UN 2421 — Nitrogen trioxide
Placard: Toxic Gas. ERG Guide 124. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
UN 2421 is Nitrogen trioxide, a toxic oxidizing gas assigned to ERG Guide 124. It can intensify fire and form corrosive acidic products with moisture.
Hazard overview: TOXIC and OXIDIZING gas; inhalation may cause severe or fatal respiratory injury. Can intensify fire and react with combustible or reducing materials. Gas may form corrosive acidic products with moisture.
Response guidance: For UN 2421, isolate the release, stay upwind and use SCBA with chemical protection for entry. Monitor air, protect against frostbite/corrosive gas and control runoff from vapor suppression under ERG 124.
Firefighter training notes: Training for UN 2421 should emphasize toxic gas isolation, air monitoring, Level A/SCBA entry, corrosive or oxidizing behavior, frostbite, vapor control and runoff management. Use ERG 124, SDS and local SOP.
Regulatory context: Nitrogen trioxide is regulated as a hazardous material for transport and emergency response. Storage, exposure, spill reporting, waste and fire-code duties depend on quantity, concentration and jurisdiction; verify shipping papers, SDS and local authority requirements.
Storage & handling: Nitrogen trioxide cylinders should be secured in a cool, ventilated gas-storage area away from heat, physical damage and incompatible materials. Provide leak detection, ventilation and emergency planning according to SDS and local code.
UN 2421 Quick Details
Common Hazards of UN 2421
- TOXIC and OXIDIZING gas; inhalation may cause severe or fatal respiratory injury.
- Can intensify fire and react with combustible or reducing materials.
- Gas may form corrosive acidic products with moisture.
- Liquefied gas contact can cause frostbite and chemical burns.
- Vapors may collect in low or poorly ventilated areas depending on release conditions.
- Containers exposed to fire may vent, rupture or rocket.
- Runoff or water used for vapor control may become acidic and toxic.
Chemical Identity & Physical Properties
Blue liquid or reddish-brown gas at room temperature with a pungent, acrid odor. Exists in equilibrium with nitrogen dioxide and nitric oxide.
| 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 |
Fireground Response Guidance — UN 2421
Extinguishing Media
PPE Requirements
Use positive-pressure SCBA and fully encapsulating chemical protective clothing for close entry or unknown concentrations. Protect against corrosive gas and liquefied-gas frostbite.
Isolation & Evacuation
First Actions for a UN 2421 Incident
- Call 911 and the emergency response number on the shipping paper, if available.
- Keep unauthorized personnel away and establish incident command.
- Stay upwind and keep people out of low or poorly ventilated areas.
- Treat the release as an inhalation hazard until monitoring proves otherwise.
- Keep combustibles, reducing agents and incompatible materials away from the release.
- Do not touch leaking cylinders or liquefied gas without proper training and PPE.
- Ventilate confined spaces only after monitoring and only if properly trained and equipped.
- Use ERG Guide 124, SDS, cylinder markings and air monitoring to set isolation, evacuation and entry decisions.
📋 Copy & Share Field Card
UN 2421 — Nitrogen trioxideUse 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.