Human health risk assessment of 2-mercaptobenzothiazole in drinking water

2-Mercaptobenzothiazole (MBT) is used as a vulcanization accelerator in rubber products that come into contact with potable drinking water. When such products are evaluated for contact with potable water and submitted for ANSI/NSF Standard 61 certification, any chemical extracting from these products must be below an appropriate action level of exposure. As defined by Standard 61, a total allowable concentration (TAC) is the maximum concentration of a nonregulated contaminant allowed in a public drinking water supply, and the single product allowable concentration (SPAC) is 10% of the TAC. Currently, MBT has a TAC of 40 microg/L and a SPAC of 4 microg/L. A comprehensive health effects evaluation of MBT was performed to determine whether these action levels should be revised. Epidemiological investigations indicate that workers occupationally exposed to MBT have an increased risk of death from bladder cancer. Genotoxicity investigations in bacterial and mammalian test systems provide some evidence indicating that MBT has the potential to induce mutations and chromosomal aberrations. Toxicity studies in rats and mice chronically exposed to MBT identified increases in various tumours, such as adrenal gland tumours, pituitary gland tumours, liver tumours and renal pelvis tumours. The biological significance of most of these tumours is questionable due to a variety of factors, such as a lack of dose-response between tumour incidence and dose, and the effect of test article vehicle (corn oil) upon tumour rates. Potential human health effects of exposure to MBT can be predicted from an NTP 2-year cancer study in rats, as well as epidemiological investigations in occupationally exposed workers. A comprehensive review of the epidemiological and toxicological dataset for MBT indicates that the induction of renal pelvis transitional cell tumours is the most sensitive and relevant health effects endpoint upon which to base a revised TAC and SPAC. A multistage model was used to extrapolate to low-dose MBT exposures due to the genotoxicity and clastogenicity of MBT. A TAC of 600 microg/L was derived for MBT, and was based upon an LED10 of 157.681 mg/kg per day. A SPAC of 60 microg/L was derived by multiplying the revised TAC by 0.10, and rounding to one significant figure. These revised action levels are based upon the most sensitive health effects endpoint, as well as current cancer risk assessment methodology.