The risks of chlorination by-products lab animals exposed to very high levels of THMS have an increased risk of cancer. Several studies on humans have also found a link between long-term exposures to high levels of chlorination by-products and a higher risk of cancer. For instance, a recent study showed an increased risk of bladder and possibly colon cancer in people who drank chlorinated water for 35 years or more.

1. Health
Canada
Santé
Canada It’s Your Health
DRINKING WATER
CHLORINATION
The Issue rine is needed to disinfect at colder tem-
peratures. THM levels are also low
Chlorine is a disinfectant added to drink- when wells or large lakes are the drink-
ing water to reduce or eliminate microor- ing water source, and higher when rivers
ganisms, such as bacteria and viruses, or other surface waters are the source,
which can be present in water supplies. because they generally contain more
The addition of chlorine to our drinking organic matter.
water has greatly reduced the risk of
waterborne diseases.
The Benefits of Chlorine
Background Current scientific data shows that the
benefits of chlorinating our drinking
For more than a century, the safety of water (less disease) are much greater
Canadian drinking water supplies has than any health risks from THMs and
been greatly improved by the addition of other by-products. Although other disin-
chlorine. Disinfecting our drinking water fectants are available, chlorine remains
ensures it is free of the microorganisms the choice of water treatment experts.
that can cause serious and life-threaten- When used with modern water filtration
ing diseases, such as cholera and methods, chlorine is effective against vir-
typhoid fever. To this day, chlorine tually all microorganisms. Chlorine is
remains the most commonly used drink- easy to apply and small amounts of the
ing water disinfectant, and the disinfec- chemical remain in the water as it travels
tant for which we have the most scientific in the distribution system from the treat-
information. ment plant to the consumer’s tap,This
level of effectiveness ensures that
Chlorine is added as part of the drinking microorganisms cannot recontaminate
water treatment process. However, chlo- the water after it leaves the treatment
rine also reacts with the organic matter, plant.
naturally present in water, such as
decaying leaves. This chemical reaction
forms a group of chemicals known as Alternatives to
disinfection by-products. The most com-
mon of these by-products are tri-
Chlorination
halomethanes (THMs), which include A number of cities use ozone to disinfect
chloroform. The amount of THMs found their water, because ozonation does not
in drinking water depends on a number produce THMs. Although ozone is a very
of things, including the season and the effective disinfectant, it breaks down
source of the water. For example, THM quickly and cannot be used to maintain
levels are generally lower in winter than disinfection in the distribution system.
in summer, because the amount of natu-
ral organic matter is lower and less chlo-

2. Small amounts of chlorine or
other disinfectants still must be
High levels of THMs may also
have an effect on pregnancy. A
Minimizing Your
added. Renovating water treat- California study found that preg- Risk
ment plants so they can use nant women who drank large
ozone can be expensive. amounts of tap water with high Drinking water that meets the
THMs had an increased risk of guidelines does not usually need
Examples of other disinfectants miscarriage. These studies do extra treatment. There is no easy
include chloramines and chlorine not prove that there is a link way that you can remove all dis-
dioxide. Chloramines are weaker between THMs and cancer or infection by-products, but here
disinfectants than chlorine, but miscarriage. However, they do are some steps you can take.
are very effective in the distribu- show the need for further
tion system. Chlorine dioxide can • Activated carbon filters can
research in this area to confirm
be used in the treatment plant, be used to remove chlorine
potential health effects.
but it is not very effective in the and its by-products. If you
distribution system . choose to use such a filter, it
How the is essential to follow the man-
All chemical disinfectants used in ufacturer’s instruction
drinking water can be expected Government Protects
to form by-products that could • The manufacture and sale of
affect human health. In general,
Your Water Supply water treatment devices are
we know less about the by-prod- not regulated in Canada.
The federal and provincial gov-
ucts of other disinfectants than Health Canada recommends
ernments share responsibility for
about chlorination by-products. the use of products that are
safe drinking water. Provincial
certified to health-based stan-
governments are generally
dards. The Standards Council
responsible for the day-to-day
The Risks of provision of drinking water to their
of Canada can provide infor-
mation on the certification
Chlorination By- constituents. The water they pro-
process.
vide must meet their own drinking
Products water requirements, which are
Lab animals exposed to very
based on the Guidelines for
Canadian Drinking Water Quality.
Need More Info?
high levels of THMs have an
These guidelines are published For more information on drinking
increased risk of cancer. Several
by Health Canada and developed water quality issues in Canada,
studies on humans have also
in collaboration with all provincial please visit Health Canada’s
found a link between long-term
and territorial governments. Water Quality Activities Web Site
exposure to high levels of chlori-
Health Canada publishes an at www.hc-sc.gc.ca/waterquality
nation by-products and a higher
annual table summary of the
risk of cancer. For instance, a
guidelines, which is available on
recent study showed an
its Web Site.
increased risk of bladder and
possibly colon cancer in people Health Canada has established a
who drank chlorinated water for guideline for THMs of 0.1 mil-
35 years or more. ligrams per litre. The cancer risk
at this level over a lifetime is con-
sidered extremely low. The guide-
lines for THMs and other chlori-
nation by-products are currently
under review by a task group
whose work is coordinated by
Health Canada.