Cancer risk assessment from trihalomethanes in drinking water

Transcription

1 Science of the Total Environment 387 (2007) Cancer risk assessment from trihalomethanes in drinking water Gen-Shuh Wang a,, Ya-Chen Deng a, Tsair-Fuh Lin b a Department of Public Health, National Taiwan University, Taipei, Taiwan b Department of Environmental Engineering, National Cheng-Kung University, Tainan, Taiwan Received 8 March 2007; received in revised form 6 July 2007; accepted 19 July 2007 Available online 28 August 2007 Abstract This study intends to calculate the lifetime cancer risks resulting from intakes of trihalomethanes (THMs) in drinking water based on the presence of each THM species. The slope factors for each THM species are used, combined with exposure model and Monte Carlo simulations, to calculate the cancer risks with consideration of different exposure routes (oral ingestion, inhalation and dermal absorption). The results revealed that the highest risk comes from the inhalation exposure to chloroform during showers, which also dominates the total risk associated with chloroform exposure. For dichlorobromomethane and chlorodibromomethane, inhalation exposure also plays an important role for total risks; however, contribution from the oral consumption cannot be ignored for these two compounds. Bromoform contributes the least cancer risk among the four THM species, with a risk factor two orders of magnitude smaller than the other three THM species. For all of the four THM species, exposure from dermal absorption is not significant when compared with oral ingestion and inhalation exposures. This study also uses the THMs data collected from Taiwan to calculate the cancer risks associated with THM exposures in different areas of Taiwan. Due to the variations of the THMs compositions, it is observed that higher concentrations of total THMs do not necessarily lead to higher cancer risks. Areas with higher bromide concentration in raw water and often with higher total THM concentration may actually give lower cancer risk if the THMs formed shift to bromoform. However, this also leads to the violation of THM standards since bromoform has much higher molecular weight than chloroform. Based on the results of the cancer risks calculated from each THM species, the regulatory issue of the THMs was also discussed Elsevier B.V. All rights reserved. Keywords: Bromide; Disinfection by-products; Exposure; Lifetime cancer risk; Trihalomethanes 1. Introduction Disinfection is the last step in the water treatment processes for the protection of public health. In Taiwan, chlorine is used as the primary disinfectant because of its low cost and its convenience for application in water purification. However, the presence of THMs in chlorinated drinking water can pose a severe health threat Corresponding author. 17 Xu-Zhou Road, Room 734, Taipei, Taiwan. Tel.: ; fax: address: gswang@ntu.edu.tw (G.-S. Wang). due to its potential carcinogenicity. Numerous epidemiological studies have been conducted to investigate the correlations between chlorination by-products and several diseases (McDonald and Komulainen, 2005; WHO, 2005). It has also been reported that the THMs in drinking water will increase the cancer risks of bladder and cause reproductive defects (WHO, 2005; Yang et al., 1998). Recently several studies have been conducted to assess the potential cancer risks resulted from the exposure of the THMs in drinking water. Due to the lack of available parameters, most of the studies focus on the /$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.scitotenv

2 G.-S. Wang et al. / Science of the Total Environment 387 (2007) risks associated with chloroform; and in fewer cases chloroform and bromodichloromethane are used as the target compounds (WHO, 2005). However, the presence of bromide in raw water leads to the formation of brominated-thms, including bromodichloromethane, chlorodibromomethane, and bromoform. In water treatment systems using raw water with high bromide concentrations, the treatment processes may remove some organic precursors and hence lower the organic precursor concentrations, causing the increase of Br / organic ratio after the treatments. After chlorination, this would result in the shift of THM species from chlorinated-thms to brominated-thms. Black et al. (1996) have shown that greater degrees of risks are associated with chlorination of raw water with