Trihalomethanes In Water (THM Disinfection By-products Guide)

We independently review everything we recommend. If you buy through our links, we may earn a commission. Learn More.
Trihalomethanes In Water (THMs)

Public water systems are frequently treated with chlorination to kill harmful microbial contaminants that are known to cause disease. However, this process also creates harmful disinfection byproducts known as trihalomethanes in water (THMs). At elevated levels, THMs can lead to an increased risk of negative health effects such as reproductive issues and cancer.

In this article, we will share everything you need to know, including how to identify if you have THMs in your tap water. And more importantly, how you can get rid of them to ensure your safety.

Key Takeaways:

  • Trihalomethanes (THMs) form when chlorine disinfectant and natural organic matter react in public water distribution systems. 
  • Trihalomethanes are the most common disinfection byproduct found in tap water and have been classified as probable human carcinogens associated with an increased risk of bladder cancer.
  • The EPA requires public water systems to manage total trihalomethanes in tap water at an average level of 80 PPB or lower.
  • The best method to detect THMs and disinfection byproducts in your drinking water is to use a certified lab test that scans for total trihalomethanes and haloacetic acids.
  • The best treatment method to get rid of THMs in tap water is to use activated carbon, catalytic carbon, or reverse osmosis filtration.

What Is Trihalomethanes In Water?

Trihalomethanes (THMs) are volatile organic compounds created during the chlorination water treatment process. They are formed when natural organic matter reacts with chlorine used to treat the water. This chemical reaction forms disinfection byproducts, of which trihalomethanes are the most common.

Disinfection By-Products

During chlorination, chlorine reacts with natural organic matter to produce chemical compounds called disinfection byproducts (DBPs). Most of the attention has focused on trihalomethanes (THMs) and haloacetic acids (HAAs), the most abundant being THMs. The four trihalomethanes are chloroform, bromodichloromethane, dibromochloromethane, and bromoform. Chloroform is the most common.

They are composed of three (tri) halogen atoms (halo), either chlorine, bromine, or a combination of the two, and methane. Chloroform is a non-brominated THM whereas the other three are brominated since they also contain one or multiple bromine atoms.

How many THMs are produced during chlorination depends on a few factors like water temperature, pH, and how much organic material is present.

What Are Trihalomethanes Side Effects In Drinking Water?

Drinking Water Cup

The general population may be exposed to THMs from drinking tap water, inhaling chlorinated water, or in swimming pools and showers through dermal absorption. In the workplace, employees at pulp and paper manufacturing centers and water treatment plants may be more prone to exposure.

As with most water contaminants, pregnant women and children are more susceptible to adverse health outcomes.

Chronic exposure to high levels of THMs can cause adverse health effects like:

  • Impaired nervous system functioning
  • Impaired liver functioning
  • Adverse reproductive outcomes: A recent large Chinese meta-analysis connected maternal THM exposure to low birth weight.
  • Colon cancer: A large cohort study in women suggests that TTHM exposure increased the likelihood of developing colorectal cancer in participants.
  • Bladder cancer: Studies estimate that roughly 10% of bladder cancers in the US can be attributed to DBP exposure.

Low-level exposure to THMs has been shown to cause:

A systematic review of the toxicological effects of THMs suggests there may be a difference in chloroform and brominated THMs regarding their adverse health effects.

Based on scientific research, the WHO’s International Agency for Research on Cancer (IARC) considers DBPs potentially carcinogenic. Specifically, two of the four THMs are considered “possibly carcinogenic” in group 2B, and two are considered “not classifiable” in group 3.

  • Chloroform: group 2B
  • Bromodichloromethane: group 2B
  • Bromoform: group 3
  • Dibromochloromethane: group 3

Furthermore, the Centers for Disease Control and Prevention (CDC) is keeping a watchful eye on the effects of trihalomethanes in the water supply through biomonitoring population health data.

What Are The Regulatory Standards For Trihalomethanes?

THM limit in drinking water

Disinfectant byproducts have been on the EPA’s radar for decades now. In 1979, the EPA’s Total Trihalomethane Rule limited THMs in drinking water to less than 0.1 mg/L. This reduced THM exposure from nearly 30% to 3% by 1988.

Since the early 2000s, the EPA has employed Disinfectant Byproducts Rules (DBPR) for public health protection. These include monitoring potentially harmful levels of THMs in the water supply that water treatment plants must abide by.

These standards fall under the EPA’s National Primary Drinking Water Regulation (NPDWR) standards. Primary standards are set when a contaminant may cause either acute or long-term health effects. The NPDWR is reconsidered every six years. The results of the most recent review are anticipated in 2023.

The EPA’s classification system provides a maximum contaminant level (MCL) value and a maximum contaminant level goal (MCLG). The difference between MCL and MCLG is that the MCL is enforceable and set based on health effects and feasibility, while an MCLG is non-enforceable and is a figure in which no negative health effects are expected.

Of course, EPA standards only apply to all public water systems. However, those that have private water sources, such as well water, should test their water source regularly and stay within EPA guidelines.

Total Trihalomethane

The Environmental Protection Agency’s (EPA) maximum containment level (MCL) for total trihalomethanes (TTHMs) is an annual average of 0.08 mg/L. There is no set maximum containment level goal (MCLG).

The fourth edition of the World Health Organization’s (WHO) guidelines for drinking water quality (GDWQ) suggests keeping TTHMs below 0.1 mg/L.

