Why we disinfect at all
Chlorine — or chloramine — is why waterborne disease is rare in the US. A disinfectant residual protects the water from the treatment plant all the way to your tap. Removing disinfection entirely would trade a small, long-term concern for a large, immediate one, which is why utilities do not simply switch it off.
The by-products: TTHM and HAA5
When chlorine reacts with natural organic matter in the water, it forms disinfection by-products, chiefly total trihalomethanes (TTHM, federal limit 80 µg/L) and haloacetic acids (HAA5, federal limit 60 µg/L). Utilities monitor these and usually report them as a running annual average, which is the figure you will see on a report.
Disinfection by-products are a reason to optimize treatment, not to stop disinfecting. The goal is both low by-products and effective disinfection.
What the numbers mean
Sustained exposure above the limits is associated in studies with bladder-cancer and reproductive risks, so utilities manage the organic precursors and disinfection practice to keep by-products down. Read a within-limits result carefully: it means the system met the federal standard, which is not the same as saying there is no health consideration at all. Compliance is a legal threshold, and the individual health picture is separate — check the date and the reported value on your city water report.
If you want to reduce them at home
An activated-carbon filter (pitcher, faucet or under-sink) reduces TTHM along with taste and odor; reverse osmosis removes them too. Neither is usually necessary for a system reporting within the limits, but both are reasonable options if you prefer. To compare the two technologies, see how reverse osmosis and carbon filters differ. PurityMap may earn an affiliate commission on some product links, at no cost to you.
This article gives general information about disinfection and its by-products. It is not medical advice and does not determine whether any water is fit for a specific person. For health questions about disinfection by-products — particularly during pregnancy or with a medical condition — consult a licensed healthcare professional, and use a certified laboratory to test your own water.