higher bromide and organic carbon concentrations. Lee et al. (2004) calculated cancer risks and hazard index of THMs through different exposure routes for tap water in Hong Kong, and reported that exposure through oral ingestion had higher risk than through dermal absorption and inhalation. Similar result was reported by Tokmak et al. (2004), which concluded that the highest risk was from the exposure to chloroform through oral ingestion. However, Nuckols et al. (2005) measured the blood and exhaled air concentrations of THMs for participants before and after exposures to THMs via different activities, and reported that the greatest observed increase in blood or exhaled breath THM concentrations in participants were due to showering, bathing, and hand washing. In a case-control study of bladder cancer, Villanueva et al. (2006) evaluated longterm exposure to THMs via ingestion, inhalation and dermal absorption and showed that the level of individual exposure to trihalomethanes depends on the route of exposure. They also mentioned that evaluation of one single exposure route such as ingestion may lead to misclassification of the total THMs (TTHMs) exposure. It has also been reported that THMs are generally well absorbed, metabolized, and rapidly eliminated by mammals after oral or inhalation exposure (IPCS, 2000). The discrepancy that the importance of the three exposure pathways ranked differently in the studies may be attributed to different concentration and speciation of THMs present in the waters. Despite the distinct properties relevant to human exposure and risk, in many countries (including US, Canada, Australia, etc) THMs in drinking water are regulated based on the total concentration. Although Japan and the World Health Organization have established drinking water guideline values for each THM species, the risks for the chemicals are accounted for oral ingestion only. In fact, THMs in drinking water quality standards were not generally considered for all the exposure pathways. In this study, the cancer risks associated with THMs exposure from drinking water in Taiwan was estimated for each species and each exposure pathway. The THM data in drinking water were first collected for four different areas in Taiwan. Exposure to THMs and associated risks for three pathways (ingestion, inhalation, and dermal absorption) were then estimated. We used this approach to analyze the relative importance of each THM species in terms of cancer risk and a riskbased THM standard for drinking water is discussed. 2. Materials and methods 2.1. Sample collection and analysis Three metropolitans (Taipei, Taichung and Kaohsiung) and an island county (Kinmen) in Taiwan with different water sources were selected for this study. Water samples were collected between July, 2002 and May, 2003 from the studied areas for source water quality and the THMs concentrations in the finished water. Monthly samples were taken from major waterworks and from distribution systems during different seasons. All of the sampling processes followed the drinking water sampling criteria for volatile organic compounds (VOC) regulated by Taiwan Environmental Protection Administration (TWEPA). For THMs analysis, duplicate water samples were collected from each sampling location, and were placed in pre-cleaned 40-mL amber glass bottles with Tefloncoated rubber septa, sealed and stored at 4 C before analysis. All THMs analyses were performed within 14 days after sampling. For each sampling site, another 60-mL water sample was collected for the analysis of non-purgeable dissolved organic carbon (NPDOC). The NPDOC concentration was measured with a Total Organic Carbon analyzer (Shimadzu TOC 5000A). The samples were filtered through a 0.45 μm syringe filter to remove particles and then acidified with one drop of 2 N HCl solution prior to NPDOC analysis. The THMs measurement follows the procedures described in section 5710B of the Standard Methods (APHA, 1989). The four THMs were quantified by a purge and trap system (O.I. 4560) directly coupled to a GC/MS (Hewlett Packard 6890GC/5973MSD) using a fused silica capillary column (RTX-Volatiles, 60 m 0.32 mm 1.5 μm). For THMs analysis, quality control (QC) samples were also prepared and analyzed for each batch of samples. The QC results showed that the relative deviation of duplicates was within 20%, and the recovery of laboratory control standards were between %.

