Trihalomethanes In your drinking water

Water disinfection can be a tricky process. Disinfectants can react with certain substances in the water and form drinking water contaminants like trihalomethanes (THM). Some of these contaminants are known to pose health risks and are even believed to be carcinogenic.

THM is actually a group of chemicals but the US government is most concerned with its four chief constituents, which are collectively known as Total Trihalomethanes or TTHM. These are chloroform, bromoform, bromodichloromethane, and dibromochloromethane.

To mitigate the health risks brought about by TTHMs, the US Environmental Protection Agency (EPA) published the Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 DBP Rule), which is aimed at regulating TTHMs at a maximum allowable annual average level of 80 parts per billion (ppb).

This is lower than the previous allowable level of 100 ppb. The new standard took effect for large surface water public water systems in December 2001 and then for small surface water and all ground water systems in 2003.

Also known as the Maximum Contaminant Level or MCL, this allowable level was lowered by the EPA after studies showed that drinking water containing THMs above 80 ppb over a long period of time increased the risk of rectal, colon and bladder cancer.

The mere presence of this regulation should give us an idea how seriously the government views the TTHM problem.

A sure-fire way of achieving compliance with the Stage 2 DBP Rule is by improving your water purification system. High quality water purification systems can already achieve up to 99.99999% removal and can thus bring down total trihalomethane concentration way below detectable limits.

Harmful effects of trihalomethanes on the human body

Total trihalomethanes found in drinking water, even those taken into the body at levels higher than their MCL, do not pose immediate health risks. Rather, their harmful effects may appear after constant exposure for over many years. In most cases, the damage may be observed in the liver, kidney, or in the central nervous system.

Here are those four TTHMs and their known possible/probable chronic (long-term) health effects:


Usually having the largest concentration among the TTHMs, chloroform can cause problems in the central nervous system, liver, kidney, and heart. It is classified as a Group B2 agent by the EPA. Meaning, it is deemed probably carcinogenic to humans based on sufficient evidence from animal bioassay data but with little or no human data.

Bioassays are scientific experiments conducted on living organisms (e.g. rats or mice) to measure the effects of a substance. Scientists normally conduct bioassays when developing new drugs or monitoring environmental pollutants.


Like chloroform, bromoform is also classified under Group B2 by the EPA. Bromoform was once used to treat whooping cough in children. Sadly, there were incidents wherein accidental overdose led to death. Before they passed away, the children first appeared drowsy and then lifeless. 


Studies have shown that Bromodichloromethane can cause injury to the liver and kidneys. It can also harm the brain, leading to incoordination and sleepiness. Like the other two TTHM members, Bromodichloromethane falls under Group B2.


Dibromochloromethane is classified under Group C. Meaning, it is deemed possibly carcinogenic to humans based on limited animal evidence and little or no human data. The parts of the body that are most likely to have cancer as a result of prolonged exposure to high levels of dibromochloromethane are the liver and the kidneys.

Where do these trihalomethanes come from?

Trihalomethanes are harmful by-products arising from a process that’s supposed to eliminate harmful elements in drinking water. Specifically, THMs are formed when certain water disinfectants like chlorine, which are supposed to eliminate microbial contaminants, react with organic and inorganic substances in water.

In spite of the health risks brought by these disinfection by-products, putting a stop to the use of chlorine and other disinfectants is out of the question. The benefits of using chlorine far outweighs the damage it may cause through THMs. Chlorine can eliminate bacteria, viruses, and other microorganisms responsible for waterborne diseases like gastroenteritis, typhoid fever, cholera, and dysentery.

Chlorine can also remove many unpleasant tastes and odors caused by a number of substances present in the water such as algae, sulfides, ammonia, and nitrogenous compounds.

That is why nearly all water systems in the United States, regardless of size or source (ground water or surface water), deploy chlorine-based processes for water disinfection. For as long as chlorine remains a widely used drinking water disinfectant, health risks due to trihalomethanes will always be there.

Balancing health risks caused by trihalomethanes and waterborne diseases

In addressing health problems caused by TTHMs, the government favors a balanced approach. That is, they want water suppliers to manage water systems in such a way that the risks posed by disease-causing organisms and total trihalomethanes are both minimized.

There are two methods of deploying water filters to eliminate TTHMs from your drinking water. The first one is designed to remove the TTHMs themselves, while the second is aimed at removing the organic materials that may react with the disinfectants to form trihalomethanes.

Water Purification Systems and Filters

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