Adding chlorine (or chloramine) to drinking water is one of the great public-health wins of the last century — it wiped out waterborne killers like cholera and typhoid. The small amount of chlorine left in your tap water isn't the main concern. What it leaves behind is.
Meet disinfection byproducts
When chlorine reacts with the natural organic matter in source water (decaying leaves, algae, soil), it forms a family of disinfection byproducts (DBPs). Scientists have identified more than 600 of them. Only about nine are regulated — the trihalomethanes (TTHMs) and haloacetic acids (HAA5) you'll see on a water report.
Long-term exposure to these byproducts has been linked, most consistently, to bladder cancer, with weaker associations to colorectal and kidney cancer and to reproductive effects like low birth weight. International cancer authorities classify several trihalomethanes as probable human carcinogens. One industry estimate suggests THMs may contribute to a meaningful share of U.S. bladder-cancer cases each year.
The surprise for most families: a hot shower can expose you to more of these chemicals than a glass of water.
Why the shower matters
Trihalomethanes are volatile — they evaporate easily, especially in hot water. So a steamy shower, a bath, or running the dishwasher releases them into the air you breathe and onto the skin you absorb through. Exposure isn't just what you drink; it's inhalation and skin contact at every warm tap. That's a big reason filtering only your drinking water leaves a gap.
Is chloramine the fix? Not exactly
Some utilities switched to chloramine because it forms fewer THMs and HAAs. But it creates its own byproducts — including NDMA, a suspected carcinogen, and a compound called the chloronitramide anion that scientists only recently identified. Different, not necessarily fewer.
Legal vs. safe, again
The federal limit for total trihalomethanes is 80 ppb — a number set to hold lifetime cancer risk near one in 10,000, balancing cost and feasibility. California proposed a health goal for trihalomethanes of just 0.8 ppb, about 100 times lower. The gap between “legal” and “protective” is wide here.
What you can do
- Activated carbon is your friend. Carbon filtration is very effective at reducing chlorine and disinfection byproducts.
- Think beyond the kitchen. Because you inhale and absorb DBPs in the shower, a whole-house carbon filter — or at minimum a shower filter — addresses exposure a drinking filter can't.
- Choose certified. Look for NSF/ANSI 42 or 53 certification for the contaminants you care about.