Page 7: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Trihalomethanes
5.0 Exposure
Although extensively studied, the chemistry of the reactions between chlorine and the organic materials present in water is complex and poorly understood; however, important factors include naturally occurring organic precursor concentration, chlorine dose, contact time, water pH and temperature, and bromide ion concentration. Consequently, there is a great degree of variation in the measured concentrations of THMs in drinking water.
Levels of chloroform, the most common THM, are generally higher in chlorinated water originating from surface water compared with groundwater, because of higher organic matter in the former. The extent of formation of chloroform varies with different water treatment processes. Concentrations of chloroform in chlorinated water in treatment plants and distribution systems are approximately twice as high during summer months as during winter months. This is a consequence of the higher concentrations of precursor organic materials in the raw water during the warmer period and especially because the rate of formation of DBPs increases with rising temperatures (LeBel et al., 1997). Levels can increase as the chlorinated water moves from the water treatment plant through the distribution system, because of the continued presence of a chlorine residual. Further increases in concentrations of chloroform in water can occur in domestic hot water tanks. However, storage in the hot water tank increases the level of chloroform twice as much in the winter, when more hot water is required to maintain the shower temperature, as in the summer, so that concentrations of chloroform in the warm water used for showering are relatively constant in both seasons (Williams et al., 1995; Benoit et al., 1997).
Concentrations of THMs have been determined in drinking water supplies at a considerable number of locations across Canada (Water Quality Issues Sub-Group, 2003). Eight provinces provided 1994-2000 THM data for just over 1200 water systems serving a sampled population of over 15 million Canadians. The methods of sampling and analysis varied and were often not well described, but generally samples were taken from the midpoints and/or endpoints of the water systems, and the typical methods of analysis were either liquid-liquid extraction or purge-and-trap gas chromatography.
Based on the data received from the eight provinces, the mean THM level was about 66 µg/L in drinking water samples from all systems. Some systems had average values in the 400 µg/L range, and some systems had maximum or peak values in the 800 µg/L range. From the eight provinces, 282 water systems (23% of sampled systems), representing a sampled population of 523 186 (3.4% of sampled population served), reported having mean THM levels greater than 100 µg/L, while 506 water systems (41%), serving a sampled population of 2 509 000 (16%), reported at least one instance of THM levels being greater than 100 µg/L (Water Quality Issues Sub-Group, 2003).
System mean chloroform levels for 1994-2000 were generally less than 50 µg/L, with some single maximum or peak values in the 400 µg/L range. From those suppliers who reported chloroform data, 290 water systems (26%), serving a sampled population of 1 130 000 (8%), reported mean chloroform levels greater than 75 µg/L, while 425 water systems (39%), serving 1 740 000 (12%) consumers, had a peak concentration greater than 75 µg/L in their drinking water during this period (Water Quality Issues Sub-Group, 2003).
Mean concentrations of both BDCM and DBCM in systems were generally less than 10 µg/L, although some averages were higher, and several locations reported one-time samples in excess of 200 µg/L. From those suppliers that reported BDCM data, 87 water systems (8% of reporting systems), representing a sampled population of 285 000 (2% of population served), reported having mean BDCM levels greater than 10 µg/L, while 192 water systems (18%), serving a sampled population of 1 165 000 (8%), reported at least one instance of BDCM levels being greater than 10 µg/L (Water Quality Issues Sub-Group, 2003).
Mean concentrations of bromoform were typically less than the detection limit, or approximately 0.5 µg/L, and individual values were less than 10 µg/L. In a few systems, however, average and maximum bromoform levels exceeded 30 µg/L over this period (Water Quality Issues Sub-Group, 2003).
Generally speaking, the smaller centres with less sophisticated treatment systems had higher THM levels in their drinking water. In this 1994-2000 national survey, it was found that where the population was unreported or less than 1000, 274 of systems had average THM levels greater than 75 µg/L, and 45 systems had average BDCM levels greater than 10 µg/L. Conversely, where the population was greater than 50 000 (and where more sophisticated treatment plants would be expected), there were only four systems whose average THM levels were greater than 75 µg/L, and only one system had an average BDCM level greater than 10 µg/L. For population centres with greater than 10 000 people, the 118 systems serving 11 036 000 people had an average system THM level of 37 µg/L -- a value significantly lower than the average of 66 µg/L reported for all systems, regardless of size. For population centres with greater than 50 000 people, the 41 systems serving 9 439 000 people had an average THM level of about 27 µg/L (Water Quality Issues Sub-Group, 2003).
