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10 Water Advisory Data

Water Advisory Data

Original source: Water Advisory Data, [Public Health Unit]
Distributed by: Public Health Unit
Suggested citation (see Data Citation Notes):
Boil water advisory data, [Public health unit], date
Drinking water advisory data, [Public health unit], date

Data Notes
  • Some public health units may have their own database for tracking BWAs and DWAs.
  • The criteria for deciding to issue a BWA or DWA has not been consistent over time or across public health units. Comparisons over time or areas may be difficult.
  • Ensure that methodologies were consistent for determining the number of households affected or note inconsistencies and limitations.
  • The Protocol for the Issuance of a Boil Water or a Drinking Water Advisory applies to all treated regulated/unregulated drinking water systems that are classified under the Drinking Water Systems Regulation (Ontario Regulation 170/03 (OR 170/03)) and other drinking water systems that do not fall under this regulation. The Protocol was revised and available for comment in 2001.1 The Protocol was awaiting final approval as of April 2005.
  • OR 170/03 made under the Safe Drinking Water Act, 2002 came into force on June 1, 2003. This regulation requires that owners/operating authority and testing laboratories of drinking water systems, give notice to the medical officer of health for the health unit in which the drinking water system is located of the exceedences of any health-related parameter in the Ontario Drinking Water-Quality Standards Regulation, encompassing microbiological, chemical and radiological standards. OR 170/03 also applies to schools, private schools, day nurseries, nursing, retirement homes, social and health care facilities in the broader public sector and private sector that have their own water supply system.1
  • A boil water advisory (BWA) is issued when there is: no disinfection or detectable disinfection residual in the distributed water; unacceptable microbiological quality in the treated water; epidemiological evidence that suggests drinking water is responsible for an outbreak; significant deterioration in source water quality; or situations where operation of the system would compromise public health (e.g. back-flow).1
  • A drinking water advisory (DWA) is issued whenever there is reason to believe that a condition exists with a drinking water supply that may result in a risk to consumers that cannot be corrected by boiling the water or by disinfection. DWAs may arise for the following reasons: a chemical standard is exceeded; a radiological standard is exceeded; or other condition judged to be hazardous that cannot be rectified by boiling the water.1
  • The medical officer of heath (MOH) has the responsibility for the advisory under the Health Protection and Promotion Act and for a BWA should alert the public to bring drinking water to a rolling boil for at least a minute to remove the risk of acquiring disease caused by bacteria or protozoa.
  • Reporting drinking water advisories in terms of household-days provides information about the scale and impact of the advisory.
  • The decision to issue a BWA or DWA is not made lightly, nor is it always clear-cut. Caution should be used when comparing data across health units and over time. The Walkerton Gastroenteritis Outbreak of May 20002 has also likely influenced the decision-making process. To more clearly outline the process, the Ministry has developed a protocol for the issuance of boil water and drinking water advisories. As well, the Council of Medical Officers of Health of Ontario (COMOH) and the Association of Supervisors of Public Health Inspection of Ontario (ASPHIO) struck a Boil Water Advisory Task Group3 to develop Implementation Guidelines for Boil Water Advisories. These measures should better standardize the issuance of water advisories.
  • The number of households affected is calculated by public health inspectors who may issue advisories on a door-to-door basis or by water plants that can estimate the number of households they service. The number may or may not include businesses.
  • Some water advisories may run over more than one time frame (e.g., December to February, running over two years). This needs to be described in the interpretation of the data.


  1. Ontario Ministry of Health and Long-Term Care, Public Health Branch. Protocol for the Issuance of a Boil Water or a Drinking Water Advisory: Safe Water Program, Spring 2001. URL:
  2. Walkerton Commission of Inquiry Reports URL:
  3. Council of Medical Officers of Health of Ontario and the Association of Supervisors of Public Health Inspection of Ontario, Boil Water Advisory Task Group. Draft Implementation Guidelines for Boil Water Advisories. January 28, 2002.
  4. Ministry of the Environment. Ontario Drinking Water Quality Standards. URL:
  5. Health Canada. Guidelines for Canadian Drinking Water Quality. URL:
  6. Ministry of Health and Long-Term Care. Boil / Drinking Water Advisory. URL:


Date of Last Revision: April 24, 2006

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