Leq/day (1.5 L ingestion, 1.7 L inhalation, 0.7 L dermal) for adults. The 3.9 Leq/day value (which can be rounded to 4.0 Leq/day) is considered to be conservative, since most Canadians do not take a 30-minute bath on a daily basis. In the event that individuals spend more than 10 minutes in a shower or are exposed to TCE via other household activities, the calculated 4.0 Leq/day value (which includes inhalation and dermal exposure from a 30-minute bath) should be adequate.
4.3 Air
Studies conducted in the 1980's and 1990's have detected TCE in outdoor and indoor air in Canada. Levels of TCE in air were determined in Toronto and Montreal for 1 year (1984-1985) and in Sarnia and Vancouver for 1 month (autumn 1983). Mean levels for the four cities were 1.9, 0.7, 1.2 and 1.0 µg/m³, respectively, with maxima of 8.6, 1.7, 3.6 and 3.4 µg/m³, respectively (Environment Canada, 1986). In another survey, mean concentrations of TCE in ambient air at 11 urban sites and 1 rural site in Canada (1988-1990) ranged from 0.07 to 0.45 µg/m³ (Vancouver and Calgary, respectively), with an overall mean value of 0.28 µg/m³ and a maximum single value of 19.98 µg/m³ reported in Montreal (Dann, 1993).
More recent U.S. data are similar to the levels measured in Canada. In 1998, ambient air measurement data from 115 monitors located in 14 states indicated that TCE levels ranged from 0.01 to 3.9 µg/m³, with a mean of 0.88 µg/m³. Mean TCE air concentrations (1985-1998) for rural, suburban, urban, commercial and industrial land uses were 0.42, 1.26, 1.61, 1.84 and 1.54 µg/m³, respectively (U.S. EPA, 1999a).
The mean air concentration in approximately 750 homes from 10 Canadian provinces surveyed in 1991 was 1.4 µg/m³, with a maximum value of 165 µg/m³ (Otson et al., 1992). In two homes tested, it was reported that showering with well water containing extremely high levels of TCE (40 mg/L) increased levels of TCE in bathroom air from <0.5 to 67-81 mg/m³ in less than 30 minutes (Andelman, 1985). However, it should be noted that TCE concentrations in Canadian water supplies are usually less than 1 µg/L. Therefore, the Leq/day values outlined above appear reasonable.
4.4 Food
The U.S. EPA (2001a) concluded that exposure to TCE from food was probably low and that there were insufficient food data for reliable estimates of exposure. The daily intakes of TCE in food for Canadian adults (20-70 years old) and children (5-11 years old) were estimated to range from 0.004 to 0.01 µg/kg bw per day and from 0.01 to 0.04 µg/kg bw per day, respectively (Department of National Health and Welfare, 1993). These numbers were based on TCE concentrations from U.S. food surveys from the mid- to late 1980s as well as Canadian food consumption data. In recent decades, severe restrictions have been placed on the use of TCE in food processing in North America, and the disposal of TCE is more carefully controlled in other industrial sectors. Therefore, there is no reason to suppose that these values would have increased in the interim.
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