The EPA doesn’t actually have set regulatory levels for the individual THMs. However, some states have set their own regulatory levels based on the EPA’s guidance, but enforce lower values.

In 2020, California’s Office for Environmental Health Hazard Assessment (OEHHA) announced public health goals (PHG) for THMs in drinking water. The PHG is the set level at which no adverse health effects are estimated to occur after a lifetime of exposure.

  • Chloroform: Public health goal of 0.4 parts per billion (ppb) = 0.0004 ppm (parts per million) = 0.0004 mg/L
  • Bromodichloromethane (BDCM): Public health goal of 0.06 ppb = 0.00006 ppm = 0.00006 mg/L
  • Bromoform: Public health goal of 0.5 ppb = 0.0005 ppm = 0.0005 mg/L
  • Dibromochloromethane (DBCM): Public health goal of 0.1 ppb = 0.0001 ppm = 0.0001 mg/L

When it comes to measuring trace amounts of contaminants, parts per billion (ppb) may not sound like a lot, but trace amounts can accumulate over a lifetime of exposure. The EPA provides helpful guidance on how to convert and interpret units of drinking water standards.

Signs Of Trihalomethanes In Water

Unlike many water contaminants, there are often telltale indications that your water could have elevated levels of THMs:

  • The water has a strong chlorine taste or odor.
  • Your Consumer Confidence Report shows your water supply has tested positive for trihalomethanes in the water.
  • If you have well water and frequently use shock chlorination treatment to disinfect the well.
  • If your water system has recently announced a water boil advisory.
  • Your water is from a surface water source such as a stream, lake, or reservoir.

If your water supply has any of these signs, it is likely it has high levels of disinfection by-products present. That said, the only sure way to know if your water supply has a THM problem is to test your water.

How To Test For Trihalomethanes In Drinking Water

Since water supplied by public water systems is more likely to be disinfected with chlorine, it is more likely to contain THMs. There are DIY methods to have your water tested, however, the only true way to detect trihalomethanes in your drinking water is to use a certified lab test.

Certified Lab Test

A certified lab test will provide the most accurate results to detect trihalomethanes and VOCs in your water. This type of test kit requires collecting a tap water sample and sending it to a certified laboratory to be analyzed by a professional.

I recommend the Freshnss Labs water test kit that not only scans for disinfection by-products, but also dozens of other harmful contaminants. Simply take a water sample and send it to the lab and you will get a comprehensive report within 3 business days of the lab receiving it. The report includes the exact contaminant levels detected, any plumbing or health risks, and the best treatment methods based on your data.

Laboratory Water Test Kit

Tests for THMS, and dozens of other harmful VOCs in tap water
Analyzed in a certified laboratory
Includes detailed report with EPA benchmarking and safety concerns

Note that if you purchase a lab test kit, make sure it analyzes disinfection byproducts like THMs, HAAs, and chloramine, not just chlorine. If you have low levels of chlorine, it does not mean you don’t have DBP problems. Many times people get a test that shows low levels of chlorine but has high DBP, so you need to make sure you understand the whole picture.

DIY Test Strips

There is not a real self-test option available for THMS or chlorine by-products in drinking water. However, there are test strips and digital meters that can monitor the total chlorine and free-chlorine residual concentration of your water. This can be a good first indicator of THM exposure, but it should only be the initial step in determining your water quality.

How To Remove Trihalomethanes From Drinking Water

If you are concerned with THM contamination in your water distribution system, the recommended treatment methods are:

  1. oxidation by ozone or chlorine dioxide
  2. clarification by coagulation, settling and filtration, precipitative softening, or direct filtration
  3. adsorption by powdered activated carbon or granular activated carbon

To treat your home’s water, there are both whole-house and point-of-use treatment methods to remove THMs from drinking water. When buying water filters for the home, make sure to look for an NSF certification. This seal of approval ensures that this product has been tested against its marketing claims.

Activated Carbon

Granular activated carbon (GAC) is the most common method for removing THMs; the process can remove 95 to 99.9% of trihalomethanes in water.

Activated carbon filters work by carbon adsorption, which is how carbon works similarly to a magnet to attract THM compounds to the filter. Catalytic carbon is a modified activated carbon filter that may be even more effective at removing THMs.

Activated carbon filtration can be a whole house system, an under-sink filter, a faucet filter, a pitcher filter, or as a refrigerator filter.

Reverse Osmosis

Reverse osmosis is a popular water filtration method for many contaminants. The process can also remove THMs from drinking water. Reverse osmosis works by pushing water through a semipermeable membrane to filter out particles and contaminants.

For the RO system to be most effective, it needs to include a carbon pre and post-filter to assist in the removal of THMs. The downside to RO filtration compared to activated carbon is that although reverse osmosis removes more contaminants, it can also remove helpful minerals from water whereas activated carbon filtration does not.

Frequently Asked Questions

Do water filters remove trihalomethanes?

Yes, some water filters remove THMs such as activated carbon and reverse osmosis filters. It’s important to check the specific water filter in question to ensure that it filters out THMs in addition to other contaminants.

Does boiling water remove trihalomethanes?

Yes, boiling water will remove THMs from the water. However, this will cause the water to turn to vapor, potentially leading to the inhalation of THMs.

Are there Trihalomethanes in bottled water?

Yes, there may be some levels of disinfection byproducts (DBPs) in bottled water. However, the Food and Drug Administration (FDA) requires manufacturers to keep DBP levels below a set standard to protect the health of the public.

      Enable registration in settings - general