What Is Methylene Chloride?
Methylene chloride, also called dichloromethane or DCM, is a colourless, volatile liquid with a sweet, ether-like odour. Registered under CAS number 75-09-2, it is one of the most widely used chlorinated solvents in industry because it combines strong solvency with non-flammability. Typical applications include paint stripping, metal degreasing, pharmaceutical processing and the manufacture of polycarbonate and other polymers.
Its low boiling point and high volatility mean that vapour, not liquid, is the dominant exposure route, and vapour control is therefore the central challenge of safe handling.
Physical and Chemical Properties
| Property | Value |
|---|---|
| CAS number | 75-09-2 |
| Molecular formula | CH2Cl2 |
| Molecular weight | 84.93 |
| Boiling point | approximately 39.6 degrees Celsius at atmospheric pressure |
| Liquid density | about 1.33 g/mL at 20 degrees Celsius |
| Vapour density | approximately 2.9 times that of air |
| Solubility in water | slight, about 1.3 g per 100 mL at 20 degrees Celsius |
| Flammability | non-flammable under normal conditions, although unusual high-temperature conditions can create flammable mixtures |
Because the vapour is much heavier than air, it accumulates in low-lying areas, pits, sumps and confined spaces. Any entry into such a space while DCM is present requires atmospheric testing, mechanical ventilation and a permit system.
Health and Safety Hazards
Inhalation toxicity: breathing high concentrations causes headache, dizziness, nausea and central nervous system depression. In the body, methylene chloride is metabolised to carbon monoxide, which reduces the oxygen-carrying capacity of the blood.
Cancer hazard: methylene chloride is classified as probably carcinogenic to humans by the International Agency for Research on Cancer, and several regulators treat it as a suspected human carcinogen, so long-term exposure should be minimised.
Skin and eye irritation: repeated contact defats the skin, causing dermatitis and, after prolonged exposure, chemical burns.
Oxygen displacement: heavy vapour can displace oxygen in confined or poorly ventilated areas, creating an asphyxiation risk.
Decomposition products: although the liquid does not readily burn, high temperatures generate hydrogen chloride and phosgene, so welding, cutting and other hot work near DCM equipment must be prohibited.
Engineering Controls and Ventilation
Preventing exposure at the source is more reliable than relying on protective equipment alone.
Contain vapour with closed systems, covered containers and sealed transfer points
Apply local exhaust ventilation at degreasing tanks, mixing vessels and every point where vapour is released
Use appropriately rated containers, drip trays and spill containment to minimise leaks
Position the operator upwind of open vessels and never place the head inside a tank
Verify performance with area and personal monitoring rather than assuming ventilation is adequate
Where occupational exposure limits apply, they are typically expressed as an 8-hour time-weighted average with a short-term exposure limit; the limit used in the United States is 25 ppm as an 8-hour average, while other jurisdictions set different values, so the applicable local limit should always be confirmed. Any control strategy that does not produce measurable results should be treated as a failure rather than a compromise.
Personal Protective Equipment and Emergency Response
Respiratory protection: an approved respirator with organic vapour cartridges for moderate exposure, and a supplied-air respirator for high concentrations, confined spaces or prolonged work
Skin protection: chemical-resistant gloves rated for chlorinated solvents, together with protective clothing and boots
Eye protection: chemical splash goggles or a full-face shield to prevent eye contact
Emergency facilities: eyewash stations and safety showers kept accessible wherever the solvent is handled
For inhalation, move the person to fresh air and obtain medical attention. For skin contact, remove contaminated clothing and rinse the skin thoroughly with water. For eye contact, flush with clean water for at least 15 minutes and arrange medical assessment. If the material is swallowed, do not induce vomiting and seek emergency medical help immediately.
Storage, Waste and Regulatory Compliance
Store methylene chloride in tightly closed, correctly labelled containers away from heat, open flames and strong oxidisers, in a ventilated area with secondary containment. Segregate it from incompatible materials and keep transfer operations inside a controlled zone.
Methylene chloride is regulated in many regions because of its toxicity and environmental persistence. Restrictions commonly cover consumer paint strippers, workplace exposure limits and emissions to air and water. Waste solvent must be handled as hazardous waste through licensed disposal channels. Buyers should confirm the requirements that apply in their own country and obtain current safety data sheets and certificates of analysis with each shipment.
Frequently Asked Questions
Q: What is methylene chloride used for?
It is used mainly as a solvent for paint stripping, metal degreasing, pharmaceutical processing and the manufacture of polycarbonate and similar polymers.
Q: Why is methylene chloride hazardous even though it does not burn?
Its vapour is toxic, heavier than air and metabolised to carbon monoxide in the body, and high temperatures can generate hydrogen chloride and phosgene.
Q: Which engineering controls are most important?
Closed systems, covered containers and local exhaust ventilation at every release point, supported by exposure monitoring to prove the controls work.
Q: What personal protective equipment is required?
Organic vapour or supplied-air respiratory protection, solvent-resistant gloves and clothing, and splash goggles or a face shield.
Q: How should methylene chloride waste be handled?
It must be collected as hazardous waste, stored in compatible labelled containers and sent to a licensed disposal facility.
Q: Is methylene chloride stable in storage?
Yes when kept closed, cool and away from ignition sources and oxidisers, but it can slowly hydrolyse in the presence of moisture, so drums should remain sealed and stock rotated.