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Drinking Water

Haloacetic Acids in Drinking Water

Haloacetic acids (HAAs) are a group of compounds that can form when chlorine used to disinfect drinking water reacts with naturally occurring organic matter (e.g., decaying leaves and vegetation).  The use of chlorine in the treatment of drinking water is required in order to eliminate waterborne diseases because chlorine can kill or inactivate most microorganisms commonly found in drinking water sources.  In some water systems, HAAs may form if humic acids are present and combine with chlorine. 

HAAs most commonly found in drinking water include monochloracetric acid (MCA), dichloracetric acid (DCA), trichloroacetic acid (TCA), monobromoacetic acid (MBA) and dibromoacetic acid (DBA). Haloacetic acid (HAA) levels can vary significantly over time, including seasonally, and may be impacted by the amount of organic matter in the raw water and water temperature.

A new drinking water maximum acceptable concentration for HAAs took effect January 1, 2020 under the Ontario Drinking Water Quality Standards.

Drinking water is carefully treated and monitored by the drinking water system operator, the Ministry of the Environment, Conservation and Parks (MECP) as well as the health unit to ensure it meets water quality standards. According to the 

Canadian Drinking Water Guidelines and the Ontario Drinking Water Quality Standards, the maximum acceptable concentration for HAAs is a running annual average of 0.08 mg/L (80 µg/L).

Chlorine is routinely used for drinking water disinfection because it is a very effective way to reduce bacteria, viruses and parasites. Chlorine also helps to reduce bacterial regrowth, biofilm formation and recontamination of water as it travels from the treatment plant to your home. The use of chlorine in the treatment of drinking water is required in order to eliminate waterborne diseases because chlorine can kill or inactivate most microorganisms commonly found in drinking water sources. The majority of drinking water treatment plants in Canada use some form of chlorine to disinfect drinking water. The success of chlorine use in Canada today can be measured by the dramatic reduction in waterborne diseases such as Typhoid fever, cholera and dysentery.

Based on the health effects of the individual HAAs, a single guideline for total HAAs has been established. The single guideline takes into consideration treatment technology and the ability for treatment plants to achieve the guideline. The guideline value is primarily designed to be protective of the health effects of DCA, the HAA that poses the most significant health concerns, and is found at the highest levels in drinking water. However, the guideline provides protection from the health effects of all the HAAs based on ratios expected to be found in drinking water.

There is concern that long-term exposures to elevated levels of HAAs through drinking water may pose a risk to health. However, continued research to further understand the potential association between HAAs and adverse health effects is occurring.

When HAA levels are elevated, the Ministry of the Environment, Conservation and Parks (MECP) recommends drinking water system operators take the following proactive steps to help reduce the amount of HAAs in the system:

  1. Reduce the chlorine dose or change the disinfectant (e.g., to ozone, chlorine dioxide, UV).
  2. Reduce disinfection by-product precursors (organic materials) prior to chlorination. This could include enhanced coagulation (e.g., adding granulated activated carbon) or enhanced filtration (e.g., nanofiltration).
  3. Optimize the amount of time chlorine is in contact with water to reduce reaction time, but still maintain primary disinfection.
  4. Reduce water age in the distribution system (e.g. by managing storage levels or using auto-flushers).

Generally, it is not necessary to use additional drinking water treatment devices for municipally treated water supplies.

If you choose to have a filtration system in your home, carefully monitor and maintain the equipment according to the owner's manual.  Filters may need to be replaced frequently when there are high levels of iron or other solids, sometimes referred to as turbidity.  Failure to maintain filters in good working condition can result in bacterial overgrowth and contamination of drinking water.


Chlorine is important in drinking water treatment for controlling pathogenic organisms that can make us sick. Drinking water that is disinfected with chlorine to kill bacteria and viruses such as E. coli and cholera, is a benefit that far outweighs the perceived negative health risks of HAAs at the present time.

If you are interested in receiving more information about public drinking water systems and their sampling results, please contact your drinking water provider.

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