TTHMs and Haloacetic Acids: The Disinfection Byproducts in Your Tap Water

Chlorine is added to municipal water to kill bacteria, viruses, and protozoa. This is unambiguously good — waterborne disease was a major public health crisis before chlorination became standard practice. But chlorine is chemically reactive. When it contacts naturally occurring organic matter in source water (decaying leaves, plant material, algae), it forms a class of compounds called disinfection byproducts (DBPs). The two most regulated groups are trihalomethanes (TTHMs) and haloacetic acids (HAA5).

Total trihalomethanes (TTHMs) are a group of four compounds: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. They form when chlorine reacts with humic and fulvic acids — the breakdown products of organic matter in source water. TTHMs are volatile, meaning they can off-gas from hot water (showers, hot tubs, dishwashers).

• EPA MCL for TTHMs: 80 ppb (annual average)
• Most Indiana systems run 20-60 ppb — within legal limits but detectable
• TTHM levels spike in summer when source water has more organic matter and warmer temperatures speed the reaction
• Long-term exposure at legal TTHM levels has been associated with increased bladder cancer risk

HAA5 refers to five haloacetic acid compounds: monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid. Like TTHMs, they form when chlorine reacts with organic matter, but HAAs are not volatile — they stay dissolved in the water rather than gassing off.

• EPA MCL for HAA5: 60 ppb (annual average)
• HAAs are drinking-water concerns, not showering concerns (unlike TTHMs)
• Research has linked HAA exposure to increased miscarriage risk and neural tube defects in pregnancy

Summer CCR readings almost always show higher DBP levels than winter readings. The reason: warm water speeds up the chemical reaction between chlorine and organic matter, and summer algae blooms increase the organic load in reservoirs and rivers. Utilities adjust chlorine dosing seasonally, but they can only reduce it so much without risking bacterial breakthrough.

Both compound classes are effectively removed by activated carbon.

• Granular activated carbon (GAC) whole-home systems remove TTHMs and HAAs at the point of entry. All taps benefit.
• Carbon block pitcher filters (like Brita) reduce DBPs at the point of use but don't help with showering exposure.
• Reverse osmosis systems typically include a carbon pre-filter and remove TTHMs and HAAs as part of their full treatment profile.
• Boiling does not remove TTHMs — it actually concentrates them as water evaporates. Boiling eliminates biological contamination only.
• Softeners do not remove TTHMs or HAAs.

Some utilities (including parts of Indianapolis and Grand Rapids) have switched from free chlorine to chloramines (chlorine combined with ammonia) as a secondary disinfectant. Chloramines produce fewer TTHMs but more iodoacids and other DBPs. Chloramines also do not gas off easily, meaning they persist longer in plumbing. Standard activated carbon does not remove chloramines well — catalytic carbon is more effective. If your utility uses chloramines (check your CCR), make sure any carbon system you install uses catalytic carbon media.