Health Canada

Santé Canada It’s Your Health

The Issue Chlorine is a disinfectant added to drink-ing water to reduce or eliminate microor-ganisms, such as bacteria and viruses,which can be present in water supplies.The addition of chlorine to our drinkingwater has greatly reduced the risk ofwaterborne diseases.

BackgroundFor more than a century, the safety ofCanadian drinking water supplies hasbeen greatly improved by the addition ofchlorine. Disinfecting our drinking waterensures it is free of the microorganismsthat can cause serious and life-threaten-ing diseases, such as cholera andtyphoid fever. To this day, chlorineremains the most commonly used drink-ing water disinfectant, and the disinfec-tant for which we have the most scientificinformation.

Chlorine is added as part of the drinkingwater treatment process. However, chlo-rine also reacts with the organic matter,naturally present in water, such asdecaying leaves. This chemical reactionforms a group of chemicals known asdisinfection by-products. The most com-mon of these by-products are tri-halomethanes (THMs), which includechloroform. The amount of THMs foundin drinking water depends on a numberof things, including the season and thesource of the water. For example, THMlevels are generally lower in winter thanin summer, because the amount of natu-ral organic matter is lower and less chlo-

rine is needed to disinfect at colder tem-peratures. THM levels are also lowwhen wells or large lakes are the drink-ing water source, and higher when riversor other surface waters are the source,because they generally contain moreorganic matter.

The Benefits of ChlorineCurrent scientific data shows that thebenefits of chlorinating our drinkingwater (less disease) are much greaterthan any health risks from THMs andother by-products. Although other disin-fectants are available, chlorine remainsthe choice of water treatment experts.When used with modern water filtrationmethods, chlorine is effective against vir-tually all microorganisms. Chlorine iseasy to apply and small amounts of thechemical remain in the water as it travelsin the distribution system from the treat-ment plant to the consumer’s tap,Thislevel of effectiveness ensures thatmicroorganisms cannot recontaminatethe water after it leaves the treatmentplant.

Alternatives toChlorinationA number of cities use ozone to disinfecttheir water, because ozonation does notproduce THMs. Although ozone is a veryeffective disinfectant, it breaks downquickly and cannot be used to maintaindisinfection in the distribution system.

DRINKING WATERCHLORINATION

Small amounts of chlorine orother disinfectants still must beadded. Renovating water treat-ment plants so they can useozone can be expensive.

Examples of other disinfectantsinclude chloramines and chlorinedioxide. Chloramines are weakerdisinfectants than chlorine, butare very effective in the distribu-tion system. Chlorine dioxide canbe used in the treatment plant,but it is not very effective in thedistribution system .

All chemical disinfectants used indrinking water can be expectedto form by-products that couldaffect human health. In general,we know less about the by-prod-ucts of other disinfectants thanabout chlorination by-products.

The Risks ofChlorination By-ProductsLab animals exposed to veryhigh levels of THMs have anincreased risk of cancer. Severalstudies on humans have alsofound a link between long-termexposure to high levels of chlori-nation by-products and a higherrisk of cancer. For instance, arecent study showed anincreased risk of bladder andpossibly colon cancer in peoplewho drank chlorinated water for35 years or more.

High levels of THMs may alsohave an effect on pregnancy. ACalifornia study found that preg-nant women who drank largeamounts of tap water with highTHMs had an increased risk ofmiscarriage. These studies donot prove that there is a linkbetween THMs and cancer ormiscarriage. However, they doshow the need for furtherresearch in this area to confirmpotential health effects.

How theGovernment ProtectsYour Water SupplyThe federal and provincial gov-ernments share responsibility forsafe drinking water. Provincialgovernments are generallyresponsible for the day-to-dayprovision of drinking water to theirconstituents. The water they pro-vide must meet their own drinkingwater requirements, which arebased on the Guidelines forCanadian Drinking Water Quality.These guidelines are publishedby Health Canada and developedin collaboration with all provincialand territorial governments.Health Canada publishes anannual table summary of theguidelines, which is available onits Web Site.

Health Canada has established aguideline for THMs of 0.1 mil-ligrams per litre. The cancer riskat this level over a lifetime is con-sidered extremely low. The guide-lines for THMs and other chlori-nation by-products are currentlyunder review by a task groupwhose work is coordinated byHealth Canada.

Minimizing YourRisk Drinking water that meets theguidelines does not usually needextra treatment. There is no easyway that you can remove all dis-infection by-products, but hereare some steps you can take.

• Activated carbon filters canbe used to remove chlorineand its by-products. If youchoose to use such a filter, itis essential to follow the man-ufacturer’s instruction

• The manufacture and sale ofwater treatment devices arenot regulated in Canada.Health Canada recommendsthe use of products that arecertified to health-based stan-dards. The Standards Councilof Canada can provide infor-mation on the certificationprocess.

Need More Info?For more information on drinkingwater quality issues in Canada,please visit Health Canada’sWater Quality Activities Web Siteat www.hc-sc.gc.ca/waterquality