The importance of exposure to chloroform and BDCM via inhalation and dermal absorption from tap water during showering and bathing was evaluated. A modifying factor for each compound, in terms of litre-equivalents per day (Leq/day), was estimated by evaluating the relative contribution of inhalation and dermal exposures associated with showering and bathing.
Krishnan (2003) determined Leq/day values for dermal and inhalation exposures of adults and children (6-, 10-, and 14-year-olds) during showering and bathing with tap water containing chloroform (5 µg/L) and BDCM (5 µg/L).Footnote 3 The Leq/day values for a 10-minute shower and a 30-minute bath were calculated using the physiologically based pharmacokinetic (PBPK) model-generated data on the absorbed fraction (Corley et al., 1990, 2000; Haddad et al., 2001; Price et al., 2003). The "absorbed fraction" for the dermal and inhalation exposures took into consideration the dose that was absorbed following exposure as well as that portion that was exhaled in the following 24 hours.
Calculations done for chloroform and BDCM accounted for inter-chemical differences in water-to-air factor (based on differences in Henry's law constants), fraction of dose absorbed during inhalation and dermal exposures, and skin permeability coefficient. Complete (100%) absorption of ingested chloroform and BDCM in drinking water was assumed for all subpopulations; this was supported by the available information on the extent of hepatic extraction of these THMs (Corley et al., 1990; DaSilva et al., 1999).
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 total exposure values for drinking water were for adults in the 30-minute bath scenario: 4.11 Leq/day (1.5 L ingestion, 1.7 L inhalation, 0.91 L dermal) and 3.55 Leq/day (1.5 L ingestion, 0.67 L inhalation, 1.38 L dermal) for chloroform and BDCM, respectively. Both values are 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 chloroform or BDCM via other household activities or additional bathroom time, the above-calculated Leq/day values (which account for inhalation and dermal exposures from a 30-minute bath) are considered to be adequate for assessment.
Data from the United States and Canada were sufficient to serve as a basis for estimating the minimum, midpoint, and maximum concentrations of chloroform in 131 of the 181 foods for which per capita daily intake rates (i.e., g/day) are available. The midpoint concentrations were greater than 100 µg/kg in 12 food items (i.e., butter, margarine, vegetable fats and oils, baby food cereal, pizza, marine fish, fresh fish, crackers, pancakes, veal, beef roast, and cheese). The highest concentrations of chloroform have frequently been measured in dairy products (Environment Canada and Health Canada, 2001).
Maximum concentrations of 2200 µg chloroform/kg and 3 µg BDCM/kg were detected in the fat of nine species of fish from six areas of the Norwegian coastline that were contaminated principally by discharges from pulp and paper plants, but also by agricultural runoff, chemical plants, and other industries. Bromoform and DBCM were detected in only one sample, at concentrations of 115 and 9 µg/kg, respectively (Ofstad et al., 1981). Neither chloroform nor BDCM was detected in composite samples of meat/fish/poultry (quantitation limits were 18 and 4.5 ng/g, respectively) or oil/fat (quantitation limits were 28 and 8.3 ng/g, respectively) from 39 different foods in the United States (Entz et al., 1982). In the composite sample of dairy foods, concentrations of chloroform and BDCM were 17 and 1.2 µg/L, respectively.
THM concentrations in six different cola and non-cola beverages (five samples of each) in New Jersey ranged from 3.2 to 44.8 µg/L (Abdel-Rahman, 1982). Concentrations of chloroform and BDCM in unspecified beverage composites from the United States averaged 32 and 1.0 µg/L, respectively (Wallace et al., 1984). Chloroform concentrations are approximately 10 times higher in cola soft drinks than in non-cola soft drinks, even for similar water sources (Abdel-Rahman, 1982; Entz et al., 1982; Wallace et al., 1984). This may be due to the method of extraction of the cola or the presence of caramel in these soft drinks. Chloroform was detected in 11 of 13 beverages sampled in Ottawa, at a maximum concentration of 14.8 µg/kg in a fruit drink (Environment Canada and Health Canada, 2001).
