Disinfection byproducts (TTHMs and HAA5) in drinking water

Disinfection is a public-health win. DBPs are the side effect.

Drinking-water chlorination is one of the most consequential public- health interventions in history. Before chlorination became routine in the early 20th century, U.S. cities suffered recurring typhoid, cholera, and dysentery outbreaks. Chlorination effectively ended waterborne epidemic disease in the developed world.

The cost is chemistry: chlorine reacts with naturally occurring organic matter (decaying leaves, soil organic carbon, algal matter) in source water to form chlorinated byproducts. The two regulated families:

• Total trihalomethanes (TTHMs): chloroform, bromodichloromethane, dibromochloromethane, bromoform. Mostly volatile.
• Haloacetic acids (HAA5): monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, dibromoacetic acid. Non-volatile.

Why they matter for health

The most consistent epidemiologic signal is bladder cancer: long-term exposure to elevated TTHMs is associated with a 15-30% relative increase in bladder cancer risk in pooled analyses. The finding has reproduced across multiple cohorts. Other cancers (colorectal in particular) show weaker associations.

Reproductive outcomes (low birth weight, miscarriage) have been studied with mixed results.

Absolute risk at the MCL is modest. The point of the regulation is to keep the population-level cancer burden small, not to suggest immediate danger from drinking treated water. The much larger public-health risk would be removing disinfection.

EPA regulation

• TTHMs MCL: 0.080 mg/L (80 µg/L), locational running annual average.
• HAA5 MCL: 0.060 mg/L (60 µg/L), locational running annual average.
• MRDLs (disinfectant residuals): 4.0 mg/L for free chlorine and chloramines, 0.8 mg/L for chlorine dioxide.

“Locational running annual average” means a single sample at a single moment in time can exceed the MCL without putting the utility in violation. The annual average is what counts. This obscures sharp seasonal spikes in some utilities (typically late summer, when source water has more organic matter and warmer temperatures drive faster reaction).

Reducing exposure

If your utility has elevated DBPs:

• Granular activated carbon (GAC) filtration at the tap is the most effective household measure. GAC removes both TTHMs and HAA5. Look for NSF/ANSI 53 certification with a VOC reduction claim.
• Reverse osmosis also removes DBPs.
• Boiling removes volatile TTHMs (they evaporate) but concentrates non-volatile HAA5. Not a complete solution.
• Refrigerating overnight in an open container lets the volatile fraction evaporate but does not help HAA5.

Switching disinfectants (free chlorine to chloramines, or to UV with a chloramine residual) is a utility-level decision that affects DBP profiles substantially. It is not a household choice.

Sources

• US EPA, “Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules”: https://www.epa.gov/dwreginfo/stage-1-and-stage-2-disinfectants-and-disinfection-byproducts-rules
• IARC Monograph 101 on Chlorinated Drinking-Water (2014): https://publications.iarc.fr/Book-And-Report-Series/Iarc-Monographs-On-The-Identification-Of-Carcinogenic-Hazards-To-Humans/Some-Chemicals-Present-In-Industrial-And-Consumer-Products-Food-And-Drinking-water-2013
• US EPA, “Drinking Water Regulations”: https://www.epa.gov/dwreginfo/drinking-water-regulations-and-contaminants