CANCER RISK ANALYSIS AND ASSESSMENT PROCEDURE OF TRIHALOMETHANES IN THE DRINKING WATER TREATMENT PLANT

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1 Š Ò š š Journal of the Chinese Institute of Environmental Engineering, Vol. 15, No. 4, pp (2005) CANCER RISK ANALYSIS AND ASSESSMENT PROCEDURE OF TRIHALOMETHANES IN THE DRINKING WATER TREATMENT PLANT Han-Keng Lee, Yir-Yarn Yeh, * Wei-Ming Chen and Yi-Jie Huang Department of Water Resource Engineering Feng-Chia University Taichung 407, Taiwan, R.O.C. Key Words : DBPs, Monte Carlo simulation, Multistage of Benchmark model, BMD, sensitive analysis ABSTRACT This study conducts risk assessment for an array of health effects that may result from exposure to disinfection by-products (DBPs), and that require an analysis of the relationship between exposure and health-related outcomes. Trihalomethanes (THMs) species have been verified the principal DBPs in the process on the disinfection of drinking water. Consequence of epidemiology and animal studies exhibited THMs species are carcinogens, caused the perspective cancers of bladder, colon, rectum and liver etc. All of the data collection is supported from Taiwan Water Corporation (TWC) since 1998 to 2002 of Taiwan regions. Utilization the statistical analysis and Monte Carlo Simulation (MCS) to estimate the cancer risk analysis and assessment. With regard to the order of mean concentration distribution of total THMs of Taiwan is external island (48.39 µg/l), southern (17.28 µg/l), northern (12.11 µg/l) and middle segment (9.59 µg/l), the predominated THMs species in external island is tribromomethane (TBM), and other segments are trichloromethane (TCM), respectively. Performed by the multistage type of Benchmark model, it can evaluate the dose-response assessment. Consequence exhibited in the 95% confidence level, the Benchmark dose (BMD) and the quantity of Lower-bound confidence limit of the BMD (BMDL) of TCM are 1.69 and 1.15 mg/kg/day, dibromochloromethane (DBCM) are 1.88 and 1.20 mg/kg/day, dichloromethane (DCBM) are 2.28 and 1.35 mg/kg/day, and TBM are 17.6 and 10.3 mg/kg/day, respectively. Compared the exposure with the reaction dose concentration of THMs species in drinking water, it s a rather lower probability of cancer risk. In the exposure assessment calculated by MCS, consequence exhibited the principal pathway is inhalation, and the next is ingestion and dermal intake pathways. The quantity of average risk in male and female are ~ and ~ in northern, ~ and ~ in middle, ~ and ~ in southern, and and in external island, respectively. Mainly, in this study, exposure time and body weight are the effective parameters in the sensitive analysis. The more of exposure time is extend, the more of cancer risk is endured. In proportion to body weight, it s displayed the negative correlation that concerned the sustain risk probability of unit dose. INTRODUCTION Chlorination has been the major strategy for the drinking water disinfection that an economic and effective method to eliminate microorganisms which may induce tremendous public health of serious waterborne infectious diseases in Taiwan. Research consequence of [1-2] had exhibited the DBPs were formed in the disinfection process. Nowadays, such disinfection process is most adopted chemical disinfectant in drinking water treatment commonly [3-5]. Directly, the principal sources of drinking water are from river approximated 59.3%, from groundwater is 21.7% and from reservoir is 19.0% respectively [5]. But it s approximated 36.0% rivers had polluted that is predominant in water supply presently. How to provide safety drinking water is more important because the popularized rate of drinking water supply is * To whom all correspondence should be addressed. address: yyyeh@fcu.edu.tw

2 234 Journal of the Chinese Institute of Environmental Engineering, Vol. 15, No. 4 (2005) Table 1. The ranges of variables, means, maximum, minimum and standard deviations of subscriber. Location Northern! Midland! Southern! Eastern! External! Yi-Lan 2 Ji-Long Tai-Pei Tao-Yuan Shin-Chu Miao-Li Tai-Chung Nan-Tou Chang Hua Yun-Lin Chia-Yi Tai-Nan Kao-Chiong Ping-Dong Hua-Lian 2 Tai-Dong Peng-Hu Temperature Mean Max Min Stdev ph Mean Max Min Stdev Number! Trichloromethane TCM, µg/l Mean f f Max f f Min f f Stdev f f Dichlorobromomethane DCBM, µg/l Mean f f Max f f Min f f Stdev f f Number! Dibromochloromethane DBCM, µg/l Mean f f Max f f Min f f Stdev f f Tribromomethane TBM, µg/l Mean f f Max f f Min f f Stdev f f Total Trichloromethane TTHM, µg/l Mean f f Max f f Min f f Stdev f f Number! Note: 1 Mean: average value, Max: maximum value, Min: minimum value, Stdev: standard deviation and Number: sampling number. 2 Yi-Lan and Hua-Lian lacked TTHMs.

