☣️ UN 3491 • CLASS 6
UN 3491 — Poisonous by inhalation liquid, water-reactive, flammable, n.o.s. (Inhalation Hazard Zone B)
Placard: Toxic. ERG Guide 155. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
🚒☣️
⚠️ Verification required:
Broad or variable material category; verify exact product, SDS and shipping papers.
⚠️ 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.
UN 3491 covers poisonous-by-inhalation liquid, n.o.s. in Inhalation Hazard Zone B, with additional water-reactive flammable hazards.
Hazard overview: The primary risk is severe inhalation toxicity. The liquid may also be flammable and corrosive, and vapors can collect in low or confined areas. Water contact may release flammable, toxic, or corrosive gases.
Response guidance: Isolate immediately, keep water away from the material, eliminate ignition sources, stay upwind, and use dry media or other SDS-approved agents only.
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UN 3491 Quick Details
UN 3491
Product name: Poisonous by inhalation liquid, water-reactive, flammable, n.o.s. (Inhalation Hazard Zone B)
DOT Class: 6
Placard type: Toxic
ERG Guide: 155 (check current ERG)
Initial isolation: ERG 155: Initial isolation 60m (200 ft) all directions; Protective Action Distance daytime 0.8 km (0.5 mi), nighttime 2.4 km (1.5 mi) for small spills; Large spills: day 2.5 km (1.6 mi), night 6.4 km (4.0 mi)
Common Hazards of UN 3491
- HIGHLY FLAMMABLE: Will be easily ignited by heat, sparks or flames.
- Vapors form explosive mixtures with air: indoors, outdoors and sewers explosion hazards.
- Most vapors are heavier than air. They will spread along the ground and collect in low or confined areas
- Vapors may travel to source of ignition and flash back.
- Those substances designated with a (P) may polymerize explosively when heated or involved in a fire.
- Substance will react with water (some violently) releasing flammable, toxic or corrosive gases and runoff.
- Corrosives in contact with metals may evolve flammable hydrogen gas.
- Containers may explode when heated or if contaminated with water.
Chemical Identity & Physical Properties
UN 3491 is a variable liquid depending on the specific chemical, typically colorless to pale colored with a sharp, penetrating, or irritating odor.
| Also known as | PIH liquid water-reactive flammable n.o.s.Toxic inhalation hazard liquid Zone BWater-reactive poisonous flammable liquidTIH Zone B water-reactive flammable |
| Appearance | Variable liquid depending on specific chemical; typically colorless to pale colored with sharp, penetrating, or irritating odor. Liquid at room temperature, highly volatile with toxic vapors. |
| Flash Point | Below 23C (73F) - highly flammable liquid |
| Boiling Point | Variable depending on specific substance; typically below 100C (212F) |
| Vapor Density | Heavier than air (vapor density >1); vapors settle in low areas |
| Water Reactivity | Reacts with water producing toxic and flammable gases; may ignite spontaneously or release heat |
Chemical details are general reference only. Always verify with current SDS, ERG, and SOP/SOG.
Fireground Response Guidance — UN 3491
Extinguishing Media
Dry chemical, CO2, dry sand only; DO NOT use water, water spray, or foam due to water reactivity
PPE Requirements
⚠️ Level A required (fully encapsulated suit with SCBA); positive pressure SCBA mandatory; chemical-resistant gloves and boots; no skin contact permitted due to inhalation toxicity
Level A with fully encapsulating suit and SCBA is recommended because of the inhalation hazard.
Isolation & Evacuation
ERG 155: Initial isolation 60m (200 ft) all directions; Protective Action Distance daytime 0.8 km (0.5 mi), nighttime 2.4 km (1.5 mi) for small spills; Large spills: day 2.5 km (1.6 mi), night 6.4 km (4.0 mi)
Always confirm protective actions with the current edition of the Emergency Response Guidebook (ERG Guide 155).
First Actions for a UN 3491 Incident
- CALL 911. Then call the emergency response number on the shipping paper, if available.
