What is Chloroform* and Why Does It Matter?

Chloroform (trichloromethane) isn't something most people think about when they turn on the tap. But it shows up in the drinking water of 657 water systems across the country — and 588 of those systems exceed the EWG's health guideline for safe exposure.

So where does it come from? Chloroform isn't added to water on purpose. It forms as a byproduct when water treatment plants use chlorine to kill bacteria and viruses. When chlorine reacts with naturally occurring organic matter — leaves, soil, algae — it creates a group of chemicals called trihalomethanes (THMs). Chloroform is the most common of these. In other words, the same treatment process that makes your water safe from pathogens can create a new chemical concern.

The health risks depend on how much chloroform you're exposed to and for how long. At the levels found in U.S. drinking water, short-term exposure is unlikely to cause immediate harm. Long-term exposure is a different story. The EPA classifies chloroform as a possible human carcinogen, and the EWG goes further, warning that even low-level chronic exposure may increase cancer risk — particularly for the liver, kidneys, and bladder. The national average concentration across affected systems is 13.589 parts per billion (ppb), but some systems have recorded levels as high as 70.2 ppb. That upper end is more than 70 times the EWG's health guideline of 0.4 ppb (EWG Tap Water Database). Children and pregnant women face the greatest risk from repeated exposure over time.

Here's where the regulatory picture gets complicated. The EPA sets the legal limit for total trihalomethanes — the group chloroform belongs to — at 80 ppb. That standard was designed to balance cancer risk against the practical costs of water treatment. The EWG health guideline for chloroform alone sits at just 0.4 ppb, based on a one-in-a-million cancer risk standard. That's a significant gap. Most water utilities are operating within legal limits, but legal doesn't always mean risk-free. Many public health researchers argue the EPA's standard hasn't kept pace with current science on cancer risk (EWG, 2021).

Geographically, chloroform contamination is widespread, but some states carry a heavier burden. Alabama leads the country with 88 affected systems, followed by New York (68), Colorado (40), Texas (37), and California (37). Why these states? A few factors drive the pattern. Warmer water temperatures speed up the chemical reactions that form chloroform. Higher levels of natural organic matter in source water — common in the South and parts of the West — give chlorine more material to react with. Older water infrastructure can also contribute, since longer water transit times in aging pipes allow more byproducts to form. Alabama's combination of warm temperatures, organic-rich surface water, and aging systems makes it particularly vulnerable.

The good news is that chloroform is one of the more filterable contaminants in tap water. Activated carbon filters — the kind found in many pitcher filters and under-sink systems — can reduce chloroform levels meaningfully. For the most thorough removal, a reverse osmosis (RO) system is the gold standard. RO systems remove up to 99% of trihalomethanes, including chloroform, by pushing water through a semi-permeable membrane. Echo Water's reverse osmosis systems are designed specifically to address contaminants like these, giving your family water that meets a much stricter standard than what comes out of the tap. If you're in Alabama, New York, Colorado, Texas, or California, it's worth checking your local water quality report — also called a Consumer Confidence Report — to see exactly what levels your utility is reporting. You can also look up your zip code on the EWG Tap Water Database for a more detailed picture. Knowing what's in your water is the first step. Filtering it out is the second.

Regulatory Standards for Chloroform*

Cities With the Highest Chloroform* Levels

States Most Affected by Chloroform*

Frequently Asked Questions

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