3 88 G.-S. Wang et al. / Science of the Total Environment 387 (2007) Table 1 Input parameters for exposure assessment Input parameters Unit Values Distribution Reference Oral ingestion THMs in water (C W ) μg/l See Table 3 Log normal This study Ingestion rate (IR) L/day 2 USEPA (1997) Inhalation in shower THMs in air (C air ) (mg/l) Model calculations Little (1992) Ventilation rate (VR) m 3 /h 0.84 (male) USEPA (1997) 0.66 (female) Absorption efficiency in alveoli (AE) 50% Lee et al. (2002) Bathroom volume (V S ) m Chen et al. (2003) Water flow rate (Q L ) L/min 5 Little (1992) Air flow rate (Q G ) L/min 50 Little (1992) Water temperature (T) C 44 Lee et al. (2002) Dimensionless Henry's law constants (H) 0.12 (CHCl 3 ) RAIS (2005) (CHCl 2 Br) (CHClBr 2 ) (CHBr 3 ) Overall mass transfer coefficient (K OL A) a (L/min) 7.4 (CHCl 3 ) Little (1992) a 5.9 (CHCl 2 Br) 4.6 (CHClBr 2 ) 3.7 (CHBr 3 ) Exposure time (ET) min/day 18.9 Triangular Lee et al. (2002) Dermal absorption Skin surface area (SA) m 2 (4BW+7)/(BW+90) USEPA (1997) Fraction of skin in contact with water (F) 0.8 Lee et al. (2002) Permeability coefficient (PC) cm/h (CHCl 3 ) RAIS (2005) (CHCl 2 Br) (CHClBr 2 ) (CHBr 3 ) Exposure time (ET) h/day 18.9 Lee et al. (2002) Conversion factor (CF) L/cm Exposure duration (ED) year 29 Lee et al. (2002) Exposure frequency (EF) day/year 365 Lee et al. (2002) Average time (AT) day Lee et al. (2002) Body weight (BW) kg 64.8±10.0 (male) Normal TWDOH (2002) 56.3±9.1 (female) a The K OL A of chloroform was from Little (1992). K OL A for the other three THMs were calculated based on the K OL of each THM species from: 1/K OL = 1/(H*K G )+1/(K L ), where H is the Henry's constant, K G and K L represent the gas- and liquid-film mass transfer coefficient for each compound Exposure assessment Based on the THMs data collected in this study, an exposure assessment was conducted to evaluate the potential THMs uptake via oral ingestion, inhalation, and dermal absorptions. For inhalation, exposure during shower is assumed as the major exposure route. Previous study has shown that inhalation exposure to chloroform in cooking was minor when compared to the inhalation exposure during shower (Lin and Hoang, 2000). The inhalation exposure model, which was developed based on two-resistance theory proposed by Little (1992), was used in this study to calculate the THMs volatilized from the drinking water into the shower room. The equations for calculation of chronic daily intakes are shown below: Oral Ingestionðmg=kg dayþ ¼ ½C W IR EF ED CFŠ= ½BW ATŠ Dermal absorptionðmg=kg dayþ ¼ ½C W SA F PC ET EF ED CFŠ = ½BW ATŠ Inhalation Intakeðmg=kg dayþ ¼ ½C air VR AE ET EF ED CFŠ = ½BW ATŠ

4 G.-S. Wang et al. / Science of the Total Environment 387 (2007) For inhalation intake, C air is calculated by: C air ¼ ðy s ðþþy t si Þ=2; where Y si Y s (t) and is the initial THM concentration in the shower room (assumed as 0 mg/l). is the THM concentration in the shower room at time t (min). Y s ðþ¼ t ½1 exp ð bt ÞŠða=bÞ b ¼ fðq L =HÞ½1 exp ð NÞŠþQ G g=v S a ¼ fq L C W ½1 exp ð NÞŠg=V S N ¼ ðk OL AÞ=Q L where N is a dimensionless coefficient that calculated from K OL. Table 1 gives the parameters and the input values used in this study for exposure assessments. Detailed description for the parameters used in this study can be seen elsewhere (Lin and Hoang, 2000; Lee et al., 2002; USEPA, 1997) Risk calculation The lifetime cancer risks of different THM species were calculated by incorporating exposure assessment and toxicity values (slope factors). In general, the lifetime cancer risk was calculated as: Cancer risk ¼ Lifetime daily THM intake THM slope factor Total exposure cancer risk ¼ Risk oral ingestion þ Risk inhalation þ Risk dermal absorption : The primary sources of the slope factors were Integrated Risk Information System (IRIS, 2005) and Risk Assessment Information System (RAIS, 2005). For the slope factors of inhalation for dichlorobromomethane and chlorodibromomethane, which are not available in IRIS and RAIS, they are assumed to be equal to the slope factors for oral ingestion. This is based on the assumption that THMs are generally well absorbed by mammals after oral or inhalation exposure (IPCS, 2000). A sensitivity analysis will also be made (refer to Section 3.3.) to justify the impact of using these slope factors on the level of inhalation risks. Table 2 summarizes the slope factors used for THMs via different exposure routes. In order to calculate the lifetime cancer risk due to the exposure to each THM species, Monte Carlo simulation with Crystal Ball software