Dichloromethane (Methylene Chloride)

Dichloromethane (methylene chloride) is a chlorinated solvent historically used as the active ingredient in heavy-duty paint strippers, aerosol carb cleaners, brake cleaners, and some adhesives. Its solvent power against varnish, lacquer, polyurethane, acrylic, and carbon deposits is exceptional — few alternatives match it on the heaviest deposits. The chemistry profile that makes it effective also makes it acutely and chronically hazardous: it is a Prop 65 listed carcinogen (CRT listed 1988-04-01), an IARC Group 2A "Probably carcinogenic to humans" substance, NTP "Reasonably Anticipated to be a Human Carcinogen," and an OSHA Specifically Regulated Substance with substance-specific PELs.

Federal regulatory direction has been progressively restrictive since 2019. EPA's TSCA Section 6(h) rule restricted consumer methylene chloride in paint-stripping uses. The 2024 TSCA Section 6 final rule extends restrictions to most other consumer and commercial uses, with a phased compliance schedule. Carburetor cleaning is not on the explicitly-prohibited-uses list but the chemistry profile is shared with applications already restricted.

Acutely, dichloromethane is uniquely dangerous in confined-space scenarios because it is metabolized to carbon monoxide in humans — elevated carboxyhemoglobin from a single high-vapor exposure can be fatal, and the mechanism is not always anticipated by users familiar with non-chlorinated solvents. The vapor is denser than air, so it accumulates in enclosed low spots (basements, pits, partially-ventilated garages). Required PPE for consumer-strength application includes a chemical respirator with organic-vapor cartridge and full facepiece (NIOSH/MSHA-approved); chemical-resistant gloves with verified dichloromethane permeation data (nitrile fails rapidly — Viton/butyl/polyvinyl alcohol are the typical chemistry-resistant options); chemical-resistant clothing; and an impervious apron.

Federally VOC-exempt under EPA 40 CFR 51.100(s) because of negligible photochemical reactivity in the atmosphere — the exemption is an atmospheric ozone-formation accounting decision and operates independently of the substance's classification as a carcinogen and aquatic toxin. California does not categorically follow this exemption for consumer-product VOC limits in all categories, which is one reason dichloromethane-based consumer products are typically not sold in California even when federally compliant.