Chloromethane (methyl chloride)
Chloromethane is a gas made mostly by nature, from tropical plants, wildfires and the oceans, and it turns up in under 1 percent of US tap water samples. Its known harms come from breathing very large amounts, which affect the nervous system. There is almost no research on drinking it, and agencies have not classified it as a human carcinogen.
EPA's non-cancer screening level, our line
28 ppb
There is no legal limit for chloromethane. It turns up in under 1 percent of US tap water samples, and there is almost no research on drinking it.
Chloromethane (methyl chloride): what is it, and where does it come from?
Chloromethane, also called methyl chloride, is a colorless gas: one carbon atom with one chlorine.
Nature makes huge amounts of it, from tropical plants and forests, burning vegetation, the oceans and wood-rotting fungi. Industry also uses it to make silicones and other chemicals, and chemical plants, PVC waste, burning plastic and cigarette smoke release more.
It reaches water from industrial releases, landfill leachate and contaminated groundwater, and small amounts may form when drinking water is chlorinated.
In EPA's third national survey of unregulated contaminants (UCMR 3, 2013 to 2015), 0.8 percent of samples had it at or above the 0.2 ppb reporting level, in 2.8 percent of water systems, and no sample was above 269 ppb. Because it evaporates quickly, levels in water stay low.
Chloromethane (methyl chloride): why does it matter?
Almost everything known about chloromethane's health effects comes from breathing very high amounts: refrigerator leaks decades ago, workplace accidents, and animal inhalation studies. At those levels it clearly harms the nervous system, and some people had lasting problems.
There is essentially no data on drinking it: ATSDR found a single short oral animal study, and EPA could not set an oral safe dose. Cancer evidence is weak. Male mice breathing high levels for two years got kidney tumors, but female mice and rats did not, and IARC and EPA both say it cannot be classified as a human carcinogen.
EPA's 28 ppb is a lifetime screening level built from mild nervous system effects in workers. At the low levels sometimes found in tap water, the air you already breathe every day likely gives you more. The research behind the line is thin, so a reading over it may or may not be harmful.
In EPA's third national survey (2013 to 2015), 283 of 36,845 samples (0.8 percent) had chloromethane at or above 0.2 ppb, in 138 of 4,916 water systems; only 8 systems had a sample above 2.69 ppb, and none were above 269 ppb.
ATSDR found only one short oral animal study, so no agency could set an oral safe dose; the known harms, to the nervous system, come from breathing high amounts.
ATSDR (CDC), Toxicological Profile for Chloromethane, September 2023
Male mice breathing high levels for two years developed kidney tumors, but female mice and rats did not; IARC and EPA both say chloromethane cannot be classified as a human carcinogen.
ATSDR (CDC), Toxicological Profile for Chloromethane, September 2023
Chloromethane (methyl chloride): what do we grade it against?
The table below lists each level that applies to Chloromethane (methyl chloride), what it is based on and who set it, and says which ones are enforceable.
| Contaminant | Health-based level | Source (health-based) |
|---|---|---|
| Chloromethane (ppb) | 28 EPA non-cancer lifetime screening level, our line | US EPA, CCL 4 contaminant information sheet, 2016 |
| Chloromethane, cautious cancer screen (ppb) | 2.69 EPA screen from a 1981 male-mouse study its later reviews declined to use | US EPA, CCL 4 and UCMR 3, 2016 |
Your result is graded against EPA's 28 ppb. The 2.69 ppb cancer screen rests on one 1981 study of male mice breathing chloromethane, and EPA's own later assessments declined to derive any cancer value from it.
What helps against Chloromethane (methyl chloride)?
The approaches below are the ones people use for Chloromethane (methyl chloride) at home. Each entry says what it does and what it cannot do, and not every one helps.
Direct fixes
Activated carbon, the standard treatment for industrial solvents
Carbon filtration is the usual tool for volatile industrial chemicals in drinking water.
We found no filter test data specific to chloromethane. Check that a filter's performance sheet names it before relying on it.
Common questions about Chloromethane (methyl chloride)
Where does chloromethane come from?
Mostly from nature: tropical plants and forests, burning vegetation, the oceans and wood-rotting fungi. Industry adds more, and it can reach water from industrial releases, landfills and contaminated groundwater. Small amounts may also form when water is chlorinated.
Does the air give me more than my tap?
Likely, at the low levels sometimes found in tap water. Outdoor air carries a small background of chloromethane everywhere, and breathing it all day adds up to more than a few liters of water at a typical detection. That comparison is our own rough estimate, not an agency figure.
Sources cited for Chloromethane (methyl chloride)
Every number on this Chloromethane (methyl chloride) page traces to one of the 3 sources below: 3 government or regulatory.