was run in an EXCEL 2000 platform. In executing this software, a distribution type of input parameters was used. For example, both mean and standard deviation of THM concentrations and body weight were provided so that the simulations can be used to predict the probability of the cancer risks (Lee et al., 2002). To obtain a representative risk distribution, 5000 times of simulation were conducted in the Monte Carlo calculations for each studied area. 3. Results and discussion 3.1. Concentrations and speciations of THMs in Taiwan The THMs concentration of the water samples analyzed in each sampling area are given in Table 3. Data in Table 3 indicated that the THMs in finished water increase with the increasing concentrations of the organic precursors (shown as DOC). This is in accordance with the general consensus that higher THM concentration is expected for the source water with higher natural organic matter (Singer, 2000). The lowest DBPs concentrations were observed in Taipei and Taichung, where the DOCs in source waters were low. Table 3 also showed large variations in DOC and THMs concentrations in Kaohsiung and Kinmen; where eutrophicated water was used as the raw water. The mean total THMs concentrations are 11.2, 19.4, 42.4 and 62.5 μg/l in Taipei, Taichung, Kaohsiung and Kinmen, respectively. Due to the presence of bromide in the raw water, brominated-thm species were observed in most of the samples analyzed. In general, about 30% 48% of THMs formed after chlorination was presented as brominated-thms in Taipei, Taichung and Kaohsiung. However, the ratio of brominated-thms increased to about 75% in Kinmen Island because of the relatively high bromide concentration in the raw water ( 0.7 mg/l). Compared with the high bromide concentration in Kinmen, the bromide concentrations were approximately 0.04, 0.1 and 0.2 mg/l in Taipei, Taichung and Kaohsiung, respectively (Huang et al., 2005; Wu, 2000). Since the brominated portions of THMs in all the four studied areas in Taiwan are not negligible, as shown in Table 3, it is necessary to consider the long-term cancer risks of both chlorinated and brominated-thms.

5 90 G.-S. Wang et al. / Science of the Total Environment 387 (2007) Table 2 Slope factors of trihalomethanes for risk calculation Chemicals Slope factor (SF) [(mg/kg-day) 1 ] Oral Dermal Inhalation CHCl (IRIS) (RAIS) (IRIS) CHCl 2 Br (IRIS) (RAIS) CHClBr (IRIS) (RAIS) CHBr (IRIS) (RAIS) (IRIS) IRIS: Integrated Risk Information System (IRIS, 2005). RAIS: Risk Assessment Information System (RAIS, 2005). Oral exposure slope factors of bromodichloromethane and chlorodibromomethane were used for inhalation exposure. The data obtained from THM sampling were used for further exposure and risk assessments. Probability distributions for the THMs concentrations in different areas are simulated using Monte Carlo simulations for 5000 times based on the data in Table 3. In the simulation, a log normal distribution was assumed for all the four areas. Fig. 1 illustrates the simulated probability distribution for THMs in the four studied area. These distributions were used for the input data of exposure as well as risk assessment Cancer risk of THMs in Taiwan The lifetime cancer risk from exposures to THMs in drinking water was estimated based on the THM concentrations measured from the four representative areas in Taiwan, as shown in Table 3 and Fig. 1. As indicated in Table 3, the TTHMs in the finished water in Taipei and Taichung were relatively low, and that in Kinmen was the highest. THM values in Table 3 also showed that about 75% of TTHMs in Kinmen is brominated-thms. For comparison, about 53 70% of TTHMs is chloroform in the other three areas, reflecting a very different species distribution of THMs between Kinmen and the other three areas. As an example of the Monte Carlo Simulation of risk calculation, Fig. 2 gives the estimated cancer risks of total THMs via different exposure pathways for male in Kaohsiung. Since the distribution of the cancer risks is log normal, geometric means were calculated for different areas (see Table 4), the modes and the 95th percentiles were also provided. As expected, the lifetime cancer risks from exposure to THMs increase with the increase in THMs concentrations. However, the formation of the various species due to the presence of bromide results in different risk distributions. The lowest lifetime cancer risks in Taipei were due to the lowest THMs concentrations in drinking water. In Taichung the higher chloroform concentration gives higher inhalation risk than the inhalation risks in