3 Han-Keng Lee et al.: Cancer Risk Analysis and Assessment Procedure of Trihalomethanes 235 in the Drinking Water Treatment Plant Table 2. Basic processes involved in carcinogenesis of USEPA. Classification Level Evidence descriptions Group A Human Carcinogen Sufficient human evidence for causal association between exposure and cancer Group B1 Probable Human Carcinogen Limited evidence in humans Group B2 Probable Human Carcinogen Inadequate evidence in humans and sufficient evidence in animals Group C Possible Human Carcinogen Limited evidence in animals Group D Not Classifiable as to Human Carcinogenicity Inadequate evidence in animals Group E No Evidence of Carcinogenicity in Humans At least two adequate animal tests or both negative epidemiology and animal studies reached 88.68% (18,497,289 persons, 2004, Web site of TWC [6] ). However, evaluating to animal and epidemiology studies, developmental toxicity and adverse effect are main consideration of potential risk or hazard to human. The result of animal studies demonstrated the evidence of liver, kidney, intestinal tumor genesis, urinary bladder, rectum and colon cancer [7-9] and some associated effects of intrauterine growth and retardation [10], low birth weight, small for gestational age, CNS defects, oral cleft defects and cardiac defects [11], retarded fetal growth [12] and spontaneous abortion [13] are caused by disinfection water. Epidemiologic studies have been conducted that examined the possible associations between consumption of chlorinated drinking water and cancer mortality, risk or incidence [4, 9,14-23]. The results suggest a positive association between consumption of chlorinating drinking water and cancer of the rectum, lung, bladder and kidney. As a consequence for many drinking water contaminants, there is the potential for exposure and uptake not only by ingestion but also through dermal absorption or by inhalation [24]. Since 1990, scientists proposed that inhalation and dermal absorption be considered in the risk assessment of drinking water [25-27]. The risk assessment paradigm as generally practiced was elaborated by the US National Research Council (NRC) in 1983 [28]. Evaluated procedures of framework contained hazard identification, doseresponse assessment, exposure assessment and risk characterization, mainly. The purpose of this study conferred the conception of risk assessment to process the derived THMs species in the drinking water of Taiwan. MATERIALS AND METHODS I. THMs Species Data This study defined geographical distribution divided Taiwan into five parts (northern, midland, southern, eastern and external island) and total data are 4,940 included temperature, ph and total THMs species in drinking water were obtained from the annual reports (since 1998~2002) of the TWC in the subscriber level (Table 1). II. Cancer Risk Analysis and Assessment 1. Hazard identification USEPA has developed a flow chart of procedure (Fig.1) and categorization scheme which contains two broad categories: sufficient and insufficient evidence in Table 2. Previous studies [29] defined THMs species includes trichloromethane (chloroform, TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM) and tribromomethane (TBM), and animal and epidemiology studies consequence exhibited THMs species considered weight of evidence and cancer guideline description in this study are Group B2 is TCM, BDCM, DBCM, respectively and Group C is TBM [30]. Chlorinated drinking water typically contains chloroform, along with other THMs species and a wide variety of other DBPs by U.S. EPA. 2. Dose-response Assessment Cancer risk analysis and assessment utilized the scientific method to qualitative and quantitative the hazard materials when human exposure in the environment [28]. Specially, the purpose of assessment developed for the management of risk-based to ensure the safety of drinking water and supply the procedure to control the water quality. Hazard occurred probability and effect of the environmental pollutions are the decision process of analysis and assessment. In the dose-response step, the goal is to determine the relationships between the route, dose, frequency and duration of exposure conditions and the health effects a chemical produces. Additionally, application of uncertainty or safety factors and mathematical modeling may an approach by Benchmark modeling. USEPA Benchmark model [31] supported the assessment tool in the low-dose of animal experiment that may cause the influence observably. For example, adverse effects included reproductive, developmental toxicity or death etc. USEPA risk assessment guid-

