TTHMs in Drinking Water: What They Are, EPA Limits, and How to Remove Them

TTHMs (total trihalomethanes) are chemical byproducts that form when the chlorine your utility uses for disinfection reacts with natural organic matter in the source water. The EPA limits them to 80 parts per billion (ppb), and their sibling group, haloacetic acids (HAA5), to 60 ppb, both as a running average of four quarterly samples at each place the utility tests. If your utility found them, both numbers are printed on its annual water quality report, so you can check yours for free before you spend a dollar.

People ask "What are THMs in the water?"

Aidan "It's trihalomethanes. It's what happens when chlorine interacts with organic matter in the water and causes something you don't wanna drink."

That answer is from my short video on chlorine in city water. This page is the long version: what the two lines on your water report mean, whether your numbers call for anything, and what takes these byproducts out.

What are TTHMs in water?

TTHMs are the four trihalomethanes that form when chlorine reacts with natural organic matter, and "total" means the utility adds the four together into one number. The organic matter is decayed leaves, algae and soil that made it into the river, lake or well the utility draws from. Chlorine is still the right call: it has greatly reduced waterborne disease outbreaks for over a century, and the state of Massachusetts puts it plainly, stopping disinfection is not an option because the risk of illness from untreated water is often greater than the risk from the byproducts.

The EPA has no single health goal for the group, but it sets one for each compound. A health goal (the MCLG on your report) is the level below which there is no known or expected risk.

Two things decide how much forms: how much organic matter is in the source water, and how long the chlorine has to react with it. Rivers, lakes and reservoirs carry more organic matter than deep groundwater, so surface water systems form more. The EPA has utilities sample out in the pipes, at the spots and in the month where these byproducts run highest. TTHMs usually run highest toward the outer edges of a water system.

In the video I say anyone on chlorinated city water should have some kind of carbon filtration. For chlorine taste and smell, that is still what I tell people. For TTHMs on their own, the number on your report decides it, and the table further down shows how. The video starts at the part about THMs.

What is the EPA limit for TTHMs?

The EPA limit for TTHMs is 80 ppb (0.080 mg/L), measured as a locational running annual average: the average of the last four quarterly samples at each monitoring location. You'll see it written as 80 ppb, 80 micrograms per liter or 0.080 mg/L. They are the same number.

Because the limit is an average of four quarters, one quarter can come in over 80 while the average stays under it, and the utility is still in compliance. That is why the range on your report matters as much as the average. Levels move with the seasons as the source water changes, and they can jump for a while when a utility adds more chlorine after a main break or a repair.

The rule used to be looser. Under the older Stage 1 rule the average was taken across every sample in the whole system. The Stage 2 rule, in place now, averages each monitoring location on its own, so one high spot can't hide behind low ones.

What are haloacetic acids (HAA5)?

Haloacetic acids are the second family of chlorine byproducts, and the EPA limits the five regulated ones (HAA5) to 60 ppb as the same kind of running average. They form from the same chlorine and organic matter as TTHMs, the EPA regulates both under the same rule, and utilities that disinfect with chlorine sample for both on a set schedule, every quarter for larger systems and as seldom as once a year for some small ones.

The five are monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, bromoacetic acid and dibromoacetic acid. On your report the row usually reads "Haloacetic Acids" or "HAA5", and its typical-source column says "By-product of drinking water disinfection", the same words as the TTHM row.

Three things set them apart from TTHMs:

1. They don't evaporate. Haloacetic acids do not vaporize at bathing water temperature, so the exposure that matters is drinking and cooking water, not the shower.
2. Boiling doesn't reliably lower them. In a boiling study by EPA and California utility scientists, some haloacetic acids went up 58 to 68% in boiled chlorinated water.
3. No home filter is certified for them. Massachusetts' environmental agency says there are currently no known water filters certified to reduce or remove HAA5, and NSF's list of filter claims has a TTHM claim but no HAA5 claim.

Carbon still works on them. The EPA names granular activated carbon as the best available technology for treatment plants to meet both the TTHM and the HAA5 limits, and activated carbon reduces haloacetic acids along with TTHMs. What nobody can hand you is a certificate that says by how much. Our tap test kits don't measure HAA5 either, so the number on your utility's report is the HAA5 number you have.

The EPA's required wording for HAA5 on a water report: "Some people who drink water containing haloacetic acids in excess of the MCL over many years may have an increased risk of getting cancer."

Where is TTHM on my water report?

Your TTHM and HAA5 numbers are on your utility's Consumer Confidence Report (CCR), the annual water quality report every community water system has to send customers by July 1. If you can't find yours, the EPA's CCR page for consumers has a search tool. Look in the table of detected contaminants for the rows labeled "TTHMs" or "Total Trihalomethanes" and "Haloacetic Acids" or "HAA5", sometimes under a "Disinfection Byproducts" heading. Here is what each column tells you about them:

One caveat: those samples come from monitoring points in the pipes, not from your faucet. The report is the right first step because it's free, and it isn't the last word on your tap. For a full walk-through of the rest of the report, see how to read your water quality report.