Taipei. However, the higher brominated-thms concentration in Taipei resulted in higher cancer risks for oral ingestions when compared with the oral ingestion risks in Taichung. For Kaohsiung, the relatively high THM concentrations in drinking water lead to high risk distributions. The geometric means of total risks in Kaohsiung were and for male and female, respectively. For comparison, the geometric means of the total risks in Taipei and Taichung were approximately and , respectively. In Kinmen, however, a different trend was observed. Although Kinmen has much higher THM concentrations ( 63 μg/l total THMs) than those observed in other studied areas in Taiwan (11 42 μg/l total THMs), the lifetime cancer Table 3 Mean THMs concentrations in drinking water, July, 2002 May, 2003 Sampling areas NPDOC TTHMs Mean species concentration Number TWEPA survey a (Average±SD) CHCl 3 CHCl 2 Br CHClBr 2 CHBr 3 of samples TTHM Taipei City 1.1± ± ±5.5 (78) Taichung City 1.2± ± ±7.4 (10) Kaohsiung City 1.7± ± ±17.5 (680) Kinmen County 6.1± ± ±3.3 (6) Units: NPDOC in mg/l, THMs in μg/l. a Data in this column were taken with permission from Taiwan EPA's drinking water quality survey databases (unpublished), only the data collected between July, 2002 and May, 2003 were used. Values in parenthesis are the number of samples.

6 G.-S. Wang et al. / Science of the Total Environment 387 (2007) risk distribution in Kinmen area ( and for male and female, respectively) is about the same order of magnitude as obtained in Kaohsiung. Compared with those observed in Kaohsiung, the high ratio of brominated-thms in Kinmen area was the major factor governing the relatively lower cancer risk. In Kinmen, about 75% of the trihalometanes contain bromine and 30% of the total THMs were bromoform. In most of the Taiwan area, about 30% 47% of the total THMs contains bromine and on average only approximately 10% was bromoform. Since bromoform possesses lower risks than the other three THM compounds, the overall lifetime risk in Kinmen due to the exposure to THMs were lower than expected as those obtained in Taiwan. It was also observed that male has higher risk from inhalation exposure and lower risk from oral ingestion than female. This is due to higher inhalation rate for male and lower bodyweight for female. One of the factors that may strongly alter the estimated exposure and risk in Taiwan is boiling of water. This factor is also discussed in this section. Since most people in Taiwan boil tap water before they drink it, reduction of THM concentration in drinking water and the reduction in the associated risk from oral ingestion is expected. For example, Lee et al. (2002) have reported that the boiling of water reduced essentially all of the VOCs including chloroform in Fig. 1. Simulated TTHMs concentration distributions in Taiwan after Monte Carlo calculations. Fig. 2. Estimated cancer risks of THMs via different exposure routes for males in Southern Taiwan (after 5000 random calculations with Monte Carlo simulations). drinking water. With a conservative assumption that the boiling process removes 80% of the THMs, Table 4 also showed that boiling water is useful in reducing the risks associated with oral ingestion exposure of THMs. This also increases the ratio of the final risks resulted from the inhalation pathway during shower Sensitivity analysis of risk calculations In this study, the slope factors of dichlorobromomethane and chlorodibromomethane via oral ingestion were used for those of risk calculations via inhalation exposures. In order to determine the impact of the assumed inhalation slope factors on the final cancer risks, Table 4 also showed the total cancer risks of THMs when the inhalation slope factors of the two compounds were increased or decreased by 50%. As shown in the table, inhalation is still the major exposure pathway for THMs for all the studied areas under the three scenarios. This is still true for Kinmen, where the largest changes on total risks were obtained due to the higher portion of brominated-thms. The results support the inference that the inhalation exposure is the most important pathway for the cancer risks of THMs. Based on the risks shown in Table 4, oral ingestion of THMs in water could be the major route of exposure when the water is not boiled. This is especially important for Kinmen where brominated-thms are the major THM species. However, the inhalation exposure from showering becomes the major exposure route when the water is boiled; in this condition, inhalation of chloroform is the major contributor for lifetime cancer risk.

