Cancer following chronic exposure is the primary hazard of concern for this class of DBPs. Because of the weight of evidence indicating that chloroform can induce cancer in animals only after chronic exposure to cytotoxic doses, it is clear that exposures to low concentrations of chloroform in drinking-water do not pose carcinogenic risks. The NOAEL for cytolethality and regenerative hyperplasia in mice was 10 mg/kg of body weight per day after administration of chloroform in corn oil for 3 weeks. Based on the mode of action evidence for chloroform carcinogenicity, a TDI of 10 µg/kg of body weight was derived using the NOAEL for cytotoxicity in mice and applying an uncertainty factor of 1000 (10 each for inter- and intraspecies variation and 10 for the short duration of the study). This approach is supported by a number of additional studies. This TDI is similar to the TDI derived in the 1998 WHO Guidelines for drinking-water quality, which was based on a 1979 study in which dogs were exposed for 7.5 years.
Among the brominated THMs, BDCM is of particular interest because it produces tumours in rats and mice and at several sites (liver, kidneys, large intestine) after corn oil gavage. The induction of colon tumours in rats by BDCM (and by bromoform) is also interesting because of the epidemiological associations with colo-rectal cancer. BDCM and the other brominated THMs are also weak mutagens. It is generally assumed that mutagenic carcinogens will produce linear dose-response relationships at low doses, as mutagenesis is generally considered to be an irreversible and cumulative effect.
In a 2-year bioassay, BDCM given by corn oil gavage induced tumours (in conjunction with cytotoxicity and increased proliferation) in the kidneys of mice and rats at doses of 50 and 100 mg/kg of body weight per day, respectively. The tumours in the large intestine of the rat occurred after exposure to both 50 and 100 mg/kg of body weight per day. Using the incidence of kidney tumours in male mice from this study, quantitative risk estimates have been calculated, yielding a slope factor of 4.8 × 10-3 [mg/kg of body weight per day]-1 and a calculated dose of 2.1 µg/kg of body weight per day for a risk level of 10-5. A slope factor of 4.2 × 10-3 [mg/kg of body weight per day]-1 (2.4 µg/kg of body weight per day for a 10-5 risk) was derived based on the incidence of large intestine carcinomas in the male rat. The International Agency for Research on cancer (IARC) has classified BDCM in Group 2B (possibly carcinogenic to humans.
DBCM and bromoform were studied in long-term bioassays. In a 2-year corn oil gavage study, DBCM induced hepatic tumours in female mice, but not in rats, at a dose of 100 mg/kg of body weight per day. In previous evaluations, it had been suggested that the corn oil vehicle may play a role in the induction of tumours in female mice. A small increase in tumours of the large intestine in rats was observed in the bromoform study at a dose of 200 mg/kg of body weight per day. The slope factors based on these tumours are 6.5 × 10-3 [mg/kg of body weight per day]-1 for DBCM, or 1.5 µg/kg of body weight per day for a 10-5 risk, and 1.3 × 10-3 [mg/kg of body weight per day]-1 or 7.7 µg/kg of body weight per day for a 10-5 risk for bromoform.
These two brominated THMs are weakly mutagenic in a number of assays, and they were by far the most mutagenic DBPs of the class in the GST-mediated assay system. Because they are the most lipophilic THMs, additional concerns about whether corn oil may have affected their bioavailability in the long-term studies should be considered. A NOAEL for DBCM of 30 mg/kg of body weight per day has been established based on the absence of histopathological effects in the liver of rats after 13 weeks of exposure by corn oil gavage. IARC has classified DBCM in Group 3 (not classifiable as to its carcinogenicity to humans). A TDI for DBCM of 30 µg/kg of body weight was derived based on the NOAEL for liver toxicity of 30 mg/kg of body weight per day and an uncertainty factor of 1000 (10 each for inter- and intraspecies variation and 10 for the short duration of the study and possible carcinogenicity).
Similarly, a NOAEL for bromoform of 25 mg/kg of body weight per day can be derived on the basis of the absence of liver lesions in rats after 13 weeks of dosing by corn oil gavage. A TDI for bromoform of 25 µg/kg of body weight was derived based on this NOAEL for liver toxicity and an uncertainty factor of 1000 (10 each for inter- and intraspecies variation and 10 for the short duration of the study and possible carcinogenicity). IARC has classified bromoform in Group 3 (not classifiable as to its carcinogenicity to humans).