- Keep unauthorized personnel away.
- Stay upwind, uphill and/or upstream.
- Ventilate closed spaces before entering, but only if properly trained and equipped.
- Isolate the spill or leak area in all directions using ERG guidance for the material and package size.
- For highlighted materials: see Table 1 - Initial Isolation and Protective Action Distances.
- For non-highlighted materials, increase the downwind precautionary distance as conditions require.
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📋 Copy & Share Field Card
UN 3491 — Poisonous by inhalation liquid, water-reHAZMAT RADIO NOTE — UN 3491
Product: Poisonous by inhalation liquid, water-reactive, flammable, n.o.s. (Inhalation Hazard Zone B)
Class 6 / Toxic / ERG 155
PPE: Level A required (fully encapsulated suit with SCBA); positive pressure SCBA mandatory; chemical-resistant gloves and boots; no skin contact permitted due to inhalation toxicity
ISOLATION: ERG 155: Initial isolation 60m (200 ft) all directions; Protective Action Distance daytime 0.8 km (0.5 mi), nighttime 2.4 km (1.5 mi) for small spills; Large spills: day 2.5 km (1.6 mi), night 6.4 km (4.0 mi)
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 3491 — Poisonous by inhalation liquid, water-reactive, flammable, n.o.s. (Inhalation Hazard Zone B)
Class: 6 | Placard: Toxic | ERG Guide: 155
Appearance: Variable liquid depending on specific chemical; typically colorless to pale colored with sharp, penetrating, or irritating odor. Liquid at room temperature, highly volatile with toxic vapors.
Water Reactivity: Reacts with water producing toxic and flammable gases; may ignite spontaneously or release heat
Extinguishing: Dry chemical, CO2, dry sand only; DO NOT use water, water spray, or foam due to water reactivity
PPE: Level A required (fully encapsulated suit with SCBA); positive pressure SCBA mandatory; chemical-resistant gloves and boots; no skin contact permitted due to inhalation toxicity
Isolation: ERG 155: Initial isolation 60m (200 ft) all directions; Protective Action Distance daytime 0.8 km (0.5 mi), nighttime 2.4 km (1.5 mi) for small spills; Large spills: day 2.5 km (1.6 mi), night 6.4 km (4.0 mi)
— Key Hazards —
• HIGHLY FLAMMABLE: Will be easily ignited by heat, sparks or flames.
• Vapors form explosive mixtures with air: indoors, outdoors and sewers explosion hazards.
• Most vapors are heavier than air. They will spread along the ground and collect in low or confined areas
— First Actions —
• CALL 911. Then call the emergency response number on the shipping paper, if available.
• 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/poisonous-by-inhalation-liquid-un-3491 | Always use current ERG + SOP/SOGIC BRIEFING
Use for: Incident command briefing, staging area whiteboard, or pre-entry team brief.
UN3491 Poisonous by inhalation liquid, water-reactive, flammable, n.o.s. (Inhalation Hazard Zone B) Cls6 ERG155 | allfirefighter.com/hazmat/un/poisonous-by-inhalation-liquid-un-3491SMS / 160 CHAR
Use for: Quick text to command or incoming units. Fits in a single SMS.
⚠️ Quick-reference only. Always use current ERG + SOP/SOG for incident-specific actions.
Page: https://allfirefighter.com/hazmat/un/poisonous-by-inhalation-liquid-un-3491
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Frequently Asked Questions about UN 3491
Yes. It has flammable liquid hazards in addition to inhalation toxicity.
Yes. Water contact may release flammable, toxic, or corrosive gases and may intensify the incident.
Level A with fully encapsulating suit and SCBA is recommended because of the inhalation hazard.
Treat as an extreme inhalation hazard, isolate the area, stay upwind, and evacuate downwind as directed by ERG and incident command.
UN 3491 is assigned to ERG Guide 155 for initial emergency response guidance.
Inhalation Hazard Zone B indicates a severe toxic inhalation risk; Zone A is more severe than Zone B.
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.