Page 2 - Guidelines for Canadian Drinking Water Quality: Guideline Technical Document: N-Nitrosodimethylamine (NDMA)
Part I. Overview and Application
The maximum acceptable concentration (MAC) for N-nitrosodimethylamine (NDMA) in drinking water is 0.000 04 mg/L (0.04 µg/L).
2.0 Executive summary
Levels of NDMA in Canadian drinking water are generally very low. There are no industrial or commercial uses for NDMA in Canada. NDMA can be found in both surface water and groundwater sources, but it is found in drinking water primarily from its formation during the treatment process, in particular chloramination.
This Guideline Technical Document reviews and assesses all identified health risks associated with NDMA in drinking water, incorporating all relevant routes of exposure from drinking water-namely, ingestion as well as skin absorption from showering and bathing; inhalation was not found to be significant. It assesses new studies and approaches and takes into consideration the availability of appropriate treatment technology. From this review, the guideline for NDMA in drinking water is established at a maximum acceptable concentration (MAC) of 0.000 04 mg/L (0.04 µg/L). The guideline for NDMA is established based on cancer end-points and is considered protective of all health effects.
2.1 Health effects
NDMA is considered highly likely to be carcinogenic to humans, based primarily on clear evidence of carcinogenicity in animals. Consequently, there have been few studies on other possible adverse health effects. The maximum acceptable concentration for NDMA in drinking water was established based on the incidence of liver cancer in male and female rats, through the calculation of a lifetime unit risk.
Canadians can be exposed to NDMA through its presence in water, air and food. Drinking water is considered to be only a minor source of exposure to NDMA compared with other sources. Overall, the concentration of NDMA measured in Canadian water supplies is normally well below the MAC.
The presence of NDMA in drinking water is primarily associated with water treatment. It can be formed as a result of chloramination, and to a lesser extent chlorination, as well as the use of some coagulants and anion exchange resins. Consequently, the best approaches to reduce the concentration of NDMA in drinking water are to remove the organic nitrogen precursors (including humic substances) from the source water, or to modify the disinfection strategy to minimize its formation, without compromising the efficacy of the disinfection process. Any modification to the disinfection strategy needs to consider the potential formation of other disinfection by-products in a holistic manner and include pilot testing. Once NDMA is present in drinking water, its reduction using ultraviolet (UV) irradiation is technically feasible, but may be difficult for smaller utilities.
3.0 Application of the guideline
Note: Specific guidance related to the implementation of drinking water guidelines should be obtained from the appropriate drinking water authority in the affected jurisdiction.
Drinking water operators should strive to keep NDMA concentrations low by implementing strategies to prevent its formation during treatment without compromising the effectiveness of disinfection. NDMA is considered a probable human carcinogen. Specific subpopulations, such as children and pregnant women, are not at a greater risk of developing adverse health effects from exposure to NDMA compared with the general population.
The drinking water guideline is based on lifetime exposure to NDMA from drinking water. For drinking water supplies that occasionally experience short-term exceedances above the MAC, it is suggested that a plan be developed and implemented to address these situations. For more significant long-term exceedances that cannot be addressed through treatment, it is suggested that alternative sources of drinking water be considered.
The guideline for a carcinogen is normally established at a level at which the increased cancer risk is considered to be "essentially negligible" for a person exposed to that level in drinking water over a lifetime (70 years). In the context of drinking water guidelines, Health Canada has defined this term as a range from one new cancer above background per 100 000 people to one new cancer above background per 1 million people (i.e., 10-5 to 10-6) exposed to a contaminant at the MAC over a lifetime. In the case of NDMA, the MAC is established at a concentration that would present an "essentially negligible" risk of one new cancer above background per 100 000 people (i.e., 10-5) exposed to NDMA at the MAC over a lifetime, which takes into consideration treatment limitations.
In general, monitoring of NDMA should be conducted on an annual basis. Where the characteristics of raw water or the treatment and disinfection strategies create the potential to produce NDMA, quarterly monitoring of treated water from surface and groundwater sources is recommended. This may be reduced to an annual frequency if drinking water monitoring consistently does not show the presence of NDMA in the finished water. Monitoring should be conducted at the treatment plant and at the points of the distribution system with the longest disinfectant retention time.
Factors to consider when determining the need for monitoring of NDMA include the presence of NDMA precursors or of nitrogen-containing compounds; the type of coagulant used; the type of anion exchange resins used; and the disinfection practices (i.e., chloramination or booster chloramination). Increased frequency of monitoring may be required for facilities using surface water sources during periods when water characteristics are more favourable to the formation of by-products, which will vary according to the specific system.
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