7 92 G.-S. Wang et al. / Science of the Total Environment 387 (2007) Table 4 Lifetime cancer risks from THMs in drinking water a (in 10 5 ) Routes Taipei Taichung Kaohsiung Kinmen Male Female Male Female Male Female Male Female Ingestion: Non-boiled 0.66 (0.48, 1.92) 0.76 (0.49, 2.19) 0.54 (0.35, 1.60) 0.62 (0.49, 1.83) 1.75 (1.12, 4.36) 2.01 (1.55, 4.94) 3.07 (2.73, 6.34) 3.55 (3.80, 7.30) Boiled 0.13 (0.10, 0.38) 0.15 (0.10, 0.44) 0.11 (0.07, 0.32) 0.13 (0.10, 0.37) 0.35 (0.23, 0.87) 0.40 (0.31, 0.99) 0.62 (0.55, 1.27) 0.71 (0.76, 1.46) Dermal 0.01 (0.008, 0.04) 0.01 (0.008, 0.04) 0.01 (0.008, 0.04) 0.01 (0.009, 0.05) 0.03 (0.02, 0.11) 0.03 (0.02, 0.11) 0.05 (0.04, 0.12) 0.05 (0.03, 0.13) Inhalation: Shower 1.22 (0.90, 4.99) 1.11 (0.78, 4.75) 2.01 (1.05, 7.20) 1.82 (1.24, 6.60) 4.26 (2.68, 15.99) 3.87 (2.49, 14.64) 4.32 (3.39, 12.38) 3.92 (3.04, 11.07) Total risk 2.02 (1.14, 6.51) 2.02 (1.04, 6.39) 2.69 (1.32, 8.49) 2.60 (1.60, 8.08) 6.41 (3.45, 19.25) 6.31 (4.99, 18.52) 7.77 (6.99, 17.62) 7.88 (7.10, 17.26) Total risk (boiled) b 1.41 (0.95, 5.31) 1.32 (0.86, 5.09) 2.17 (1.12, 7.52) 2.00 (1.33, 6.89) 4.76 (2.88, 16.74) 4.43 (2.71, 15.46) 5.11 (4.42, 13.40) 4.82 (4.05, 12.27) Calculated cancer risk from inhalation exposures with slope factor of CHCl 2 Br and CHClBr 2 reduced by 50% Inhalation: Shower 0.90 (0.79, 4.16) 0.82 (0.70, 3.80) 1.78 (0.88, 6.75) 1.62 (0.77, 6.23) 3.45 (2.98, 14.57) 3.13 (2.38, 13.21) Total risk (1.44, 5.54) (1.36, 5.51) (1.46, 7.81) (1.67, 7.41) (3.57, 17.84) (3.18, 16.83) Total risk (boiled) b (0.82, 4.47) (0.74, 4.14) (0.94, 6.96) (1.36, 6.47) (3.15, 15.24) (2.60, 13.94) 3.07 (2.13, 9.82) 6.64 (6.20, 14.92) 3.90 (2.78, 10.89) 2.79 (2.09, 8.80) 6.85 (5.51, 14.95) 3.73 (2.78, 10.06) Calculated cancer risk from inhalation exposures with slope factor of CHCl 2 Br and CHClBr 2 increased by 50% Inhalation: Shower 1.45 (0.80, 5.87) 1.31 (0.99, 5.36) 2.20 (1.97, 7.70) 1.98 (1.41, 6.96) 4.96 (3.50, 17.98) 4.48 (2.59, 16.29) Total risk (1.52, 7.36) (1.21, 7.23) (2.84, 9.03) (2.17, 8.42) (4.10, 21.43) (4.87, 20.41) Total risk (boiled) b (0.86, 6.21) (1.07, 5.73) (2.16, 7.99) (1.58, 7.26) (3.69, 18.71) (2.76, 17.16) 5.46 (3.81, 15.12) 9.04 (8.09, 20.45) 6.25 (4.74, 16.28) 4.92 (3.74, 13.60) 8.99 (7.07, 20.00) 5.84 (5.58, 15.02) a Cancer risks were calculated based on the THMs concentrations shown in Table 3. Data shown were geometric means of the risk distributions for each route/area. Values in the parenthesis are the modes and 95th percentiles of the calculated risk distributions. b Total Risk (boiled) represented the total cancer risks calculated with the oral ingestion of boiled water Comparison of health risk associated with THM species Table 5 summarizes the specific cancer risks (the lifetime cancer risk at exposure to 1 μg/l of each THM species) of the four THM species from different routes of exposures. With oral intake of chloroform defined as unity, the relative health risk from various exposure routes for each THM species were also calculated (shown also in Table 5). The highest risk (on 1 μg/l basis) comes from inhalation exposure to chloroform, which also dominates the total risk associated with chloroform exposure. The calculated lifetime risk from inhalation exposure of chloroform at 1 μg/l was ,whichhasalready Table 5 Specific cancer risks a for each THM species Route Oral Inhalation Dermal Total THM species Risk Relative risk a Risk Relative risk a Risk Relative risk a Risk Relative risk a CHCl CHCl 2 Br CHClBr CHBr The calculated risks shown were lifetime cancer risks based on exposure to 1 μg/l of each species. a Relative risk represents the risk compared to that from oral ingestion of chloroform.

