Chlorination By-Products Profile


Numerous IARC Monographs (view table under General Information)

Chlorination By-Products Profile

General Information

Chlorination disinfection by-products (DBPs) are formed when chlorine-based chemicals are added to water to remove harmful microorganisms. Chlorine, chloramines, and chlorine dioxide are used most often. They control microorganisms at water treatment facilities and in water distribution pipes.[1] Disinfection chemicals react with naturally present organic material in untreated water,[2] but most treatment facilities use system methods to reduce the amount of organic material prior to treatment with disinfectants.[1]

The formation of DBPs is complex and varies by chemical. For example, the level of trihalomethanes (THMs) formed increases at high pH levels and decreases at low pH levels. The opposite is true of haloacetic acids (HAAs).[2] Levels of THMs can increase as the water moves from the treatment plant through the distribution system, while mean concentrations of HAAs may decrease along that same path.[3] Other factors that influence levels of DBPs include water temperature, the amount of organic material present in the water (which tends to be higher in surface water than in ground water), chlorine dose, contact time, and bromide ion concentration.[2]

THMs and HAAs are the two major groups of disinfection byproducts in drinking water and are often found at the highest levels.[2] In general, chloroform is the most common THM,[2] while dichloroacetic acid and trichloroacetic acid are the most common HAAs.[4] Separate CAREX profiles are available for chloroform and bromodichloromethane.

Early studies suggested an association between consumption of chlorinated water and cancer of the bladder, large intestine, and rectum in humans.[2] However, the designs of these studies were not optimal and did not differentiate between specific byproducts.[2] Subsequent studies conducted in the late 1990s suggest associations between DBPs and colon, rectal, and brain cancer in humans, although the data “are not sufficiently reliable to confirm a dose-response or causal relationship,”[2] or to target specific DBPs.

Studies of specific THMs show that:

  • Chloroform can cause kidney cancer in mice and rats, malignant lymphomas in both male and female rats, and liver tumours in mice.[5]
  • Ingesting bromodichloromethane caused kidney and large intestine cancers in rats in some studies.
  • Dibromochloromethane was not found to cause cancer in rats; however carcinogenicity was suggested in female mice.[6]
  • Bromoform has been associated with cancer of the large intestine in rats.[6]
  • Ingesting high levels of the HAA dichloroacetic acid has been associated with liver cancer in mice and rats.[7]
  • Exposure to high levels of MX in drinking water is associated with cancers at multiple sites in rats, including the liver, thyroid gland, adrenal gland, lung, and pancreas in males, and in the liver, thyroid gland, adrenal gland, and mammary gland in females.[7] Increased rates of lymphoma and leukemia were also observed in female rats given MX in drinking water.[7]

IARC classification of chlorination by-products

Group/NameCAS No.IARC GroupMonograph
Chlorinated drinking waterN/A3Volume 52 (1991)
Trihalomethanes (THMs)
Chloroform67-66-32BVolume 73 (1999)
75-27-42BVolume 52 (1991)
Volume 71 (1999)
Dibromochloromethane124-48-13Volume 52 (1991)
Volume 71 (1999)
Bromoform75-25-23Volume 52 (1991)
Volume 71 (1999)
Haloacetic Acids (HAAs)
Monochloroacetic acid79-11-8N/A
Dichloroacetic acid79-43-62BVolume 84 (2004)
Volume 106 (2014)
Trichloroacetic acid76-03-92BVolume 106 (2014)
Monobromoacetic acid79-08-3N/A
Dibromoacetic acid631-64-12BVolume 101 (2013)
Bromochloroacetic acid5589-96-82BVolume 101 (2013)
Haloacetonitriles (HANs)
Trichloroacetonitrile545-06-23Volume 52 (1991)
Volume 71 (1999)
Dichloroacetonitrile3018-12-03Volume 52 (1991)
Volume 71 (1999)
Bromochloroacetonitrile83463-62-13Volume 52 (1991)
Volume 71 (1999)
Dibromoacetonitrile3252-43-52BVolume 52 (1991)
Volume 71 (2013)
Volume 101 (2012)
Haloketones (HKs)
Chloral hydrate302-17-02AVolume 63 (1995)
Volume 84 (2004)
Volume 106 (2014)
Cyanogen chloride506-77-4N/A
2,4,6-trichlorophenol88-06-22BVolume 52 (1991)
Volume 71 (1999)
(as ‘polychlorophenols’)
Volume 117 (in prep.)
MX77439-76-02BVolume 84 (2004)
Chloramine10599-90-33Volume 84 (2004)
N-nitrosomethylethylamine10595-95-62BVolume 17, Suppl. 7 (1987)
Table References: Nieuwenhuijsen, MJ[3]; IARC Volume 73 (1999)[5]; IARC Volume 52 (1991)[8]; IARC Volume 71 (1999)[6]; IARC Volume 84 (2004)[7]; IARC Volume 17, Suppl. 7 (1987)[9]; IARC Volume 101 (2012)[10]; IARC Volume 106 (2014)[11]


Regulations and Guidelines

Chloroform has been evaluated and deemed not to be a toxic substance in Canada.[4] Inorganic chloramines were also evaluated by the Canadian Environmental Protection Act (CEPA)[12] and were subsequently added to CEPA’s Toxic list under consideration for the environment and/or its diversity (not human health).[13] No other specific DBPs are identified as having been or as being under evaluation for toxicity under CEPA.

