Page 12: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Vinyl Chloride

Part II. Science and Technical Considerations (continued)

11.0 Rationale

Vinyl chloride has been classified as a human carcinogen (Group 1 carcinogen) with sufficient evidence of cancer in both humans and animals. Entry into drinking water may result from industrial discharges to source water from chemical and latex manufacturing or from leaching of the entrapped monomer from polyvinyl chloride pipe. Vinyl chloride may also be formed in groundwater and the environment through the biodegradation of synthetic solvents such as trichlorethylene, trichloroethane and tetrachloroethylene.

The effects of vinyl chloride exposure have been studied in humans and animals, with similar outcomes reported in all species. Liver and neurological effects have been observed consistently in workers as well as several animal species exposed to vinyl chloride over different exposure durations. Liver cancer is the most serious endpoint that follows oral (food or water) or inhalation exposure to vinyl chloride, based on consistencies between epidemiological and experimental animal studies. Although many other tumour types have been reported in animals following vinyl chloride exposure, there is little consistency in the observed tumour types across species.

Given the volatility of vinyl chloride, a multi-route exposure assessment was performed using PBPK modeling in order to determine any additional exposure by dermal or inhalation exposure during showering or bathing. Additional litre-equivalent (L-eq) exposures of 1.9 L-eq from dermal exposure and 0.4 L-eq from inhalation exposure during showering or bathing were determined from the human PBPK model and were added to the Canadian drinking water ingestion rate of 1.5 L/day, resulting in a total estimated L-eq daily exposure of 3.8 L-eq. This daily exposure was used in both the cancer and non-cancer risk assessment.

PBPK modeling was used to calculate excess cancer risks based on the internal dose of vinyl chloride metabolites in humans exposed to vinyl chloride through drinking water. Evidence from animal studies suggests that very young children (less than 5 weeks of age) may be more sensitive to the carcinogenic effects of vinyl chloride than adults; in fact, current scientific evidence suggests a 2-fold increased sensitivity if children under the age of 5 weeks are exposed to vinyl chloride. Therefore, the concentrations representing "essentially negligible" lifetime risk of combined liver tumours in children less than 5 weeks of age would range from 0.04 to 0.4 µg/L. Health Canada has defined this term as a range from one new cancer above background levels per 100 000 people to one new cancer above background levels per 1 million people (i.e., 10−5 to 10−6). This level is protective of both potential cancer and non-cancer effects resulting from exposure to vinyl chloride in drinking water.

Air stripping and packed tower aeration are municipal-scale treatment processes, which can remove vinyl chloride from drinking water to level below 1 µg/L. Alternative processes, such as ozonation and advance oxidation processes, have been reported to be effective for the reduction of vinyl chloride from drinking water to below 0.5 µg/L.

There are no certified residential treatment devices currently available for the reduction of vinyl chloride from drinking water. However, a study conducted on treatment devices using activated carbon filters demonstrated that they may be effective for the reduction of vinyl chloride to levels below 2 µg/L. Health Canada recommends that PVC pipes used in the distribution of drinking water be certified to NSF/ANSI Standard 61, which limits the leaching of vinyl chloride based on a harmonized regulatory limit of 2 µg/L.

A MAC of 0.002 mg/L (2 µg/L) for vinyl chloride is established based on the following considerations:

  • The MAC must be measurable. The U.S. EPA has established a PQL of 0.002 mg/L, based on the ability of laboratories to measure vinyl chloride within reasonable limits of precision and accuracy using approved methods. There is no similar process in place to establish a PQL specific to Canada.
  • The MAC must be achievable at reasonable cost. Municipal scale treatment technologies can consistently remove vinyl chloride from drinking water to 0.5 µg/L, which is below the MAC of 2 µg/L. At the residential level, although there are no treatment units currently certified to remove vinyl chloride, it is expected that the use of two or more treatment devices using technologies such as activated carbon would remove vinyl chloride below the MAC.
  • NSF/ANSI Standard 61 limits leaching of vinyl chloride into drinking water by ensuring materials meet a SPAC of 0.0002 mg/L (0.2 µg/L) or less (NSF/ANSI, 2012). The SPAC is established at one tenth the regulatory value for drinking water, as harmonized between the U.S. and Canada.

In considering both treatment achievability and the health risks associated with vinyl chloride exposure from drinking water, the Federal-Provincial-Territorial Committee on Drinking Water has established a MAC of 0.002 mg/L (2 µg/L). The estimated lifetime risk associated with ingestion of water containing vinyl chloride at the MAC is 5.0 × 10−5 which is above the range that is considered to represent "essentially negligible" risk. This value is based on analytical achievability; as it exceeds the health-based value for the protection of young children, every effort should be made to maintain vinyl chloride levels in drinking water as low as reasonably achievable (or ALARA).

As part of its ongoing guideline review process, Health Canada will continue to monitor new research in this area and recommend any change to the guideline that is deemed necessary.

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