8 G.-S. Wang et al. / Science of the Total Environment 387 (2007) exceeded the general guidance risk value for Class A carcinogens ( ). This result reveals that exposure to chloroform through inhalation during shower is the most important pathway for cancer risks from THMs. For dichlorobromomethane and chlorodibromomethene, inhalation exposure ( and , respectively) also plays a major role for total risks; however, contributions from oral consumption ( and , respectively) cannot be ignored for these two compounds. Bromoform contributes the least cancer risk ( and for oral intake and inhalation exposure, respectively) among the four THMs, with a risk factor two orders of magnitude smaller than the other three THM species. For all of the four THM species, exposure from dermal absorption is not significant when compared with oral intake and inhalation exposures. Compared with the brominated- THMs, the high inhalation risk of chloroform comes from its high Henry's constant (H), mass transfer coefficient (K OL A) and high inhalation slope factor, as shown in Tables 1 and 2. The calculated cancer risks for THMs also showed that about 80% 90% of the total risks due to exposure of THMs were from inhalation exposure, which indicates that the inhalation exposures to volatile THMs are the major pathway for the cancer risks resulted from the THMs in drinking water. This observation is consistent with the results reported by Nuckols et al. (2005), in which inhalation exposure to THMs is suggested to be an important pathway for THM. The calculated risks in Tables 4 and 5 also showed that the presence of bromide in raw water not only increased the concentrations of the THMs in finished water but also changed the potential cancer risks associated with the exposure to THMs. The final cancer risks from exposure to THMs depend on the relative distribution of the THM species in finished water, and higher TTHM concentration does not necessarily introduce higher cancer risk. When bromoform is the major brominated-thm species, the overall risk may be lower than usually expected Implications on water quality standards for trihalomethanes Currently most countries and organizations set up water quality standards for disinfection by-products based on the TTHMs. For example, 80 or 100 μg/l standard was set for TTHMs in most countries. However, the presence of bromide in raw water shifts the chlorinated-thms to brominated-thms and complicates this widely used guideline. As shown in Table 5, the four THM species give different long-term carcinogenicities, and therefore, the current water quality standards for disinfection byproducts need to be reviewed to account for the different risks resulting from different THM species. Based on the current TTHMs standards, all of the four THM species were treated as with the same health risk, and this may leads to unsuitable conclusions for the potential carcinogenicities from THMs exposure. For example, with higher bromide in raw water, the final disinfection byproducts will be dominated by brominated-thms, such as chlorodibromomethane or bromoform. Under this condition, the overall cancer risks will not be the same as those exposed to same concentration of chloroform and/or bromodichloromethane. In some cases, the lifetime cancer risk may even be reduced when a higher portion of bromoform is formed. Ironically, the TTHMs concentrations will increase due to the presence of bromide since bromine has higher atomic weight than chlorine. In order to avoid the biased contribution of the high molecular weight THM species to the total mass concentration, the TTHMs may be calculated on the basis of the chloroform by conversion of the molar concentrations for brominated-thms. To understand the total health risk associated with THM exposure from drinking water, Table 6 gives the Table 6 THM concentrations corresponding to specific cancer risks THM species This study a (10 6 ) (10 5 ) USEPA b (10 5 ) Canada c (10 5 ) WHO d (10 5 ) WHO guidelines d CHCl CHCl 2 Br CHClBr CHBr a The calculated THM concentration (in μg/l) corresponding to risks shown were based on the specific cancer risks shown in Table 5. b USEPA (2005). c Health Canada (2006). d WHO (2005). e JWWA (2003). Japanese Water Quality Standards also regulate a Total THMs of 100 μg/l. Japan standards e