In the United States, emissions from approximately 5000 materials were determined, with a small number of these products emitting chloroform, usually in trace amounts. Emissions of chloroform were detected from the following materials (with median emission levels reported in parentheses): ink and pen (10.0 µg/g), miscellaneous housewares (4.85 µg/g), photographic equipment (2.5 µg/g), rubber (0.9 µg/g), electrical equipment (0.23 µg/g), lubricant (0.2 µg/g), adhesives (0.15 µg/g), fabric (0.1 µg/g), photographic film (0.1 µg/g), tape (0.05 µg/g), and foam (0.04 µg/g) (Environment Canada and Health Canada, 2001).
The use of swimming pools results in inhalation and dermal exposure to THMs due mainly to the reaction between chlorine and organic matter. In indoor pool environments, concentrations of chloroform in plasma increase with the level of exertion of swimmers and are closely correlated with the chloroform concentrations in air and time spent swimming (Aggazzotti et al., 1990). In general, competitive swimmers are potentially exposed to higher levels of chloroform than are leisure swimmers due to higher breathing rates and longer durations of exposure (Health Canada, 1999).
The inhalation route appears to be significantly more important than the dermal route for swimmers. Levesque et al. (1994) determined that when swimmers (in indoor pools) are exposed to high concentrations of chloroform in the pool water and air, 78% and 22% of the body burden were due to inhalation and dermal uptake, respectively. Limited information suggests that users of hot tubs may have more significant dermal uptake than swimmers due to higher water temperatures (Wilson, 1995).
Estimates of total chloroform exposure for the general population and the relative contribution of drinking water to total exposure were calculated by the World Health Organization (WHO, 2005). In this estimate, mean intake of chloroform from indoor air was estimated to be 0.3-1.1 µg/kg bw per day. The average intake of chloroform (inhalation and dermal absorption) during showering was 0.5 µg/kg bw per shower. Preliminary results from a study by Benoit et al. (1998), based on four volunteers, suggested that showering for 10 minutes with warm water that has been treated with a chlorinated disinfectant is equivalent to drinking 2.7 L of cold water per day from the same water supply, on an annual average. Dermal absorption accounted for an average of 30% of the total uptake. The estimated mean intake of chloroform from ingestion of drinking water for the general population, based on an average concentration of <20 µg/L, is less than 0.7 µg/kg bw per day. The estimated intake of chloroform from foodstuffs is approximately 1 µg/kg bw per day. Outdoor air exposure is estimated to be considerably less than exposure from other sources. The total estimated mean intake is approximately 2-3 µg/kg bw per day; for some individuals living in dwellings supplied with tap water containing relatively high concentrations of chloroform, estimates of total intake are up to 10 µg/kg bw per day (WHO, 2005).
As described above, swimming pools are an additional source of exposure to chloroform among swimmers. The daily dose of chloroform resulting from a 1-hour swim (65 µg/kg bw per day) in conditions found in public indoor swimming pools is much greater than any of the exposures estimated above (Levesque et al., 1994).
The Canadian Environmental Protection Act, 1999 (CEPA) Priority Substances List assessment report on chloroform (Environment Canada and Health Canada, 2001) developed deterministic estimates of chloroform exposure for six age groups based on data on concentrations of chloroform in outdoor and indoor air acquired in national surveys in Canada and on estimates of the concentrations of chloroform in foods in Canada and the United States. Estimates of intake in drinking water were based on monitoring data from the provinces and territories. Estimates of the average daily intake of chloroform by inhalation and dermal absorption during showering were also derived for teenagers, adults, and seniors.
Based on this report, the main sources of exposure to chloroform for the general population in Canada are inhalation of indoor air and ingestion of tap water. The contributions of outdoor air and food are considerably less than the contributions from indoor air and tap water (Environment Canada and Health Canada, 2001). Most of the chloroform in indoor air is present as a result of volatilization from drinking water (WHO, 2005). A more recent study using PBPK modelling (Krishnan 2003) found the highest chloroform exposure values among adults taking a 30-minute bath daily.
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