What Is Dichloromethane?

Dichloromethane, also called methylene chloride, is a synthetic chlorinated solvent with the formula CH₂Cl₂. It is a colorless, highly volatile liquid historically used in paint stripping products, aerosol formulations, metal cleaning, pharmaceutical manufacturing, chemical processing, adhesives, and extraction processes. Its volatility makes it move readily between water and air, while its solvent properties make it useful in industry and concerning when released to soil, groundwater, or indoor environments.

In drinking water, dichloromethane is most often treated as an industrial volatile organic compound, or VOC. It is not a mineral, nutrient, or naturally occurring water-quality parameter. Its presence usually indicates a human source such as a solvent release, industrial discharge, landfill leachate, improper waste disposal, chemical storage failure, or groundwater contamination plume. It can also appear at very low levels as an analytical contaminant if sampling is not carefully performed, because dichloromethane is common in laboratory and industrial settings.

Dichloromethane is important for drinking water safety because it is toxic, mobile enough to affect groundwater, and volatile enough to create inhalation exposure from contaminated water or from vapor intrusion above contaminated groundwater. Public health agencies regulate or monitor it because long-term exposure has been associated with increased cancer risk and organ toxicity, while high short-term exposures can affect the central nervous system and increase carbon monoxide levels in the blood.

Scientific Identity

Dichloromethane is a low-molecular-weight chlorinated hydrocarbon consisting of one carbon atom bonded to two hydrogen atoms and two chlorine atoms. It belongs to the broader group of halogenated volatile organic compounds that includes chloroform, carbon tetrachloride, trichloroethylene, tetrachloroethylene, and dichloroethylenes. Its CAS number is 75-09-2, and its common industrial synonym is methylene chloride.

From a water chemistry perspective, dichloromethane is moderately soluble in water compared with many petroleum hydrocarbons, but it is also highly volatile. This combination allows it to dissolve into groundwater after a release and then partition into soil gas or indoor air under the right conditions. It has a relatively low affinity for soil organic matter compared with heavier chlorinated solvents, so it can migrate with groundwater rather than remaining tightly bound to sediments.

Dichloromethane is chemically distinct from chlorine disinfectants, although it is part of the broad halogenated organic family. It is not usually the primary disinfection byproduct of concern in treated drinking water; chloroform and other trihalomethanes are far more typical disinfection byproducts. However, dichloromethane can occasionally be discussed alongside disinfection byproducts or other chlorinated organics because they may be detected together in VOC testing panels.

How Dichloromethane Enters Drinking Water

The most significant drinking water pathway is contamination of groundwater from industrial or commercial solvent handling. Facilities that used dichloromethane for degreasing, paint removal, coating operations, extraction, or chemical manufacturing may have released it through spills, leaking tanks, floor drains, disposal pits, sewer systems, or historical waste practices. Once in the subsurface, it can dissolve into groundwater and form a contaminant plume that moves downgradient from the source.

Waste sites are another important pathway. Landfills, hazardous waste lagoons, drum disposal areas, and former manufacturing parcels may contain mixed solvents, including dichloromethane. Rainwater infiltration can leach chemicals from buried waste into groundwater. Private wells near these sites can be vulnerable because they are often not tested as frequently as regulated public water supplies.

Surface water can be affected by industrial discharges, urban runoff, accidental releases, or contaminated groundwater discharging to streams. In open surface waters, dichloromethane often volatilizes relatively quickly, but continuous inputs can maintain detectable concentrations. In enclosed or semi-enclosed water systems, contaminated raw water or distribution system intrusion may also be relevant.

Dichloromethane can also create exposure through vapor pathways. If contaminated groundwater or soil gas lies beneath a building, vapors may migrate through cracks, utility penetrations, sump pits, or foundation gaps. This is known as vapor intrusion. Vapor intrusion is not the same as drinking contaminated water, but it may occur at the same contaminated sites and can dominate total exposure in some buildings.