4 236 Journal of the Chinese Institute of Environmental Engineering, Vol. 15, No. 4 (2005) Table 3. Animal experiment data of carcinogenic derives materials of THMs species [33]. Chemicals Data Set Dose (mg/kg/day) TCM Moderate or marked fatty cysts in males plus females Data Values No. Incidence References NTP, 1987 [34] BDCM B6C3F1 mice, male NTP, 1987 [34] DBCM mouse/b6c3f1, female NTP, 1985 [35] TBM F344/Nrat, Female NTP, 1989 [36] Dose-response assessment Hazard identification Risk characterization Exposure assessment Fig. 1. Health risk assessment procedure of THMs in drinking water by USEPA. ance [32] indicated the procedures of calculating Benchmark dose (BMD, USEPA provides a more quantitative alternative to the first step in the doseresponse assessment than the current NOAEL/LOAEL process for non-cancer health effects, and is similar to that for determining the point of departure (POD) proposed for cancer endpoints) following the steps. (1) Fitness determination: utilized the fitted mathematic regression model to estimate the BMD that will addition less than 20% extra risk to calculate the Benchmark response (BMR) fitting the BMD in the low dose level. (2) Classification of data pattern: Dichotomous, Continuous and Categorical category are based on the different experiment design and dose-response data. Multistage type of Dichotomous has a well interpretation in the relation between THMs species exposure dose and hazard risk assessment (USEPA, 2000). (3) BMR value optimum: BMD value is decided by assuming BMR, USEPA (2000) suggestion the BMR value is extra ten-fold into BMD value in the model. (4) Quantity of BMD/Benchmark Dose Limit (BMDL, it refers to the corresponding lower limit of a onesided 95% confidence interval on the BMD) determination: Ajaike s Information Criterion (AIC) statistical examination may evaluate the optimum model and calculate BMD/BMDL value. Table 3 revealed animal studies result from Integrated Risk Information System (IRIS) [32], input data into Benchmark model to obtain the BMD/BMDL. 3. Exposure Assessment Exposure is defined as the contact of human with the THMs species by different pathways to evaluate the health risk assessment. Referring to the data of exposure factors handbook (USEPA, 1997 [37]), USEPA risk assessment guidance for superfund Volume-Human Health Evaluation Manual [38-39] and Risk assessment information system (RAIS, 2003) assumed the pathways as reasonable maximum exposure (RME) of THMs species are ingestion, inhalation and dermal intake to evaluate the chronic daily intake (CDI) respectively. Respectively, the study assumed the behaviors of drinking water, take a shower, and skin contact in the shower to represent the exposure pathways of ingestion, inhalation and dermal intake simplistically [40]. THMs concentration within the air influenced by many parameters, adopting the mathematic model of [41] to evaluate the concentration in the bathroom. All kinds of exposure pathways formula and parameters display on Table 4.

5 Han-Keng Lee et al.: Cancer Risk Analysis and Assessment Procedure of Trihalomethanes 237 in the Drinking Water Treatment Plant Table 4. Reference data of exposure assessment suppose date. Parameters Value References Weight of Population, C = C i P total C i Concentration of i region P i Population of water supply in the i region P total Total population of water supply in the i region P i ( CW 0.8 IR EF ED) Exposure pathway of Ingestion, CDI = ( AT BW ) Chronic daily intake, CDI mg/kg-day THMs concentration of drinking water, CW ( µg/l) Intake quantity, IR (liters/day) 2.5 [42] Average exposure time, AT (day) [43] Exposure during, ED (year) 70 [43] Exposure frequency, EF (day/year) 365 [43] Body Weight, BW (Kg) Male 64.8 ± 10 [44] Female 56.3 ± 9.09 Absorptivity of body 100% Assumption ( C Exposure pathway of Inhalation, CDI = air VR EF ET ED ( AT BW ) Q C L C air = ( 1 exp( bt ))( a / b), a = ) in (1 exp( V S N )) + Q Gs ( Q, b = L / m (1 exp( V s N )) + Q Gs, N = ( KOL A) / QL THMs vapor concentration in the bathroom, C air ; The shower time, t Mean vapor quantity of daily inhalation (adult), VR (m 3 /day) 12.3 [42] Flow velocity, Q L (L/min) [42] Air flow velocity, Q GS (L/min) 50 [41] Volume of bathroom, Vs (m 3 ) 6.6 [42] Henry constant, H and m= H 1 TCM: BDCM: DBCM: TBM: Transferred coefficient of liquid mass valid air/surface area, K OL A [41] [33] Exposure pathway of Dermal intake, CDI = CW PC SA EF ET ED ( AT BW ) ( Dermal intake permeable coefficient, PC (cm/hr) TCM: [45] BDCM: DBCM: TBM: Surface area dermal intake contact, SA (4BW+7)/(BW+90) [45] Exposure time, ET (min/day) 20 [46] ) 4. Risk characterization In this step, the toxicity and exposure assessments are summarized and integrated into quantitative and qualitative expression of risk. For carcinogenic effects, the risk is expressed as the probability that an individual will be dose-response characteristics. Under the assumption that the slope factor is a constant, the risk is directly related to the intake. Linear low-dose cancer risk equation: Risk = CDI SF (1) Where, Risk: a unitless of an individual developing cancer. CDI: chronic daily intake averaged over 70 years (mg/kg-day). SF: slope factor, expressed in mg/kg-day. Estimating risk or hazard potential requires the combination of simultaneous exposure by more than one pathway and carcinogenic effect assumes dose additivity. Assume that there are no synergistic or antagonistic interaction, the total cancer risk assumes that all carcinogens are equal, and slope factors derived from animal data are also given the same weights as factors derived from human data. Total exposure cancer risk = Risk (exposure pathway 1) + Risk (exposure pathway 2)+ + Risk (exposure pathway i) (2)

6 238 Journal of the Chinese Institute of Environmental Engineering, Vol. 15, No. 4 (2005) 5. Uncertainty and sensitivity analysis There are several types of uncertainty, and an important task in risk analysis is to determine what kinds of uncertainty are likely to affect the finding. MCS is suggested by USEPA (1997) to process the uncertainty and sensitivity analysis. Essentially, Monte Carlo Simulation (MCS) involves conducting and then comparing repeated with inputs that sample the distributions of the system parameters. This study utilized Risk view (version 4.5) software to execute the probability distribution of data and then simulate sensitivity by MCS. Dose (µg/l) RESULTS AND DISCUSSION I. Data Analysis The maximum value of subscriber in TCM level exhibited northern (TY, 67.5 µg/l), southern (KC, 57.3 µg/l), eastern (NT, 34.8 µg/l), midland (NT, 32.5 µg/l) than external island (PH, 21.0 µg/l). The BDCM level exhibited eastern (TD, 29.3 µg/l), northern (JL, 27.1 µg/l), southern (KC, 26.5 µg/l), external island (PH, 21.0 µg/l) than midland (YN, 12.7 µg/l). The DBCM level exhibited external island (PH, 37.3 µg/l), southern (TN, 35.3 µg/l), eastern (TD, 20.0 µg/l), midland (YN, 13.1 µg/l) than northern (TY, 8.2 µg/l). The TBM level exhibited external island (PH, 65.1 µg/l), eastern (TD, 41.3 µg/l), midland (YN, 37.4 µg/l), southern (TN, 26.8 µg/l) than northern (TY, 8.2 µg/l). The total THMs species exhibited external island (PH, µg/l), southern (KC, 96.2 µg/l), northern (TY, 86.0 µg/l), midland (YN, 71.9 µg/l) than eastern (TD, 66.7 µg/l) respectively. All monitoring data were corresponded to the standard of Taiwan EPA in total THMs level (100 ppb) expected external island (107.8 and ppb, respectively). Ground, sea water utilized and product bromide compounds existed is the credible reason, mainly. Partially, residual chloride concentration in the pipeline may caused the effect. Increasing total THMs concentration exist the cancer probability; this may a good topic in the risk management for analysis. Figure 1 exhibited that the assessment procedure of THMs in drinking water by USEPA and described the process below: Dose (µg/l) Dose (µg/l) II. Hazard Identification A number of epidemiological studies have been performed to investigate the occurrence of adverse effects in human exposed to TCM, BDCM, DBCM and TBM respectively. Epidemiologic and animals studies have been conducted that examined the possible associations between consumption of chlorinated drinking Dose (µg/l) Fig. 2. BMD/BMDL values from (a) TCM, (b) BDCM, (c) DBCM and (d) TBM data by first stage multistage model fit with 95% confidence limits.

