UN 1242 — Methyldichlorosilane
Placard: Dangerous When Wet. ERG Guide 139. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
UN 1242 is Methyldichlorosilane, a dangerous-when-wet Class 4 material assigned to ERG Guide 139. It reacts with water or moist air to release flammable and corrosive/toxic gases, creating fire, fuming and re-ignition hazards.
Hazard overview: UN 1242 presents water-reactive, flammable gas and corrosive fume hazards. Moisture can generate hydrogen and hydrogen chloride fumes, and the material may ignite or re-ignite if water, foam or incompatible runoff reaches it.
Response guidance: For a UN 1242 incident, responders should confirm the product using shipping papers, container markings, SDS and ERG Guide 139. Establish incident command, isolate the area, stay upwind and uphill, remove ignition sources when safe, keep vapors or runoff out of sewers and use compatible fire-control agents from a protected position.
Firefighter training notes: Training for UN 1242 should emphasize dangerous-when-wet behavior, flammable gas generation, corrosive fumes, dry-agent selection and runoff control. Common errors include applying water or foam directly, entering without SCBA and ignoring re-ignition after apparent knockdown. Use ERG 139, SDS and hazmat SOP.
Regulatory context: Methyldichlorosilane is regulated as a hazardous material for transportation and emergency response purposes. Transportation, workplace exposure, spill reporting, waste handling, storage and environmental requirements may vary by formulation, quantity and jurisdiction. Verify current requirements through shipping papers, SDS, facility documents and applicable DOT, OSHA, EPA, NFPA, state or local authority guidance.
Storage & handling: Methyldichlorosilane should be stored in tightly closed compatible containers in a cool, dry, well-ventilated area away from water, moisture, heat, ignition sources, oxidizers and incompatible materials. Protect containers from corrosion, impact and unauthorized access, with dry spill-control materials available where required.
UN 1242 Quick Details
Common Hazards of UN 1242
- DANGEROUS WHEN WET; reacts with water or moisture to produce flammable and toxic/corrosive gases.
- May ignite on contact with water or moist air.
- Reaction may release heat, hydrogen and hydrogen chloride fumes.
- May be ignited by heat, sparks or flames.
- May re-ignite after fire appears to be extinguished if moisture or incompatible agents remain.
- Containers may rupture or explode when heated or contaminated with water.
- Runoff or water application may spread fire, corrosive contamination or reactive residues.
Chemical Identity & Physical Properties
Colorless to pale yellow fuming liquid with a sharp, pungent, acrid odor. Fumes in moist air due to hydrolysis. Liquid at room temperature.
| Also known as | DichloromethylsilaneMethylsilicon dichlorideSilane, dichloromethyl-MDCS |
| CAS Number | 75-54-7 |
| Appearance | Colorless to pale yellow fuming liquid with a sharp, pungent, acrid odor. Fumes in moist air due to hydrolysis. Liquid at room temperature. |
| Flash Point | -32C (-26F) |
| Boiling Point | 41C (106F) |
| Vapor Density | 4.0 (heavier than air) |
| Water Reactivity | Reacts violently with water, producing HCl gas and heat. May ignite spontaneously. Do not use water directly on material. |
Fireground Response Guidance — UN 1242
Extinguishing Media
PPE Requirements
Use positive-pressure SCBA for vapor, fire or confined-space exposure. Chemical-resistant protective clothing, gloves, boots and eye/face protection should be selected from SDS and incident command because fumes and liquid may be corrosive, toxic and water-reactive.
Isolation & Evacuation
First Actions for a UN 1242 Incident
- CALL 911. Then call the emergency response telephone number on the shipping paper, if available.
- Keep unauthorized personnel away.
- Stay upwind, uphill and/or upstream.
- Eliminate ignition sources if it is safe to do so.
- Do not touch spilled material or damaged containers unless properly trained and equipped.
- Keep water, foam and moisture away from the released material unless incident command confirms a compatible cooling or control use.
- Ventilate closed spaces before entering, but only if properly trained, equipped and authorized by incident command.
- Isolate the spill or leak area for at least 50 meters (150 feet) in all directions.
- For large spills, fire involvement or violent reaction, expand isolation and consider downwind evacuation based on monitoring and incident command.
- Use ERG Guide 139, shipping papers, SDS and local SOP for protective actions and entry decisions.
📋 Copy & Share Field Card
UN 1242 — MethyldichlorosilaneUse for: Quick radio or face-to-face size-up. Short, structured, field-ready.
Use for: Incident command briefing, staging area whiteboard, or pre-entry team brief.
Use for: Quick text to command or incoming units. Fits in a single SMS.