What is Bromodichloromethane* and Why Does It Matter?

Bromodichloromethane is a disinfection byproduct — a chemical that forms when chlorine used to treat drinking water reacts with naturally occurring organic matter. Think of it like an unintended chemical reaction: utilities add chlorine to kill harmful bacteria, but that chlorine can combine with plant material, algae, and other organic compounds already in the water. The result is a group of chemicals called trihalomethanes (THMs), and bromodichloromethane is one of the most common and concerning members of that family. It doesn't come from industrial spills or agricultural runoff. It's created inside the water treatment process itself.

The health concerns around bromodichloromethane are serious enough to warrant attention. Animal studies have linked long-term exposure to liver, kidney, and reproductive harm. The International Agency for Research on Cancer (IARC) classifies it as a possible human carcinogen. The Environmental Working Group (EWG) flags it as a cancer risk even at very low concentrations, with their health guideline set at just 0.06 parts per billion (ppb). Across 653 water systems tested, the average detected level was 4.47 ppb — more than 74 times above the EWG health guideline. The highest recorded level reached 23.5 ppb, nearly 400 times that benchmark. Pregnant women, infants, and people with compromised immune systems face the greatest risk from ongoing exposure.

The EPA does regulate bromodichloromethane, but not on its own. Instead, it's grouped with three other trihalomethanes under a combined legal limit of 80 ppb — a standard set back in 1998. That collective limit means a utility's water can contain significant amounts of bromodichloromethane and still pass federal testing. The EWG argues, based on more recent research, that the combined regulatory limit doesn't adequately protect against cancer risk over a lifetime of drinking tap water. Of the 653 water systems where bromodichloromethane was detected, 645 of them — that's 98.8% — exceeded the EWG health guideline, even while likely staying within the EPA's legal limit. That gap between "legal" and "safe" is exactly why understanding your water matters.

Geographically, the states with the most affected water systems include Alabama (85 systems), New York (57), Colorado (39), Illinois (37), and California (37). The pattern reflects a combination of factors: water source chemistry, climate, and treatment practices. Warmer surface water — common in the South and parts of the West — tends to contain more organic matter, giving chlorine more material to react with. Older infrastructure in states like Illinois and New York can also play a role, as longer transit times through pipes allow more byproducts to form. Alabama's high count likely reflects heavy reliance on surface water sources like rivers and reservoirs, which naturally carry more organic content than groundwater.

The good news is that bromodichloromethane is very effectively removed by the right filtration technology. A certified reverse osmosis (RO) system can remove up to 99% of trihalomethanes, including bromodichloromethane, from your drinking water. Activated carbon filters — particularly those using granular activated carbon (GAC) or solid carbon block — also perform well against this contaminant. Look for filters certified by NSF International under Standard 53 (for health-based contaminant reduction) or Standard 58 (for reverse osmosis systems). Echo Water's reverse osmosis systems are independently tested to address trihalomethanes and dozens of other contaminants in a single system. If you're on city water treated with chlorine, filtering your drinking and cooking water is one of the most practical steps you can take. You can also request your utility's annual Consumer Confidence Report — it's free, publicly available, and will show you what's actually been detected in your local water supply.

Regulatory Standards for Bromodichloromethane*

Cities With the Highest Bromodichloromethane* Levels

States Most Affected by Bromodichloromethane*

Frequently Asked Questions

Related Contaminant Guides