Drinking Water Guidelines

Introduction

Canadian drinking water guidelines have been developed for a variety of microbiological, chemical, physical and radiological parameters. These guidelines, which apply to drinking water from all private and municipal water sources, are set out in a publication entitled"Guidelines for Canadian Drinking Water Quality". The guidelines are developed by the Federal-Provincial Subcommittee on Drinking Water and are published in booklet form by Health Canada. The Subcommittee, which meets twice yearly, has been developing new drinking water guidelines and revising old ones since its inception in 1983. It is composed of members from all provincial and territorial governments, as well as a member from Health Canada. Officials from Health Canada act as scientific advisors to this committee.

The Guidelines are recognized throughout Canada as the standard of water quality. They provide a convenient, reliable, yardstick against which water quality can be measured, so that problems can be quickly identified and corrected.

Since provision of drinking water is a provincial responsibility, the guidelines are not applied federally except in those areas that fall under federal jurisdiction. Virtually all provincial and territorial governments have, however, established their own measures of water quality based on the Guidelines.

Water Supply Contaminants

Canadian drinking water supplies are largely free of the disease-causing organisms found in the water supplies of many developing countries. Recently, however, there has been increasing concern over the presence in drinking water of parasites such as Giardia and Cryptosporidium which are resistant to common disinfection practices. There has also been a growing public awareness and increasing alarm over the presence of a wide range of chemical contaminants in drinking water. Improved analytical methods developed in recent years have enabled scientists to discover these previously-undetected chemicals at extremely low levels -- parts per trillion or even parts per quadrillion. (A part per quadrillion is equivalent to one cent in ten trillion dollars.) At these low levels, their mere presence in drinking water does not necessarily mean that there is any risk to health, so that public alarm is largely unjustified.

Contaminants in drinking water may come from a variety of sources; these include naturally-occurring chemicals (such as arsenic and asbestos), natural and industrial radioactive materials (such as radon), and synthetic chemicals from industrial effluents and emissions.

Contaminants such as trihalomethanes may also be formed during the process of water chlorination or other treatment processes. Substances such as lead and copper may leach from the drinking water materials used in the water distribution system itself.

The process used in the development of Canadian guidelines is well established and generally follow the approaches used internationally. Although there are some differences in the basic approaches used by different jurisdictions, Canadian guidelines are generally comparable. Canadian expertise in the area of guideline development is highly recognized and as such, we are asked to actively participate in the development and revision of many of the drinking water guidelines published by the World Health Organization.

Basis for the guidelines

The primary concern in developing drinking water guidelines is the protection of human health. Scientists begin by examining available data to determine the relationship between dose and response, and to establish a level of exposure at which no adverse health effects are observed in human or animal studies (termed the No-Observed-Adverse-Effect Level or NOAEL).

Based on this NOAEL, a maximum daily exposure level to a contaminant (expressed as an tolerable daily intake or TDI) is carefully calculated, taking into account the differences in response between animals and humans, variability between individuals or groups in the human population, and the reliability of the data.

Using this tolerable daily intake level scientists calculate a maximum acceptable concentration, or MAC, adjusting for average body weight and drinking water consumption. Because drinking water is rarely the principal source of exposure to a contaminant, an allowance is made for exposure from other sources such as food, air and soil, as well as the use of water for bathing and other domestic purposes. This allowance for other sources is large, usually accounting for 80 to 95 per cent of the calculated total daily exposure. The guideline is normally based on long-term chronic or lifetime studies, as well as special studies on reproductive hazards, genetic damage, and potential to cause cancer.

Other considerations may result in slight modification of the MAC. For example, the existence of a method to measure the chemical accurately at a very low concentration, and the availability of treatment or removal techniques are two factors which must be taken into consideration. Aesthetic characteristics such as taste, odour, staining action, corrosiveness, turbidity and colour are also considered.

What the guidelines mean in terms of health

The methods used to develop the guideline ensure that these MACs are far below (i.e. 10 to 10,000 times) exposure levels at which any adverse health effects have been observed. Exposure to water at the MAC level is expected to have no impact on health, and ingestion of water containing concentrations above the MAC for a short time is not necessarily hazardous

Conclusion

The Guidelines for Canadian Drinking Water Quality are designed to provide Canadians with access to wholesome and safe drinking water. To ensure that the guidelines are based on the most recent scientific data, the Federal-Provincial Subcommittee on Drinking Water will continue to revise the guidelines, and Health Canada will publish updates as required.

Copies of the latest version of the Guidelines are available from the Canada Communications Group - Publishing, Ottawa, K1A 0S9, or from book stores which carry government publications.