Page 14: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Trihalomethanes
12.0 Rationale
Because THMs are formed in drinking water primarily as a result of chlorination of organic matter present in raw water supplies, it is important to recognize the substantial benefits to health associated with disinfection by chlorination. The use of chlorine has virtually eliminated waterborne microbial diseases, because of its ability to kill or inactivate essentially all enteric pathogenic microorganisms. Chlorine is the most convenient and easily controlled disinfectant; it is a strong oxidant for which a residual can be maintained in the distribution system to prevent bacterial regrowth. Although the use of chlorine can lead to the formation of disinfection by-products such as THMs, efforts to manage THM levels in drinking water must not compromise the effectiveness of disinfection.
THMs and haloacetic acids (HAAs) are the two major groups of CDBPs found in drinking water and generally at the highest levels. The concentrations of these contaminants can be used as indicators of the total loading of all CDBPs which may be found in drinking water supplies. In the absence of information on other CDBPs, control and management of THMs and HAAs should reduce exposure to and risk from other by-products. When appropriate drinking water treatment strategies are implemented to reduce THMs and HAAs, the levels of other chlorinated disinfection by-products may also be reduced in the process.
Two guidelines for trihalomethanes have been established. The THM guideline is based on health effects of chloroform, and applies to the total concentration of chloroform, BDCM, DBCM and bromoform. A separate guideline for BDCM was also established; BDCM can be used as an indicator of the presence of other brominated THMs in drinking water. Animal data have consistently shown significantly higher level of toxicity for brominated DBPs than chlorinated DBPs.
New information also indicates that inhalation and dermal absorption from drinking water are important exposure routes, and should be considered, resulting in a higher overall exposure to all THMs.
Considerable progress has been made since the establishment of the previous Canadian drinking water guideline for THMs which was also based on chloroform. The weight of evidence now suggests that chloroform is a threshold carcinogen mediated through a non-genotoxic mechanisms of action resulting in sustained cytotoxicity by metabolites and ultimately persistent cellular proliferation (i.e., cancer). As such, chloroform has been reclassified from Group II (probably carcinogenic to humans) in the previous guideline to Group III (possibly carcinogenic to humans) in this assessment. Incorporation of additional exposure routes from drinking water such as inhalation and dermal absorption leads to a higher overall exposure (total of 4.11 Leq/day for ingestion and dermal and inhalation exposures from showering and bathing) to THMs than was previously recognized and results in a calculated health-based target of 80 µg/L. Chloroform was again used as a model THM for the purposes of derivation of a guideline since it is the THM for which there exists the most scientific information on which a guideline could be based, and it is also the predominant THM found in drinking water supplies.
Epidemiological evidence of a possible association between exposure to high levels of THMs and reproductive effects has also been reviewed. However, neither a dose-response pattern of increasing risk with increasing concentration of THMs nor a clear evidence of a threshold has been found.
Meeting a guideline of 80 μg/L for THMs in drinking water can present significant financial implications for treatment plants. As the increase in health risks from exposure to THMs at levels up to 100 μg/L is not expected to be significant, the Federal-Provincial-Territorial Committee on Drinking Water is establishing a MAC of 0.10 mg/L (100 μg/L) for THMs in drinking water, based on an annual average. Utilities should make every effort to achieve concentrations as low as reasonably achievable without compromising the effectiveness of water disinfection.
Because BDCM is classified in Group II (probably carcinogenic to humans), the MAC is derived based on consideration of the estimated lifetime cancer risk and best available treatment technology. Since the MAC must be measurable by available analytical methods, the Method Detection Limit (MDL) is also taken into consideration.
A MAC of 0.016 mg/L (16 µg/L) for BDCM is derived, on the basis of the following considerations:
(1) The estimated unit lifetime human cancer risk associated with the ingestion of 1 µg/L BDCM in drinking water ranges from 2.06 × 10-7 (based on adenomatous polyps and carcinoma tumours [combined] of the large intestine in female rats) to 6.33 × 10-7 (based on adenomatous polyps and carcinoma tumours [combined] of the large intestine in male rats). Therefore, the estimated lifetime human cancer risk associated with ingestion of 16 µg/L BDCM (i.e., 3.3 × 10-6 to 1.0 × 10-5 ) is within a range that is considered to be "essentially negligible."
(2) The MDL (based on the ability of laboratories to measure THMs including BDCM within reasonable limits of precision and accuracy) is 0.1-0.2 µg/L, which is well below the MAC.
(3) The MAC must be measurable and achievable. By optimization of treatment processes (i.e., improvement of specific conventional water treatment processes to remove organic [brominated] compounds prior to disinfection and addition of such processes as carbon adsorption and preoxidation), BDCM concentrations can be reduced below 16 µg/L.
Epidemiological studies, partially supported by toxicological studies, have also identified possible associations of reproductive effects (increased risk for spontaneous abortion or stillbirth) with exposure to BDCM. Recent studies suggest that BDCM targets human placental trophoblasts that produce chorionic gonadotrophin, a hormone that plays a vital role in the maintenance of pregnancy. A decrease in bioactive levels of this hormone could lead to adverse effects on pregnancy; however, only limited evidence exists on the biological plausibility of the observed BDCM-induced pregnancy loss. Although the lowest levels of exposure to BDCM associated with possible fetal loss in epidemiological studies are ≥20 µg/L, the evidence is presently insufficient to determine whether BDCM in drinking water causes reproductive effects in humans, or to base the MAC on these effects. These epidemiological studies have been carefully reviewed and considered in the guideline development, both by the Federal-Provincial-Territorial Committee on Drinking Water and by the CDBP Task Group. From a risk assessment perspective, these epidemiological studies are considered limited in their ability to quantify individual exposure to BDCM or other specific disinfection by-products. It is recommended that utilities strive to keep levels of brominated THMs as low as reasonably achievable without compromising the effectiveness of disinfection.
As part of its on-going guideline review process, Health Canada will continue to monitor new research in this area and recommend any change(s) to the guideline it deems necessary.
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