EPA legal limit
No federal limit
Maximum Contaminant Level
EWG health guideline
0.4 ppb
Science-based, stricter target
IARC classification
Group 2B — possibly carcinogenic to humans
Cancer research classification
Health effects
Chloroform is classified by IARC as a possible human carcinogen (Group 2B) based on liver and kidney tumour evidence in animal studies. Long-term exposure via drinking water has been associated with bladder cancer risk in epidemiological studies. It is also a reproductive toxicant — associated with spontaneous abortion and small-for-gestational-age birth outcomes at levels commonly found in US tap water.
Where it comes from
Formed during chlorination of drinking water — not present in source water before disinfection. Surface water utilities with high organic content (humic substances from decaying vegetation) produce more chloroform. Higher in warm months when algae and organic matter are elevated.
How it's regulated
No individual EPA MCL — regulated as part of the total trihalomethanes (TTHMs) group limit of 80 ppb (running annual average). EWG health guideline: 0.1 ppb, based on a 1-in-1-million cancer risk from lifetime exposure. NYC average (2021–2023): 34.3 ppb — 343× the EWG guideline while meeting the EPA standard.
How to filter chloroform
Not all filters address chloroform. Look for independently certified filters—NSF International certification means the removal claim has been independently verified.
Chloroform is not removed by water ionization. An NSF 53 certified activated carbon block filter is the evidence-based solution for chloroform and other TTHM compounds.
Frequently asked questions
What is chloroform?
Chloroform is the most abundant of the four trihalomethane (TTHM) compounds, accounting for the majority of total TTHM levels in chlorinated water. It forms when chlorine reacts with naturally occurring organic matter in source water. Higher surface-water content in the source — especially warm-season algae and decaying vegetation — drives higher chloroform levels.
What are the health effects of chloroform?
Chloroform is classified by IARC as a possible human carcinogen (Group 2B) based on liver and kidney tumour evidence in animal studies. Long-term exposure via drinking water has been associated with bladder cancer risk in epidemiological studies. It is also a reproductive toxicant — associated with spontaneous abortion and small-for-gestational-age birth outcomes at levels commonly found in US tap water.
Is chloroform regulated in drinking water?
No individual EPA MCL — regulated as part of the total trihalomethanes (TTHMs) group limit of 80 ppb (running annual average). EWG health guideline: 0.1 ppb, based on a 1-in-1-million cancer risk from lifetime exposure. NYC average (2021–2023): 34.3 ppb — 343× the EWG guideline while meeting the EPA standard.
Where does chloroform come from?
Formed during chlorination of drinking water — not present in source water before disinfection. Surface water utilities with high organic content (humic substances from decaying vegetation) produce more chloroform. Higher in warm months when algae and organic matter are elevated.
How do I remove chloroform from tap water?
The most effective methods for removing chloroform are: Activated carbon block filter, NSF 53 certified filter, Reverse osmosis (RO). Look for NSF-certified systems—independent certification confirms removal claims have been verified.
Is chloroform in your water?
Enter your ZIP code to see the measured level in your specific utility.