Occurrence and Exposure

Dichloromethane is most likely to be found in groundwater near industrial corridors, chemical plants, former paint-stripping or metal-cleaning operations, landfills, dry industrial waste areas, and hazardous waste cleanup sites. It may be detected together with other VOCs such as trichloroethylene, tetrachloroethylene, vinyl chloride, dichloroethylenes, chloroform, benzene-related compounds, and other solvents depending on the source history.

People can be exposed by drinking contaminated water, cooking with it, or using it to make beverages. Because dichloromethane is volatile, household uses such as showering, bathing, dishwashing, and laundering can transfer some of the compound from water into indoor air. Inhalation during showering may contribute to total exposure, especially where concentrations are elevated or ventilation is poor.

Public water systems typically monitor regulated VOCs according to national or local requirements, but testing frequency depends on system size, source type, prior detections, and jurisdiction. Private well owners are responsible for their own testing in many countries. A private well near a manufacturing site, landfill, fuel or solvent spill, military or aviation facility, or known groundwater plume should be evaluated with a laboratory VOC panel rather than a simple home screening kit.

Health Effects and Risk

Dichloromethane is a high-concern contaminant because toxicological evidence links exposure to cancer risk and non-cancer effects. Several regulatory and scientific agencies classify it as a probable or likely human carcinogen based on animal studies, mechanistic evidence, and occupational exposure data. Cancer concerns have included liver and lung tumors in experimental animals, with risk assessments using conservative assumptions for lifetime drinking water exposure.

Non-cancer health effects are also important. Dichloromethane is metabolized in the body partly to carbon monoxide, which can increase carboxyhemoglobin in blood and reduce oxygen delivery. This is especially relevant for people with cardiovascular disease, anemia, respiratory disease, pregnant individuals, infants, and others who may be more vulnerable to reduced oxygen transport. High short-term exposures can cause headache, dizziness, nausea, impaired coordination, and central nervous system depression.

Longer-term exposure has been associated with liver effects and possible kidney effects. Because drinking water exposure may include ingestion and inhalation from volatilization, risk evaluation should consider total household exposure, not just the amount swallowed. The greatest concern is not a one-time trace detection but repeated exposure above health-based limits or ongoing use of a contaminated well.

If dichloromethane is detected in drinking water, the appropriate response depends on concentration, duration, water use, and the availability of alternate water. Elevated detections should be confirmed with a properly collected laboratory sample. Sensitive populations and households using private wells near known solvent plumes should seek guidance from local health authorities, environmental agencies, or qualified water professionals.

Testing and Monitoring

Dichloromethane requires specialized laboratory analysis for volatile organic compounds. The most common approach is purge-and-trap gas chromatography with mass spectrometry or a related EPA, ISO, or nationally recognized VOC method. In the United States, laboratories commonly use methods such as EPA Method 524.2 or related drinking water VOC methods for regulated public water monitoring, while groundwater investigations may use EPA Method 8260 or similar protocols.

Sampling technique is critical because dichloromethane is volatile. Samples are typically collected in laboratory-supplied glass vials with no headspace, preserved as required, sealed with septa caps, kept cold, and shipped promptly under chain-of-custody procedures. Aerating the water, leaving air bubbles in the vial, collecting from hoses, or using inappropriate plastic containers can cause loss of VOCs or cross-contamination.

Because dichloromethane is also a common laboratory solvent, quality control is important. A credible report should include method detection limits, reporting limits, blanks, sample preservation status, and any laboratory qualifiers. If a result is near the detection limit, confirmation sampling is often appropriate before making costly treatment or real estate decisions.

For private wells, testing should include a broad VOC panel rather than dichloromethane alone, because chlorinated solvent sites rarely contain a single chemical. Where vapor intrusion is suspected, water testing should be paired with indoor air, sub-slab soil gas, or soil vapor evaluation by qualified professionals. A standard mineral test, bacteria test, or basic home strip cannot reliably detect dichloromethane.

