Does Your Water Contain Unsafe Levels of Trihalomethanes?

I try to make sure my family’s drinking water is of high quality. So, when I found out our city’s water tested positive for Trihalomethanes (THMs), I set out to find out how to minimize our exposure to these potentially harmful contaminants.

Most water utility companies use Chlorine to disinfect the water they provide their customers. While this is effective in controlling microbial contaminants in our drinking water, it can also result in the formation of by-products known as Trihalomethanes. Trihalomethanes are chlorinated organic compounds, like Chloroform, you do not want in your drinking water. Consuming water containing Trihalomethanes at levels equal to or greater than their respective Maximum Contaminant Level Goals (MCLGs) can cause adverse health effects.

Fortunately, THMs are some of the drinking water contaminants regulated by the EPA. This means public water companies are required to monitor their water supply for THMs and work to ensure the water they provide their customers does not exceed the legal limit for THMs. However, even with this standard in place, it’s still possible for your tap water to contain excessive amounts of Trihalomethanes. Using data from the EPA, I estimated as many as 3.86 million Americans were exposed to water with potentially unsafe levels of Trihalomethanes at least once in 2019.

How THMs Can Affect Water Quality and Your Health

During the water treatment process, Chlorine is used to kill off bacteria and other microbes. When Chlorine interacts with inorganic matter (e.g., rocks, minerals, etc.) or organic matter (e.g., algae) contained in the water supply, Trihalomethanes can form.

What are Trihalomethanes, exactly? Trihalomethanes are chemical compounds, including Chloroform, and Fluoroform, generally used in industrial applications as solvents and refrigerants.

Trihalomethanes commonly found in public water systems include Bromodichloromethane, Bromoform, Chloroform, and Dibromochloromethane. These four harmful chemicals are known collectively as “Total Trihalomethanes (TTHMs).”

How can drinking water that contains Trihalomethanes affect your health? If you consume water that contains elevated levels of THMs over a prolonged period, you may develop problems with your liver, kidneys, or central nervous system, and you may have an increased risk of getting cancer.

At what levels can Trihalomethanes cause health problems? THMs are known to cause adverse health effects when their concentrations in drinking water are at or above the levels outlined in the table, below:

ContaminantMaximum Contaminant
Level Goal (MCLG)
Chloroform0.07 mg/L
Dibromochloromethane0.06 mg/L


In an effort to protect our health, the EPA established legally enforceable standards to limit the amount of TTHMs in our drinking water. Public water companies are required to ensure the concentration of TTHMs in the water they provide their customers is kept at or below 0.08 mg/L.

Given available treatment technology, water utility companies should be able to provide drinking water that meets this quality standard. However, more than 780 public water systems violated the regulatory limit for TTHMs in 2019.

Does Your Drinking Water Contain Harmful Levels of THMs?

Public water companies are required to provide customers with an annual water quality report, also known as a Consumer Confidence Report (CCR). Inside this report, you’ll find important information about your drinking water, including where it comes from, and whether it tested positive for any regulated contaminants like THMs.

Most Populated Towns with Violations for TTHMs in 2019

Water System
Orlando, FL
Orlando Utilities Commission (7 WPS)
Newark, NJ
Newark Water Department
Chandler, AZ
Chandler City of
Roanoke, VA
Western Virginia Water Authority
Hot Springs, AR
Hot Springs Utilities

If you’re curious to know what levels of THMs were found in your city’s drinking water, grab a copy of the latest CCR and look for the section of the report that covers “By-Products of Drinking Water Disinfection.” Here, you’ll find test results for THMs as well as other contaminants like Haloacetic Acids (HAA5).

How to Interpret Your Water Quality Report

When looking at your water quality report, you can expect to see test results for TTHMs reported in Parts per Billion (ppb). 1 mg/L equals 1,000 ppb, so 0.08 mg/L is equivalent to 80 ppb.

Below is an example of what you might see on your annual water quality report if TTHMs are detected in your town’s drinking water:

Example Water Quality Report

MCLG (1)MCL (2)Average
Your Water
TTHM [Total Trihalomethanes]

Definitions from the EPA:
1. Maximum Contaminant Level Goal (MCLG) – The level of a contaminant in drinking water below which there is no known or expected risk to health. MCLGs allow for a margin of safety and are non-enforceable public health goals.
2. Maximum Contaminant Level (MCL) – The highest level of a contaminant that is allowed in drinking water. MCLs are set as close to MCLGs as feasible using the best available treatment technology and taking cost into consideration. MCLs are enforceable standards.

In this example, water samples contained an average of 83 ppb of TTHMs, exceeding the legal limit (MCL) for this group of contaminants.

Will You Be Notified If Your Water Contains Unsafe Levels of TTHMs?

In addition to providing you with an annual quality report, your water company is required by the EPA to notify you if they identify a problem with your drinking water.

If your water company delivers water that could negatively impact your health, they have to notify you of the situation within the timeframe set by the EPA. Depending on the severity of the issue, the company is given 24 hours to 30 days to provide this notice.

If your city’s water exceeds the Maximum Contaminant Level (MCL) for TTHMs, your water company must notify you within 30 days of the violation. Typically, you will receive this notice via the media or through the mail.

What Can You Do to Remove THMs from Your Tap Water?

Fortunately, if you want to limit your family’s exposure to THMs, there are several affordable yet effective options for removing these contaminants from your tap water.

But how can you tell which products work and which ones don’t? Well, if you want peace of mind, I recommend choosing a water treatment product that is certified by NSF International (NSF) to be effective at removing THMs from water.

What does the NSF certification represent? When a product is NSF certified to remove THMs, you can rest assured that:

  • the manufacturer’s contaminant reduction claims have been verified;
  • the system was tested to confirm it adds nothing harmful to the water;
  • the system has been found to be structurally sound;
  • the product’s advertising, literature, and labeling have all been verified as accurate;
  • and there is testing in place to determine whether the quality of the product is consistent over time.

Do all NSF certified water filters work against THMs? Water treatment devices can earn certification for meeting one or more NSF standards or protocols. But only those products that meet NSF standard NSF/ANSI 53 for THMs are certified to reduce the amount of THMs that are in your water.

One such product, the Clean Water Machine by Aquasana, is NSF certified to remove up to 95% of the THMs found in your tap water. The device sits on your countertop and does not require a lengthy installation process so you can set it up in a matter of minutes.

3 steps for selecting the right water treatment system for your family:

  1. Review your area’s annual water quality report to find out what’s in your drinking water.
  2. Determine which contaminants you’d like to reduce from your water.
  3. Select a water treatment product that is NSF certified to work effectively against those contaminants you’d like to reduce from your tap water.

Just remember, no matter which water treatment product you choose, you need to make sure to perform the routine maintenance suggested by the manufacturer. This will help keep the device in proper working order and limit your family’s exposure to THMs.



I’ve moved several times over the years, so I know just how stressful it can be to relocate. I want to help put your mind at ease. That’s why I research and write about all the things I think you should consider when moving to a new town.

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