5) Haloacetic acids
The induction of mutations by DCA is very improbable at the low doses that would be encountered in chlorinated drinking-water. The available data indicate that DCA differentially affects the replication rates of normal hepatocytes and hepatocytes that have been initiated. The dose-response relationships are complex, with DCA initially stimulating division of normal hepatocytes. However, at the lower chronic doses used in animal studies (but still very high relative to those that would be derived from drinking-water), the replication rate of normal hepatocytes is eventually sharply inhibited. This indicates that normal hepatocytes eventually down-regulate those pathways that are sensitive to stimulation by DCA. However, the effects in altered cells, particularly those that express high amounts of a protein that is immunoreactive to a c-Jun antibody, do not seem to be able to down-regulate this response. Thus, the rates of replication in the pre-neoplastic lesions with this phenotype are very high at the doses that cause DCA tumours to develop with a very low latency. Preliminary data would suggest that this continued alteration in cell birth and death rates is also necessary for the tumours to progress to malignancy. This interpretation is supported by studies that employ initiation/promotion designs as well.
On the basis of the above considerations, it is suggested that the currently available cancer risk estimates for DCA be modified by incorporation of newly developing information on its comparative metabolism and modes of action to formulate a biologically based dose-response model. These data are not available at this time, but they should become available within the next 2-3 years.
The effects of DCA appear to be closely associated with doses that induce hepatomegaly and glycogen accumulation in mice. The lowest-observed-adverse-effect level (LOAEL) for these effects in an 8-week study in mice was 0.5 g/litre, corresponding to approximately 100 mg/kg of body weight per day, and the NOAEL was 0.2 g/litre, or approximately 40 mg/kg of body weight per day. A TDI of 40 µg/kg of body weight has been calculated by applying an uncertainty factor of 1000 to this NOAEL (10 each for inter- and intraspecies variation and 10 for the short duration of the study and possible carcinogenicity). IARC has classified DCA in Group 3 (not classifiable as to its carcinogenicity to humans).
TCA is one of the weakest activators of the peroxisome proliferator activated receptor (PPAR) known. It appears to be only marginally active as a peroxisome proliferator, even in rats.
Furthermore, treatment of rats with high levels of TCA in drinking-water does not induce liver tumours. These data strongly suggest that TCA presents little carcinogenic hazard to humans at the low concentrations found in drinking-water.
From a broader toxicological perspective, the developmental effects of TCA are the end-point of concern. Animals appear to tolerate concentrations of TCA in drinking-water of 0.5 g/litre (approximately 50 mg/kg of body weight per day) with little or no signs of adverse effect. At 2 g/litre, the only sign of adverse effect appears to be hepatomegaly. Hepatomegaly is not observed in mice at doses of 0.35 g of TCA per litre in drinking-water, estimated to be equivalent to 40 mg/kg of body weight per day.
In another study, soft tissue anomalies were observed at approximately 3 times the control rate at the lowest dose administered, 330 mg/kg of body weight per day. At this dose, the anomalies were mild and would clearly be in the range where hepatomegaly (and carcinogenic effects) would occur. Considering the fact that the PPAR interacts with cell signalling mechanisms that can affect normal developmental processes, a common mechanism underlying hepatomegaly and the carcinogenic effects and developmental effects of this compound should be considered.
The TDI for TCA is based on a NOAEL estimated to be 40 mg/kg of body weight per day for hepatic toxicity in a long-term study in mice. Application of an uncertainty factor of 1000 (10 each for inter- and intraspecies variation and 10 for possible carcinogenicity) to the estimated NOAEL gives a TDI of 40 µg/kg of body weight. IARC has classified TCA in Group 3 (not classifiable as to its carcinogenicity to humans).
Data on the carcinogenicity of brominated acetic acids are too preliminary to be useful in risk characterization. Data available in abstract form suggest, however, that the doses required to induce hepatocarcinogenic responses in mice are not dissimilar to those of the chlorinated acetic acids. In addition to the mechanisms involved in the induction of cancer by DCA and TCA, it is possible that increased oxidative stress secondary to their metabolism might contribute to their effects.