Treatment Methods

Activated carbon is generally the preferred drinking water treatment for dichloromethane when the system is properly designed, sized, installed, and maintained. Granular activated carbon adsorbs organic molecules onto a high-surface-area carbon bed. For VOCs such as dichloromethane, carbon can be effective, but performance depends strongly on influent concentration, empty bed contact time, flow rate, water temperature, competing organic chemicals, natural organic matter, and carbon replacement schedule.

Dichloromethane is smaller and more volatile than many hydrophobic organic chemicals, so it may break through carbon faster than heavier solvents. This means undersized carbon filters, old refrigerator cartridges, taste-and-odor filters, and poorly maintained units should not be assumed to provide reliable protection. A treatment design should include certified VOC reduction claims where applicable, adequate contact time, routine cartridge or media replacement, and post-treatment monitoring when contamination is significant.

Point-of-use activated carbon at a kitchen tap can be appropriate when the goal is reducing ingestion from drinking and cooking water. Point-of-entry treatment may be more appropriate when concentrations are high enough that showering, bathing, laundry, or other whole-house uses could contribute meaningful inhalation exposure. For homes near solvent plumes, treatment decisions should also consider vapor intrusion, which cannot be solved by a sink filter.

Regulations and Guidelines

Dichloromethane is regulated or guideline-listed in many jurisdictions because of its toxicity and potential carcinogenicity. In the United States, the U.S. Environmental Protection Agency regulates dichloromethane, also called methylene chloride, under the Safe Drinking Water Act for public water systems. The federal Maximum Contaminant Level is 0.005 mg/L, equivalent to 5 micrograms per liter, and the health-based goal is set more protectively because of cancer concern. Public water systems must monitor according to EPA and state implementation requirements.

The World Health Organization has published a drinking water guideline value for dichloromethane in its Guidelines for Drinking-water Quality. WHO values are health-based guidance and are not automatically enforceable unless adopted by a country or local authority. National standards may differ from WHO guidance because of local risk-management decisions, analytical feasibility, occurrence data, and regulatory policy.

Other countries and regions may set their own maximum acceptable concentrations, parametric values, or advisory levels for dichloromethane. These limits can vary by country, province, state, or water supply category. Private wells are often not covered by the same enforceable monitoring rules that apply to public systems, even when health-based benchmarks exist.

Where dichloromethane is detected near industrial sites, regulatory concern may extend beyond drinking water. Environmental agencies may evaluate groundwater cleanup levels, vapor intrusion screening levels, soil gas, indoor air, discharge permits, and hazardous waste obligations. A drinking water result should therefore be interpreted in the broader context of site history, nearby plumes, and local environmental regulations.

Related Contaminants

Frequently Asked Questions

Is dichloromethane the same as methylene chloride?

Yes. Dichloromethane and methylene chloride are two names for the same chemical, CH₂Cl₂. Laboratory reports, safety data sheets, and regulatory documents may use either name, so both should be checked when reviewing water test results.

Can I smell or taste dichloromethane in drinking water?

Not reliably. Dichloromethane has a sweet, solvent-like odor at sufficiently high concentrations, but health-relevant drinking water levels can be far below odor recognition. Absence of taste or smell does not mean the water is safe.

Is boiling water a good way to remove dichloromethane?

No. Boiling can volatilize dichloromethane and release it into indoor air, potentially shifting exposure from ingestion to inhalation. If elevated levels are confirmed, use an alternate water source or a properly designed treatment system rather than boiling.

Should I use a point-of-use or point-of-entry filter?

A point-of-use activated carbon unit may be suitable for low-level contamination when the concern is drinking and cooking water. Point-of-entry treatment may be needed when levels are high enough that showering, bathing, and other household uses could release meaningful vapors. Site-specific professional advice is recommended for confirmed contamination.

What should private well owners do if dichloromethane is detected?

Confirm the result with a properly collected VOC sample from an accredited laboratory, review nearby industrial or waste-site sources, test for related VOCs, and consult local health or environmental officials. If concentrations exceed applicable standards or health guidance, use bottled or treated water until a reliable long-term solution is installed and verified.