9 94 G.-S. Wang et al. / Science of the Total Environment 387 (2007) concentration of each THM species corresponding to 10 6 and 10 5 cancer risks. Note that all the three exposure pathways, oral ingestion, inhalation, and dermal absorption were considered, and the calculation was based on the parameters in Tables 1 and 2. For comparison, the THM concentrations from the drinking water standards and guidelines in different countries and organizations were also provided for the same risk level. It is evident that the suggested THM standards obtained in this study is smaller than all the corresponding THM concentration from other countries (Table 6) at the same estimated cancer risk level. Although most countries have already established a contribution factor of 20% 80% to account for the percentage of total daily exposure that is attributable to oral ingestion of tap water, our estimated concentrations (with safety factor=1) are still lower than those listed in all the standards and guidelines. Since most of the studies/ standards only account for the exposure route of oral ingestion, the importance of accounting for inhalation route is obvious, as suggested in the previous sections. Therefore, there is a need to consider all the exposure pathways as well as each THM species into the regulation of THM concentrations in drinking water standards. 4. Conclusions The specific cancer risks (lifetime cancer risk at exposure to 1 μg/l of each THM species) of the four THM species from different routes of exposure were estimated in this study. The results revealed that the highest risk comes from inhalation exposure to chloroform. The calculated lifetime risk from inhalation exposure of chloroform at 1 μg/l was , which has exceeded the general guidance risk value ( ). For dichlorobromomethane and chlorodibromomethene, inhalation exposure ( and , respectively, per μg/l) also plays the major role for total risks; however, contributions from the oral consumption ( and , respectively, per μg/l) cannot be ignored for these two compounds. Bromoform contributes least health risk ( and for oral intake and inhalation exposure, respectively, per μg/l) among the four THMs, with a risk factor two orders of magnitude smaller than the other three THM species. For all of the four THM species, exposure from dermal absorption is not significant. This study also calculated the cancer risks in different areas of Taiwan. Due to the variations of the THMs species, it is concluded that higher concentrations of THMs do not necessarily lead to higher risks. The reason is due to the relatively lower total risks of the brominated-thms from different pathways. Since bromoform gives lower total risk than chloroform, the areas with higher brominated-thms in finished water may actually give lower cancer risk than the areas with lower chlorinated-thms. When the water is not boiled, the total risk calculation showed that the oral ingestion of THMs could be the major route of exposure when brominated-thms are the major THM species in water. However, the inhalation exposure from showering becomes the major exposure route when water is boiled; while inhalation of chloroform is the major contributor for lifetime cancer risk. Although lower risk is expected, the presence of higher bromoform concentration in water may lead to the violation of THM standards since bromoform has much higher molecular weight than chloroform. 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