Full Report: PDF
Report Contents
Health Unit Profiles
» Group A - Population Health
 –  Teen Pregnancy
 –  Low Birth Weight
 –  Breastfeeding Duration
 –  Postpartum Contact
 –  Smoking Prevalence
 –  Youth Lifetime Smoking
 –  Adult Heavy Drinking
 –  Youth Heavy Drinking
 –  Physical Activity Index
 –  Healthy Body Mass Index
 –  Fruit and Vegetable
 –  Fall-Related Hospitalizations
     Among Seniors
 –  Enteric Illnesses Incidence
 –  Respiratory Infection
     Outbreaks in Long-Term
     Care Homes
 –  Chlamydia Incidence
 –  Immunization Coverage of
     Hepatitis B
 –  Immunization Coverage of
     Measles, Mumps and Rubella
 –  Adverse Water Quality
» Group B - Governance and
   Accountability Indicators
Case Studies
Supporting Documents
Initial Report on Public Health
Adverse Water Quality Incidents

The adverse water quality incidents indicator identifies the number of adverse water quality incidents from drinking water systems subject to O.Reg 170/03/O.Reg 252/05 and unregistered drinking water systems. An adverse drinking water incident occurs when a water sample test result exceeds the Ontario Drinking Water Quality Standards 118 or an operator observes that the system may not be providing safe water.

Contaminated drinking water can lead to serious health concerns. Most water-related health problems are caused by microbial or chemical contamination and can result in illnesses ranging from mild gastroenteritis, to disease outbreaks including E. coli infections, giardiasis and cryptosporidiosis.

Public health programs and services, together with programs through the Ministry of the Environment, aim to prevent or reduce the occurance of water-borne illness:

  • through timely and effective detection and identification of water contaminants and illnesses, their associated risk factors and emerging trends
  • mitigating water-borne illness
  • using evidence to influence the development of healthy public policy to reduce the burden of water-borne illnesses of public health importance
  • ensuring public awareness of drinking water safety and the importance of source protection

Boards of health must ensure that the medical officer of health, or designate, is available 24 hours a day, 7 days a week to receive and respond to reports of adverse water quality incidents. Public health units are also involved in providing input into the development of legislation and regulations in order to ensure that the minimum standards for public water systems reflect evidence informed best practices.

The safety of drinking water is a major concern throughout Ontario, especially since the Walkerton contaminated drinking water incident in 2000.

There are about 2,855 drinking water systems in Ontario as of February 2008 governed under the Safe Drinking Water Act 119 Regulation 170/03. 120 These include year-round residential systems as well as those that supply water to designated facilities such as schools, daycares and nursing homes. An additional 18,000 small drinking water systems fall under Ontario Regulation 318/08 121 (Transitional- Small Drinking Water Systems) and Ontario Regulation 319/08, 122 (Small Drinking Water Systems).

There are wide variations in the size and complexity of the drinking water systems that fall under the various regulations. These variations have a direct bearing on the water sampling and testing frequency and, ultimately, the number of adverse water quality incidents which may occur

In 2007 there were a total of 4,458 adverse drinking water incidents in Ontario for all system types. Based on 36 public health units in Ontario the highest number of adverse drinking water incidents was 446 and the lowest number was 13 for the calendar year of 2007. The wide variation of reported adverse water quality incidents among public health units reflects the number of regulated drinking water systems within each health unit as well as the size of the population served by the systems. Systems serving larger populations have greater sampling frequency requirements.

Indicator Definition


Number of adverse water quality incidents from drinking water systems subject to O.Reg 170/03/O.Reg 252/05 and unregistered drinking water systems.

Data Source(s):

Drinking Water Programs Branch, Ministry of the Environment


Number of adverse water quality incidents from drinking water systems subject to O.Reg 170/03/O.Reg 252/05 and unregistered drinking water systems for the 2007 calendar year


  • Exceedances from schools and day cares subject to O.Reg 243/07 not included in this summary
  • O.Reg 170/03, a.k.a. Drinking Water Systems included year round residential systems as well as designated facilities including schools, daycares and nursing homes
  • O.Reg 252/05 a.k.a. Non-Residential and Non-Municipal Seasonal Residential Systems That Do Not Serve Designated Facilities

118 O. Reg. 169/03. Retrieved April 23, 2009 from:
119 Safe Drinking Water Act, S.O. 2002, c.32. Retrieved April 23, 2009 from:
120 O. Reg. 170/03. Retrieved April 23, 2009 from:
121 O. Reg. 318/08. Retrieved April 23, 2009 from:
122 O. Reg. 319/08. Retrieved April 23, 2009 from: