Chlorinated water

Chlorinated water has chlorine (a chemical) added to it. Chlorine disinfects the water and makes it suitable to drink.


Clean drinking water protects Canadians from illnesses caused by bacteria and other microbes in untreated water. These illnesses include cholera, giardiasis and sickness caused by the bacteria Escherichia coli (E.coli). E. coli can cause diarrhea, abdominal cramps and sometimes more serious illness.


Chlorination by-products

Chlorine is a strong chemical, but the chlorine itself is not the main cause for concern in drinking water. When chlorine interacts with organic matter (such as dead leaves and soil) in untreated water, it forms new chemicals that remain in the water. These are called chlorination by-products. It is chlorination by-products that can increase cancer risk.


The amount of chlorination by-products formed during water treatment depends on the amount of chlorine used and on the amount of organic matter in the water.

How you are exposed to chlorine by-products

You’re exposed to chlorine by-products when you use water treated with chlorine. This includes drinking chlorinated water and breathing in water vapour during showering, swimming and other activities.

Chlorination by-products and cancer

The International Agency for Research on Cancer (IARC) classifies some chlorination by-products as possible causes of cancer. Find out more about how cancer-causing substances are classified.


For decades, researchers have studied the long-term effects of using chlorinated tap water. Most studies have shown that when people are exposed to chlorinated water over long periods of time, it slightly increases their risk of bladder cancer. Some studies have also found links to colorectal cancer, but we need more research to be sure of this link.


Several chlorination by-products are possible causes of cancer. Studies have shown that trihalomethanes (THMs) and haloacetic acids (HAAs), which are the most common chlorination by-products, can cause cancer in laboratory animals.


Many scientists believe that other chlorination by-products may also increase your risk of cancer. More research is needed about these and other chlorination by-products.

Tips to reduce your exposure

You may be able to reduce your exposure to chlorination by-products by: 

  • using activated carbon filter systems in your water pitcher
  • attaching a filter to your tap
  • using devices that filter all the water in your house

However, levels of chlorination by-products may not be reduced by all types of products mentioned here. And you can still be exposed to by-products when you shower or swim in a pool treated with chlorine.


If you choose to use these types of products, make sure they are certified to National Sanitation Foundation (NSF) standards. Also change the filters according to the manufacturer’s instructions to avoid contamination from bacteria and other microbes.

Suggested links for more information

General information about water chlorination
Learn more about the benefits and possible health risks associated with water chlorination and the role of government in regulating drinking water standards in Canada and the US.


Health Canada – Drinking water chlorination
Health Canada – Guidelines for Canadian drinking water quality – chlorine guideline technical document
CAREX Canada – Carcinogen profile: chlorination by-products


Drinking water protection
Materials prepared by Canadian and international governmental agencies that provide information and guidance on keeping our drinking supplies safe.


Health Canada – Canadian drinking water guidelines
Health Canada – From source to tap: guidance on the multi-barrier approach to safe drinking water
Health Canada – Frequently asked questions about bottled water
Canadian Water and Wastewater Association
World Health Organization – Drinking water quality


Alternatives to chlorination
This Health Canada page lists methods for disinfecting water that you may consider as alternatives to chlorination.


Health Canada – Water treatment devices for disinfection of drinking water