Trihalomethanes (THMs)
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Case-control study of colon and rectal cancers and chlorination by-products in treated water.

To assess the relationship between chlorination by-products and colon and rectal cancers, a case-control study was conducted in southern Ontario, Canada. Cases were residents 30-74 years of age, who had a diagnosed primary cancer of the colon or rectum between September 1992 and May 1994 and were identified from the Ontario Cancer Registry. Eligible controls were selected randomly from a database of residential listings.

The final analysis included 767 colon cases, 661 rectal cases and 1545 controls. Among males, long-term exposure (over 35 years) to a THM level above 75 micrograms/L was associated with a doubled colon cancer risk.

Full article at

Drinking Water Source and Risk of Brain Cancer   / Taken from an article published by the National Cancer Institute

A population-based case-control study of brain cancer was conducted to assess risk associated with drinking water source and chlorination by-products in Iowa. Analyses were carried out with information on lifetime residential history, drinking water sources, beverage intake, and other potential risk factors collected by mail questionnaire from 291 cases and 1,983 controls. Exposure to chlorination by-products in drinking water was estimated by combining questionnaire data with historical information from water utilities and trihalomethane levels. After multivariate adjustment, odds ratios for brain cancer were 1.0, 1.1, 1.6, and 1.3 for exposure to chlorinated surface water for 0, 1 to 19, 20 to 39, and 40 or more years, respectively (p trend = 0.1). Similar results were found with estimates of average lifetime levels of trihalomethanes, with a stronger association among men with above-median consumption of tap water. These findings suggest a possible excess risk of brain cancer related to intake of chlorination by-products. (Cantor KP, Lynch CF, Hildesheim ME, Dosemeci M, Lubin J, Alavanja M, Craun G. Drinking water source and chlorination byproducts in Iowa. III. Risk of brain cancer. Am J Epidemiol 1999;150:552-560)


Disinfection Byproducts / taken from the California water plan 1994

In its journey to the sea, water dissolves organic compounds present in the soil as a result of plant decay. This organic material includes humic and fulvic acids, and other organic compounds. High levels of these compounds can be present in drainage from wooded or heavily vegetated areas and from soils high in organic content, such as the peat soils which are present in parts of the Delta and other places in California.

Disinfectant chemicals are applied to drinking water to kill pathogenic organisms. Chemicals such as chlorine, which are capable of efficiently killing such organisms, are highly reactive and can cause unwanted chemical reactions to occur. Trihalomethanes are a class of synthetic organic chemicals produced in drinking water when chlorine, used as a disinfectant, comes into contact with naturally occurring organic material dissolved in the water. Where present, bromide (a type of salt found in sea water) enters the reaction to produce bromine-containing trihalomethane compounds.

The organic matter and salts in Delta waters are by themselves not harmful and only become so when they undergo reaction during water treatment. However, trihalomethanes are suspected of causing cancer in humans. Maximum Contaminant Levels of trihalomethanes in drinking water have been established by the U.S. Environmental Protection Agency and California Department of Health Services, in accordance with the federal and State Safe Drinking Water laws. The current MCL for THMs in drinking water is 0.10 mg/L. The regulations establishing the MCLs are being reviewed, and the stricter standard of 0.08 mg/L is expected to be promulgated.

Trihalomethanes formed in drinking water due to polyvinyl chloride (PVC) piping

Drinking water contaminant

From the Environmental Protection Agency's (EPA) website

Disinfection Byproducts

Disinfection byproducts form when disinfectants added to drinking water to kill germs react with naturally-occuring organic matter in water.

Total Trihalomethanes. Some people who drink water containing trihalomethanes in excess of EPA's standard over many years may experience problems with their liver, kidneys, or central nervous systems, and may have an increased risk of getting cancer. 

Haloacetic Acids. Some people who drink water containing haloacetic acids in excess of EPA's standard over many years may have an increased risk of getting cancer.

