☣️ UN 1242 • CLASS 4
UN 1242 — Methyldichlorosilane
Placard: Dangerous When Wet. ERG Guide 139. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
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⚠️ This page is a quick-reference aid. For real incidents: stage upwind, isolate, deny entry, request Hazmat early, and consult the current ERG + SOP/SOG.
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UN 1242 Quick Details
UN 1242
Product name: Methyldichlorosilane
DOT Class: 4
Placard type: Dangerous When Wet
ERG Guide: 139 (check current ERG)
Initial isolation: ERG 139: Isolate spill 50m all directions. If tank/rail car involved in fire, isolate 800m all directions and consider evacuation.
Common Hazards of UN 1242
- Produce flammable and toxic gases on contact with water.
- May ignite on contact with water or moist air.
- Some react vigorously or explosively on contact with water.
- May be ignited by heat, sparks or flames.
- May re-ignite after fire is extinguished.
- Some are transported in highly flammable liquids.
- Containers may explode when heated.
- Runoff may create fire or explosion hazard.
Chemical Identity & Physical Properties
| 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. |
Chemical details are general reference only. Always verify with current SDS, ERG, and SOP/SOG.
Fireground Response Guidance — UN 1242
Extinguishing Media
Dry chemical, CO2, dry sand, soda ash. Do NOT use water or foam directly on liquid.
PPE Requirements
⚠️ Level A or B minimum with SCBA required. Full chemical-resistant suit for spill response. Respiratory protection mandatory due to HCl vapor generation.
Isolation & Evacuation
ERG 139: Isolate spill 50m all directions. If tank/rail car involved in fire, isolate 800m all directions and consider evacuation.
Always confirm protective actions with the current edition of the Emergency Response Guidebook (ERG Guide 139).
First Actions for a UN 1242 Incident
- CALL 911. Then call emergency response telephone number on shipping paper. If shipping paper
- Keep unauthorized personnel away.
- Stay upwind, uphill and/or upstream.
- Ventilate closed spaces before entering, but only if properly trained and equipped.
- Isolate spill or leak area in all directions for at least 50 meters (150 feet) for liquids and at least 25 meters
- For highlighted materials: see Table 1 - Initial Isolation and Protective Action Distances.
- For non-highlighted materials: increase the immediate precautionary measure distance, in the downwind
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📋 Copy & Share Field Card
UN 1242 — MethyldichlorosilaneHAZMAT RADIO NOTE — UN 1242
Product: Methyldichlorosilane
Class 4 / Dangerous When Wet / ERG 139
PPE: Level A or B minimum with SCBA required. Full chemical-resistant suit for spill response. Respiratory protection mandatory due to HCl vapor generation.
ISOLATION: ERG 139: Isolate spill 50m all directions. If tank/rail car involved in fire, isolate 800m all directions and consider evacuation.
ACTION: Stage upwind · Isolate · Deny entry · Request HazmatRADIO
Use for: Quick radio or face-to-face size-up. Short, structured, field-ready.
=== IC HAZMAT BRIEFING ===
UN 1242 — Methyldichlorosilane
Class: 4 | Placard: Dangerous When Wet | ERG Guide: 139
Appearance: Colorless to pale yellow fuming liquid with a sharp, pungent, acrid odor. Fumes in moist air due to hydrolysis. Liquid at room temperature.
Water Reactivity: Reacts violently with water, producing HCl gas and heat. May ignite spontaneously. Do not use water directly on material.
Extinguishing: Dry chemical, CO2, dry sand, soda ash. Do NOT use water or foam directly on liquid.
PPE: Level A or B minimum with SCBA required. Full chemical-resistant suit for spill response. Respiratory protection mandatory due to HCl vapor generation.
Isolation: ERG 139: Isolate spill 50m all directions. If tank/rail car involved in fire, isolate 800m all directions and consider evacuation.
— Key Hazards —
• Produce flammable and toxic gases on contact with water.
• May ignite on contact with water or moist air.
• Some react vigorously or explosively on contact with water.
— First Actions —
• CALL 911. Then call emergency response telephone number on shipping paper. If shipping paper
• Keep unauthorized personnel away.
• Stay upwind, uphill and/or upstream.
• Ventilate closed spaces before entering, but only if properly trained and equipped.
SOURCE: allfirefighter.com/hazmat/un/1242 | Always use current ERG + SOP/SOGIC BRIEFING
Use for: Incident command briefing, staging area whiteboard, or pre-entry team brief. Full chemical + response details.
UN1242 Methyldichlorosilane Cls4 ERG139 | ERG 139: Isolate spill 50m all directions. If tank/rail car involved in fire, is | allfirefighter.com/hazmat/un/1242SMS / 160 CHAR
Use for: Quick text to command or incoming units. Fits in a single SMS. Includes link to full page.
⚠️ Quick-reference only. Always use current ERG + SOP/SOG for incident-specific actions.
Page: https://allfirefighter.com/hazmat/un/1242
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Frequently Asked Questions about UN 1242
Produce flammable and toxic gases on contact with water. May ignite on contact with water or moist air. Some react vigorously or explosively on contact with water. May be ignited by heat, sparks or flames. May re-ignite after fire is extinguished. Some are transported in highly flammable liquids. Containers may explode when heated. Runoff may create fire or explosion hazard.
CALL 911. Then call emergency response telephone number on shipping paper. If shipping paper Keep unauthorized personnel away. Stay upwind, uphill and/or upstream.
Level A or B minimum with SCBA required. Full chemical-resistant suit for spill response. Respiratory protection mandatory due to HCl vapor generation.
Water reactivity: Reacts violently with water, producing HCl gas and heat. May ignite spontaneously. Do not use water directly on material.. Recommended extinguishing: Dry chemical, CO2, dry sand, soda ash. Do NOT use water or foam directly on liquid..
ERG Guide 139 recommendation: ERG 139: Isolate spill 50m all directions. If tank/rail car involved in fire, isolate 800m all directions and consider evacuation.
No. This is a training/quick-reference aid only. Always consult the current ERG Guide 139 and your department SOP/SOG for incident-specific protective actions.
Sources (high level): DOT/PHMSA marking & class concepts + ERG usage principles.
This page does not reproduce ERG guide text—always consult the current ERG for incident-specific protective actions.