☣️ UN 2421 • CLASS 2
Nitrogen trioxide
Placard: Toxic Gas. 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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Quick details
UN 2421
Class: 2
Placard type: Toxic Gas
ERG: Guide 124 (check current ERG)
Isolation: ERG 124: isolate 100m in all directions; evacuate 800m downwind if tank/railcar involved in fire
Chemical & Response Details
| Also known as | Dinitrogen trioxideN2O3Nitrous anhydrideNitrogen sesquioxide |
| CAS Number | 10544-73-7 |
| Appearance | Blue liquid or reddish-brown gas at room temperature with a pungent, acrid odor. Exists in equilibrium with nitrogen dioxide and nitric oxide. |
| Flash Point | Not applicable (oxidizing gas/liquid) |
| Boiling Point | 3.5°C (38°F) |
| Vapor Density | 2.6 (heavier than air) |
| Water Reactivity | Reacts with water forming nitrous and nitric acids; corrosive reaction |
| Extinguishing | Do not use water or foam; dry chemical or CO2 for small fires only |
| PPE | ⚠️ Level A required; SCBA with full face protection; chemical-resistant suit due to corrosive and toxic nature |
| Isolation | ERG 124: isolate 100m in all directions; evacuate 800m downwind if tank/railcar involved in fire |
Chemical details are general reference only. Always verify with current SDS, ERG, and SOP/SOG.
Common hazards (high level)
- TOXIC and/or CORROSIVE; may be fatal if inhaled or absorbed through skin.
- Fire will produce irritating, corrosive and/or toxic gases.
- Contact with gas or liquefied gas may cause burns, severe injury and/or frostbite.
- Runoff from fire control or dilution water may cause environmental contamination.
- Substance does not burn but will support combustion.
- Vapors from liquefied gas are initially heavier than air and spread along ground.
- These are strong oxidizers and will react vigorously or explosively with many materials including fuels.
- May ignite combustibles (wood, paper, oil, clothing, etc.).
First actions (field-minded)
- CALL 911. Then call emergency response telephone number on shipping paper. If shipping paper
- Keep unauthorized personnel away.
- Stay upwind, uphill and/or upstream.
- Many gases are heavier than air and will spread along the ground and collect in low or confined areas
- Ventilate closed spaces before entering, but only if properly trained and equipped.
- Isolate spill or leak area for at least 100 meters (330 feet) in all directions.
- See Table 1 - Initial Isolation and Protective Action Distances.
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UN 2421 — Nitrogen trioxide HAZMAT RADIO NOTE — UN 2421
Product: Nitrogen trioxide
Class 2 / Toxic Gas / ERG 124
PPE: Level A required; SCBA with full face protection; chemical-resistant suit due to corrosive and toxic nature
ISOLATION: ERG 124: isolate 100m in all directions; evacuate 800m downwind if tank/railcar involved in fire
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 2421 — Nitrogen trioxide
Class: 2 | Placard: Toxic Gas | ERG Guide: 124
Appearance: Blue liquid or reddish-brown gas at room temperature with a pungent, acrid odor. Exists in equilibrium with nitrogen dioxide and nitric oxide.
Water Reactivity: Reacts with water forming nitrous and nitric acids; corrosive reaction
Extinguishing: Do not use water or foam; dry chemical or CO2 for small fires only
PPE: Level A required; SCBA with full face protection; chemical-resistant suit due to corrosive and toxic nature
Isolation: ERG 124: isolate 100m in all directions; evacuate 800m downwind if tank/railcar involved in fire
— Key Hazards —
• TOXIC and/or CORROSIVE; may be fatal if inhaled or absorbed through skin.
• Fire will produce irritating, corrosive and/or toxic gases.
• Contact with gas or liquefied gas may cause burns, severe injury and/or frostbite.
— 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.
• Many gases are heavier than air and will spread along the ground and collect in low or confined areas
SOURCE: allfirefighter.com/hazmat/un/2421 | Always use current ERG + SOP/SOGIC BRIEFING
Use for: Incident command briefing, staging area whiteboard, or pre-entry team brief. Full chemical + response details.
UN2421 Nitrogen trioxide Cls2 ERG124 | ERG 124: isolate 100m in all directions; evacuate 800m downwind if tank/railcar | allfirefighter.com/hazmat/un/2421SMS / 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/2421
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FAQ
TOXIC and/or CORROSIVE; may be fatal if inhaled or absorbed through skin. Fire will produce irritating, corrosive and/or toxic gases. Contact with gas or liquefied gas may cause burns, severe injury and/or frostbite. Runoff from fire control or dilution water may cause environmental contamination. Substance does not burn but will support combustion. Vapors from liquefied gas are initially heavier than air and spread along ground. These are strong oxidizers and will react vigorously or explosively with many materials including fuels. May ignite combustibles (wood, paper, oil, clothing, etc.).
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 required; SCBA with full face protection; chemical-resistant suit due to corrosive and toxic nature
No. This is a training/quick-reference aid only. Always consult the current ERG Guide 124 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.