There are a significant number of data on the effects of dibromoacetic acid (DBA) on male reproduction. No effects were observed in rats at doses of 2 mg/kg of body weight per day for 79 days, whereas an increased retention of step 19 spermatids was observed at 10 mg/kg of body weight per day. Higher doses led to progressively more severe effects, including marked atrophy of the seminiferous tubules with 250 mg/kg of body weight per day, which was not reversed 6 months after treatment was suspended. A TDI of 20 µg/kg of body weight was determined by allocating an uncertainty factor of 100 (10 each for inter- and intraspecies variation) to the NOAEL of 2 mg/kg of body weight per day.
6) Chloral hydrate
Chloral hydrate at 1 g/litre of drinking-water (166 mg/kg of body weight per day) induced liver tumours in mice exposed for 104 weeks. Lower doses have not been evaluated. Chloral hydrate has been shown to induce chromosomal anomalies in several in vitro tests but has been largely negative when evaluated in vivo. It is probable that the liver tumours induced by chloral hydrate involve its metabolism to TCA and/or DCA. As discussed above, these compounds are considered to act as tumour promoters. IARC has classified chloral hydrate in Group 3 (not classifiable as to its carcinogenicity to humans).
Chloral hydrate administered to rats for 90 days in drinking-water induced hepatocellular necrosis at concentrations of 1200 mg/litre and above, with no effect being observed at 600 mg/litre (approximately 60 mg/kg of body weight per day). Hepatomegaly was observed in mice at doses of 144 mg/kg of body weight per day administered by gavage for 14 days. No effect was observed at 14.4 mg/kg of body weight per day in the 14-day study, but mild hepatomegaly was observed when chloral hydrate was administered in drinking-water at 70 mg/litre (16 mg/kg of body weight per day) in a 90-day follow-up study. The application of an uncertainty factor of 1000 (10 each for inter- and intraspecies variation and 10 for the use of a LOAEL instead of a NOAEL) to this value gives a TDI of 16 µg/kg of body weight.
Without appropriate human data or an animal study that involves a substantial portion of an experimental animal's lifetime, there is no generally accepted basis for estimating carcinogenic risk from the HANs.
Data developed in subchronic studies provide some indication of NOAELs for the general toxicity of dichloroacetonitrile (DCAN) and dibromoacetonitrile (DBAN). NOAELs of 8 and 23 mg/kg of body weight per day were identified in 90-day studies in rats for DCAN and DBAN, respectively, based on decreased body weights at the next higher doses of 33 and 45 mg/kg of body weight per day, respectively.
A WHO Working Group for the 1993 Guidelines for drinking-water quality considered DCAN and DBAN. This Working Group determined a TDI of 15 µg/kg of body weight for DCAN based on a NOAEL of 15 mg/kg of body weight per day in a reproductive toxicity study in rats and incorporating an uncertainty factor of 1000 (10 each for intra- and interspecies variation and 10 for the severity of effects). Reproductive and developmental effects were observed with DBAN only at doses that exceeded those established for general toxicity (about 45 mg/kg of body weight per day). A TDI of 23 µg/kg of body weight was calculated for DBAN based on the NOAEL of 23 mg/kg of body weight per day in the 90-day study in rats and incorporating an uncertainty factor of 1000 (10 each for intra- and interspecies variation and 10 for the short duration of the study). There are no new data indicating that these TDIs should be changed.
LOAELs for trichloroacetonitrile (TCAN) of 7.5 mg/kg of body weight per day for embryotoxicity and 15 mg/kg of body weight per day for developmental effects were identified. However, later studies suggest that these responses were dependent upon the vehicle used.
No TDI can be established for TCAN.There are no data useful for risk characterization purposes for other members of the HANs.
The mutagen MX has recently been studied in a long-term study in rats in which some carcinogenic responses were observed. These data indicate that MX induces thyroid and bile duct tumours. An increased incidence of thyroid tumours was seen at the lowest dose of MX administered (0.4 mg/kg of body weight per day). The induction of thyroid tumours with high-dose chemicals has long been associated with halogenated compounds. The induction of thyroid follicular tumours could involve modifications in thyroid function or a mutagenic mode of action. A dose-related increase in the incidence of cholangiomas and cholangiocarcinomas was also observed, beginning at the low dose in female rats, with a more modest response in male rats. The increase in cholangiomas and cholangiocarcinomas in female rats was utilized to derive a slope factor for cancer. The 95% upper confidence limit for a 10-5 lifetime risk based on the linearized multistage model was calculated to be 0.06 µg/kg of body weight per day.