7 Han-Keng Lee et al.: Cancer Risk Analysis and Assessment Procedure of Trihalomethanes 239 in the Drinking Water Treatment Plant water and cancer mortality, risk or incidence. Previous research results suggest a positive association between consumption of chlorinating drinking water and cancer of the rectum, lung, bladder and kidney. III. Dose-response Assessment This study adopted the multistage type of Benchmark model that approved by USEPA (2003) [30] to process the dose-response assessment. It had well demonstrated to interpret the chronic toxicity and carcinogenic potential of total THMs species at low dose situation. Figure 2 exhibited the BMD/BMDL value of total THMs species that calculated from the Benchmark model by way of animal data (95% confidence limits and first stage model fit). Furthermore, result shown that the range of BMD/BMDL value in first/second multistage model. Generally, the BMD values of 1st stage are higher than 2nd one, but the BMDL values are similar between 1st and 2nd stage model. USEPA proposed the standard BMD and BMDL values are TCM is 1.69, 1.15; BDCM is 2.28, 1.35; DBCM is 1.88, 1.20; and BMD is 17.6, 10.3 mg/kg/day respectively. IV. Exposure Assessment The THMs data processed the weighted averages method via regional population and particular years (since 1998~2002) to acquire the statistic analysis actually. Utilized the MCS method to evaluate the CDI dose it got the result of probability distribution by 1,000 frequency calculation. Consequence exhibited the order of CDI values in the ingestion pathway are southern, external island, middle than northern segment, female (CDI range is ~ mg/kg-day) received higher CDI endured than male (CDI range is ~ mg/kg-day) because of average body weight. In the respired estimated, consequence exhibited the order of CDI values in the ingestion pathway are external island, middle, northern than southern region for female (CDI range is ~ mg/kg-day) and southern, northern, external island than middle segment for male (CDI range is ~ mg/kg-day) because of the shower behavior, it s different between segment and sex. In the skin contact estimated, consequence exhibited the order of CDI values in the dermal intake pathway are southern, external island, northern than middle segment equivalently, female (CDI range is ~ mg/kg-day) are higher than male (CDI range is ~ mg/kg-day). V. Risk Characterization Furthermore, it can obtain the slope factor of total THMs species from dose-response curve in the low Fig. 3. Special distribution via different exposure pathways in CDI (a) and risk (b) assessment. dose situation by the linear model immediately. Assumption the cancer risk is CDI multiplication slope factor and total cancer risks are including different exposure pathways. The consequence of total cancer risk evaluation is southern, northern, middle than external island segment. Average value for female is ~ and ~ for men respectively. Figures 3 and 4 exhibited the space distribution of THMs species via different exposure pathways to estimate the CDI value, risk evaluation and contribution percentage simultaneously. Southern segment exist the higher risk characterization. Moreover, the inhalation pathway revealed the magnificent in the risk evaluation of total THMs species, the next is ingestion and dermal intake pathways respectively. TCM is the main contribution to risk assessment in Taiwan area (50% approximately); TBM is predominance in external island (50% approximately). Sea and groundwater contain bromide concentration is the principal reason. VI. Sensitivity Analysis To select the parameters of species THMs, body weight, intake quantity and exposure time processing the sensitivity analysis in the 20% extra risk and in the local area and external island had analysis in radar plots exhibited in Fig. 5. No matter, research regions

8 240 Journal of the Chinese Institute of Environmental Engineering, Vol. 15, No. 4 (2005) (a) Fig. 4. Different contribution percentage of THMs species and exposure pathways in cancer risk assessment ((a) is male and (b) is female). displayed the negative correlation in exposure time and positive correlation in body weight. TCM concentration is the predominant influence parameter in local areas expected to external island is influence by DBCM concentration and intake quantity (Table 5). CONCLUSIONS This investigation was including statistical analysis, epidemiology data and cancer risk analysis and assessment procedures of THMs species in drinking water of scriber of Taiwan. It may establish an estimated procedure for the decision-making in policy of