Bromate. Some people who drink water containing bromate in excess of EPA's standard over many years may have an increased risk of getting cancer.

Chlorite. Some infants and young children who drink water containing chlorite in excess of EPA's standard could experience nervous system effects.  Similar effects may occur in fetuses of pregnant women who drink water containing chlorite in excess of EPA's standard.  Some people may experience anemia.




Environmental Health Issues
Water - Cancer & Chlorine

Is the chlorine in our drinking water acting as catalyst triggering tumor development both in atherosclerosis and cancer? The addition of chlorine to our drinking water started in the late 1890ís and had wide acceptance in the United States by 1920. Joseph Price, M. D, wrote a fascinating yet largely ignored book in the late 1960ís, entitled Coronaries Cholesterol. Chlorine, Dr Price believes, is the primary and essential cause of atherosclerosis is chlorine. "Nothing can negate the incontrovertible fact the basic cause of atherosclerosis and resulting entities, such as heart attacks and most common forms of stokes is chlorine. The chlorine contained in processed drinking water." (1)

This conclusion is based on experiments using chlorine in the drinking water of chickens. The results: 95% of the chickens given chlorine added to distilled water developed atherosclerosis within a few months.

Atherosclerosis, heart attacks and the resulting problems of hardening of the arteries and plaque formation is really the last step in a series of biochemical malfunctions. Price points out it takes ten to twenty years before symptoms in humans become evident In many ways, this is reminiscent of cancer which can take twenty to thirty years to develop.

Can chlorine be linked to cancer too? In the chlorination process itself, chlorine combines with natural organic matter decaying vegetation to form potent cancer causing trihalomethanes (THMís) or haloforms. Trihalomethanes collectively include such carcinogens as chloroforms, bromoforms carbon tectachloride, bischlorothane and others.

Read the full article at


Consuming chlorinated drinking water is associated with a 20 to 40% increase in the incidence of colon and rectal cancer, according to the results of a Norwegian study in 1992 .

Taken from

While adding chlorine-type compounds to drinking water protects the public from several kinds of harmful bacteria such as Shigella, Salmonella, and vibrio cholera, chlorine can form cancer-causing compounds in drinking water.

The most taken-for-granted chemical in our water supply is chlorine, which has been used for nearly one hundred years to "purify" drinking water. The disinfection of drinking water with chlorine is standard practice throughout the United States. The amount of chlorine used is particularly high during the summer; since warm conditions favor bacterial overgrowth, chlorination is heaviest in southern parts of the country.

While there is little doubt that chlorine played a key role in eradicating waterborne infectious diseases such as typhoid and cholera, new evidence indicates that drinking chlorinated water increases the risk of cancer for the roughly 200 million Americans who drink it. Consuming chlorinated drinking water is associated with a 20 to 40% increase in the incidence of colon and rectal cancer, according to the results of a Norwegian study published in the International Journal of Epidemiology in 1992.


Chlorination of drinking water in Spain and bladder cancer

Drinking water chlorination generates trihalomethanes and other by-products with mutagenic and carcinogenic properties in animal experiments. Epidemiological studies have associated trihalomethanes to an increased risk of bladder cancer


NTP Factsheets - Year 2002   - Safe Drinking Water Program

Taken from

The trihalomethanes are one of the major families of DBPs found in chlorinated water.

Chloroform, which is the most prevalent trihalomethane, was shown almost 20 years ago to be carcinogenic in rodents. The DOD in collaboration with NTP is evaluating health effects of this chemical in Medaka to determine dose-response relationships in this fish model relative to rodents.

Bromodichloromethane has also been shown to be carcinogenic in rodents in studies conducted by the NTP and by EPA. Additional NTP drinking water studies will characterize intestinal, renal, and liver responses in rats and mice. Bromodichloro-methane is also being studied by the NTP in transgenic mouse models (Tg.AC and p53
def) and in the Medaka fish model by DOD.



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