The World Health Organization reports that MX and chloral hydrate occur at concentrations well below those at which toxic effects may occur. They have not established guideline values for these DBPs.[12]

Drinking water quality guidelines[14]

Canadian Guidelines[14]Level (µg/L)
TrihalomethanesMAC: 100(a) ALARA
Haloacetic acidsMAC: 80(b) ALARA
US EPA Guidelines[15]Level (µg/L)
Haloacetic acids60(c)
World Health Organization[16]Level (µg/L)
2,4-6 Trichlorophenol200
TrihalomethanesSum of the ratio of the concentration
of each to its respective guideline
should not exceed 1.
µg/L = microgram per litre
MAC = maximum acceptable concentration
ALARA = as low as reasonably achievable
(a) Compliance is based on a location sampling on a quarterly basis at minimum (on average) and at the point in the distribution system with the highest potential THM levels.[2]
(b) MAC based on the annual average of samples taken (at minimum on a quarterly basis) in the distribution system.[2]
(c) Compliance is based on a location’s annual average calculation, which averages each sampling location in the distribution system.[15]
(d) The guideline describes water quality that is acceptable for lifelong consumption.[16]

Environmental Exposures Overview

Ingestion, inhalation and dermal absorption are all routes of exposure for the general public.[17]

Canadians are exposed to DBPs by drinking treated water. People who drink treated water from above-ground reservoirs, lakes, or streams with no pre-treatment filtration of organic matter may be exposed to higher levels of DBPs.

Exposure may also occur by breathing air while bathing or showering or using chlorinated pools and hot tubs. It is also possible via dermal absorption during these activities. Inhalation is reported to be a more significant route of exposure for swimmers, while exposure via dermal absorption is more important for hot tub users due to higher water temperatures.[2]

Health Canada conducted a national survey of DBP levels in treatment plants and distribution systems in 1993.[18] A more recent analysis of data from 1990 to 2004 looked at 135 large (> 5,000 people served) and 312 small (< 5,000 people served) water treatment plants in Canada. It found 12% of large systems and 44% of small systems had total HAAs above the guideline of
80 µg/L.[19]

The Municipal Water and Wastewater Survey is conducted annually by Environment Canada.[20] Responding is voluntary, so the data are limited.

In 2009, Statistics Canada completed a survey of DBP levels in approximately 2,600 drinking water plants in Canada for the years 2005, 2006 and 2007. Levels of total HAAs, total THMs, and bromodichloromethane were reported by all surveyed facilities. Drinking water plants that serve fewer than 300 people were not included.[21]

For more information, see the environmental exposure estimates for chlorination by-products.


1. US Environmental Protection Agency (EPA). Safe Drinking Water Act, Drinking Water Treatment (2004)
3. Nieuwenhuijsen, MJ. “Exposure Assessment in studies of chlorination disinfection by-products and birth outcomes” Exposure Assessment in Occupational and Environmental EpidemiologyOxford University Press. (2003)
5. International Agency for Research on Cancer (IARC). Monograph summary, Volume 73 (1999) (PDF)
6. International Agency for Research on Cancer (IARC). Monograph summary, Volume 71 (1999) (PDF)
7. International Agency for Research on Cancer (IARC). Monograph summary, Volume 84 (2004) (PDF)
8. International Agency for Research on Cancer (IARC). Monograph summary, Volume 52 (1991) (PDF)
9. International Agency for Research on Cancer (IARC). Monograph summary, Volume 17, Suppl. 7 (1987) (PDF)
10. International Agency for Research on Cancer (IARC). Monograph summary, Volume 101 (2012) (PDF)
11. International Agency for Research on Cancer (IARC). Monograph summary, Volume 106 (2014) (PDF)
13. Environment and Climate Change Canada. CEPA List of Toxic Substances (1999)
16. World Health Organization. Guidelines for Drinking-water Quality. Fourth edition (2011) (PDF)
17. US Centers for Disease Control and Prevention (CDC). National Biomonitoring Program. Disinfection By-Products (Trihalomethanes) (2013)
20. Environment Canada. Municipal Water and Wastewater Survey (2016)

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