drinking water safety. The following conclusions are obtained from this study: (1) According to the statistical data of Taiwan Water Supply Corporation from 1998 to 2002, TCM concentration is the major species of DBPs in the local regions of Taiwan, and the external island is TBM comparatively. The ratio of bromide compound has the increasing tendency in northern and southern regions in the recently years. Besides, it related with the pollution of water resources or the groundwater is pullulated by sea-water. Further (b) Fig. 5. Radar plots for sensitivity analysis in Taiwan area (a) and external island (b). more, by way of statistical data and Arc View plot (Fig. 6), it can interpret southern has higher risk than northern Taiwan. (2) Multistage type of Benchmark model [47] is utilized for dose-response assessment to estimate the BMD/BMDL values of THMs species. Lower quantity of BMD/BMDL show that dose opposite to cancer risk. Exposure assessment appeared inhalation pathway is predominant, moreover are ingestion and dermal intake pathways. Southern region exist high cancer risk in assumed pathways, high temperature is the main reason. Furthermore, female has higher CDI value than male in Taiwan. (3) Risk is the conception of probability, how to quantified and emerged it in value is important for population and study of decision-making. Fortunately, Benchmark model and MCS Risk supply the mythology. The standard of total THMs species in Taiwan is 100 ppb presently, if the standard may decide by TCM, BDCM, DBCM, and TBM separately. It may establish a control management of individual material from reducing the harmful risk is the main purpose. (4) Consequence of sensitivity analysis exhibited body weight and exposure time are provided with

9 Han-Keng Lee et al.: Cancer Risk Analysis and Assessment Procedure of Trihalomethanes 241 in the Drinking Water Treatment Plant Table 5. Sensitivity analysis of cancer risk assessment in spatial distribution and THMs influence parameters. Regions Northern Middle Southern External island Parameters TCM ( µg/l) BDCM ( µg/l) DBCM ( µg/l) TBM ( µg/l) BW 1 ( ) IR 2 (liters/day) Ji-Long (JY) Tai-Pei (TP) Tao-Yuan (TY) Shin-Chu (SC) Miao-Li (ML) Tai-Chung (TC) Nan-Tou (NT) Chang-Hua (CH) Yun-Lin (YL) Chia-Yi (CY) Tai-Nan (TN) Kao-Chiong (KC) Ping-Dong (PD) ET 3 (day) Peng-Hu (PH) Abbreviation: 1 BW: body weight; 2 IR: intake quantity; 3 ET: exposure time, 4 Number exhibited the extent of sensitivity, 1 is the most sensitivity. influence in cancer risk analysis and assessment. Besides, TCM and bromide compounds have influenced in all regions and external island respectively. Because many parameters influence the estimation results, multiple evaluation method may provide much more parameters into the sensitivity analysis. (5) System dynamic maybe a methodology for decision-maker to formulate a procedure considered the parameters of economic, politics, and feasibility technology to reduce the limit standard value obviously. Suppose an accepted policy of safety drinking water and optimum social cost is the next objective. ACKNOWLEDGEMENTS The authors gratefully acknowledge the funding of this study by the National Science Council (NSC Z ), Republic of China, and the assistance of the Taiwan Water Corporation in this study. REFERENCES ± Fig. 6. Quantity of cancer risk assessment of Taiwan (Arc View plot). 1. Rook, J. J., Formation of Haloforms During Chlorination of Natural Water, Water Treat. Exam., 23, (1974). 2. Bellar, T. A., J. J. Lichtenberg and R. C. Kroner, The Occurrence of Organic Halides in Chlorinated Drinking Waters, J. Am. Water Works Assoc., 66 (12), (1974). 3. Houston, A. C., Studies in Water Supply, 1st ed., Macmillan & Co. Publishers, London (1913). 4. Yang, C. Y., H. F. Chiu, M. F. Cheng and S. S. Tsai, Chlorination of Drinking Water and Cancer Mortality in Taiwan, Environ. Res., 78, 1 6 (1998). 5. Hsu, C. H., W. L. Jeng, R. M. Chang, L. C. Chien and B. C. Han, Estimation of Potential Lifetime

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12 244 Journal of the Chinese Institute of Environmental Engineering, Vol. 15, No. 4 (2005) t Þ v ÓŒ! *! Þ! { y Œ ôë k Ô Þ v Â!! Ú (Trihalomethanes THMs) Ît ôëã ÚôË Disinfection by- products DBPs Ìuù Î om Ðt Þ ùù Î (Trichloromethane, TCM) (Dichlorbromomethane, BDCM) (Dibromochloromethane, DBCM) (Tribromomethane, TBM) ÚÚ dº Û THMs «Î 100 µg/l ÓŒñ Òt h 12 á 137 á t 1998 t 2002 n œ ng Ú h t n THMs j Ò Â vh êt THMs  x «wî Þ v s ÓŒ d k THMs  TCMw 60%Î l êk TBMw 60%Î l k 95% j Þ Ò x k ë v Þ Î Ú ë k Þ v Þ» Þ k ñ Õ Þ à wîç{ié ð Ó Œ dº gk Þ Ñ p Ø dºt THMs hýtdº  75%~95% Þ Ýt (< )