Trihalomethanes get most of the attention, but they’re only half the disinfection-byproduct story. The other half is haloacetic acids — HAA5, the group of five — formed the same way, by the same chlorine, carrying their own cancer risk.
What they are. Haloacetic acids are a group of five related compounds formed when chlorine (or chloramine) reacts with natural organic matter in the water. Unlike trihalomethanes, they’re not volatile — they don’t evaporate, they don’t off-gas in the shower. They stay in the water, which makes them an ingestion concern: drinking and cooking.
Where they come from. Same origin as TTHMs — disinfection doing its job. Chlorine meets organic matter, byproducts form, and they ride the distribution system to your tap. Levels run highest in warm months and in surface-water systems. Another city-water story.
The health picture, honestly. Long-term exposure above the legal limit carries an increased risk of cancer — that’s the EPA’s stated concern. The legal limit is 60 parts per billion (0.060 mg/L). Note the difference from TTHMs: because HAA5 isn’t volatile, the shower isn’t the exposure route — the glass is.
How to test. Your utility’s annual water quality report lists HAA5 right alongside TTHMs. Independent labs run it too. And like TTHMs, if your utility has sent violation notices, they’re telling you the numbers already.
What removes them. Activated carbon — point-of-use carbon adsorbs haloacetic acids, though it loads up and breaks through as the bed fills, so filter changes matter more here than usual. A quality under-sink carbon block system handles the drinking and cooking water. Whole-house carbon helps too.
What does NOT remove them. Boiling won’t touch them (they’re not volatile). Water softeners, UV, and standard pitcher filters aren’t the answer. And don’t assume your TTHM treatment automatically covers HAA5 — check what the system is actually rated for.
What I’d buy: Treatment type: an under-sink carbon block system, NSF listed, for drinking and cooking water — with disciplined filter changes, because carbon loads up on HAA5 and loses effectiveness as it fills. If TTHMs are also on your report, a whole-house carbon system covers both at once.
HAA5 is the quiet half of the disinfection-byproduct pair — no smell, no shower vapor, just in the glass. Carbon filtration handles it. Read your utility’s report, treat the drinking water, change the filters on time.
Curious what’s in your tap? Start with What’s In Your Water?
Leave a Reply