Is my TTHM number a problem?

Your TTHM number is a problem when the average or the top of the range is over 80 ppb. Under 40 ppb is comfortably low: that's the line where the EPA lets a utility test less often. For HAA5 the same two lines are 60 and 30 ppb.

Have your water report in hand?

Send Aidan both numbers

Text a photo of the TTHM and haloacetic acid rows. Aidan will tell you whether they call for anything, including when the answer is nothing.

Text Aidan443-277-2204 Call800-460-5810Aidan

8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.

Are TTHMs in drinking water safe?

Below the limit, the risk from TTHMs is small, and it builds over years of drinking, not days. The wording the EPA requires on a water report says it this way: "Some people who drink water containing trihalomethanes in excess of the MCL over many years may experience problems with their liver, kidneys, or central nervous system, and may have an increased risk of getting cancer."

Note the two conditions: above the limit, and over many years. Massachusetts' environmental agency adds that drinking water somewhat above the limits for a limited time, while a utility fixes the problem, is not likely to significantly raise health risks for most people. A CDC study notes that exposure to these byproducts has been associated with bladder and colorectal cancer in humans.

The people to think about first are children, who drink more water for their size than adults. If your report shows 15 ppb, this isn't your problem. If it shows a level near 80 with a range over it, year after year, it's worth fixing at your house.

Do TTHMs get into shower air?

Yes: TTHMs evaporate out of hot water, so a hot shower adds to what you take in by breathing them and through your skin. A CDC study had adults shower for 10 minutes, bathe for 10 minutes, or drink a liter of the same tap water. The highest blood levels of THMs came from the shower, and the lowest from drinking. Massachusetts' fact sheet says the same: TTHM exposure from breathing and skin "can be significant, especially when showering and bathing." Haloacetic acids don't evaporate, so this part is about TTHMs only.

That is the real reason a tank where the water enters the house does more than an under-sink filter when your TTHMs run high. A shower-head filter is an option, but I recommend filtering the whole house, and NSF certifies shower filters for free chlorine only. Three free steps help with any shower: run the bathroom fan, keep showers shorter, and turn the water heater down a little.

What removes TTHMs from water?

Activated carbon is the established way to take TTHMs out of water, and the more carbon the water passes through, the more contact time it gets. The EPA names granular activated carbon as the best available technology for treatment plants, defined as a carbon bed that holds the water for 10 or 20 minutes. Massachusetts lists carbon and reverse osmosis as the home filters that reduce TTHM exposure.

For a whole house, the tank I recommend for most homes is the Clack 2.5 non-backwashing carbon filter ($1,695). It's a 13 by 54 inch Vortech tank with 2 cubic feet of Centaur catalytic carbon made by Calgon. The tank is the 2.5 cubic foot size, and the extra room lets the carbon lift as water flows up through it. Because it's a large tank, it won't really interrupt your flow rates. The Clack 1.5 ($1,495) is the same head and the same carbon in a smaller 10 by 54 tank. Every whole house carbon filter we sell uses the same Centaur carbon.

Here is the honest part. None of the carbon tanks or reverse osmosis units we sell is certified for TTHM or HAA5 reduction, and Calgon lists Centaur for chloramine and hydrogen sulfide without publishing a THM number for it. Carbon is the technology the EPA names for both byproducts. A home tank gives the water less contact time than a treatment plant's carbon bed, which is why the size of the tank matters, and why a tap test after it's in is how you know it's working on your water.

The tank ships empty. The carbon ships separately and goes in at install through the funnel that comes with it, with the center tube taped over first so no carbon gets inside it. There's no drain line and no electricity, just an inlet and an outlet on the cold line where the water comes into the house. I recommend having a plumber put it in, and an average plumber does it in about an hour and a half; I'm a phone call away for your plumber. If you'd rather do it yourself, text me with any questions.

For drinking and cooking water only, an under-sink reverse osmosis system does the job at the kitchen faucet: the MAW-50 (from $275) or the quick-change Pure-75 ($595). Neither is NSF system-certified, and neither does anything for the shower. If you rent and can't put in a whole house system, I tell people to drink from a refrigerator dispenser with a carbon filter, or buy bottled water; check that the filter is certified for TTHMs (see the Brita question). For more on the trade-off, see carbon filter vs reverse osmosis.

How do I test my tap for TTHMs?

Your utility's report is free and comes first; a certified lab test at your own faucet tells you what's coming out of your tap. You can also ask your utility for its VOC (volatile organic compound) results, which include the trihalomethanes.

Two of our kits measure TTHMs at your tap. The VOC Water Test Kit ($169) measures total trihalomethanes and each of the four compounds. The Advanced City Water Test Kit ($279) measures them along with lead, copper and the rest of a 109-item panel. Our standard City Water Test Kit ($199) does not include disinfection byproducts, and none of our kits measures HAA5.

Of the tests my customers have sent in since June that found TTHMs (12 of them), half came in under 22 ppb and the highest was 79; none was over 80. For how strips, meters and lab tests compare, see how to test your tap water.

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