UN 3520 — Chlorine, adsorbed
Placard: Toxic Gas. ERG Guide 173. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
UN 3520 is Chlorine, adsorbed, a toxic oxidizing corrosive adsorbed chlorine entry assigned to ERG Guide 173. Low-area vapor and acid formation are key hazards.
Hazard overview: TOXIC, CORROSIVE and OXIDIZING chlorine gas adsorbed on a porous solid; inhalation may be fatal. Released chlorine is heavier than air and may spread into low or confined areas. Reacts with moisture to form hydrochloric and hypochlorous acids, causing corrosive injury.
Response guidance: For UN 3520, isolate downwind/low areas, treat as a life-threatening gas release and use Level A/SCBA for entry. Verify exact gas with shipping papers and ERG 173.
Firefighter training notes: Training for UN 3520 should emphasize toxic-gas recognition, Zone A isolation, Level A/SCBA entry, air monitoring, source-control limits and decontamination. Use ERG 173, SDS and local SOP.
Regulatory context: Chlorine, adsorbed is regulated as a hazardous material for transport and emergency response. Storage, reporting, exposure, waste and incident-notification duties depend on quantity, package type, gas identity and jurisdiction; verify shipping papers, SDS and authority guidance.
Storage & handling: Chlorine, adsorbed should be stored in secured compatible adsorbed-gas packages away from heat, impact, corrosion, moisture where incompatible and unauthorized access, with ventilation and emergency planning.
UN 3520 Quick Details
Common Hazards of UN 3520
- TOXIC, CORROSIVE and OXIDIZING chlorine gas adsorbed on a porous solid; inhalation may be fatal.
- Released chlorine is heavier than air and may spread into low or confined areas.
- Reacts with moisture to form hydrochloric and hypochlorous acids, causing corrosive injury.
- Chlorine can intensify fire involving combustibles and react with fuels or reducing agents.
- Gas contact can burn eyes, skin and respiratory tissue.
- Containers exposed to fire may release toxic/corrosive gas.
- Runoff or vapor-control water may be acidic and chlorine-contaminated.
Chemical Identity & Physical Properties
Greenish-yellow gas with a pungent, irritating odor. When adsorbed, it is stabilized on a porous solid substrate in cylinders or containers.
| Also known as | Molecular chlorineBertholiteChlorine gas adsorbedCl2 |
| CAS Number | 7782-50-5 |
| Appearance | Greenish-yellow gas with a pungent, irritating odor. When adsorbed, it is stabilized on a porous solid substrate in cylinders or containers. |
| Flash Point | Not applicable (nonflammable gas, strong oxidizer) |
| Boiling Point | -34C (-29F) |
| Vapor Density | 2.5 (heavier than air) |
| Water Reactivity | Reacts with water forming hydrochloric and hypochlorous acids; corrosive mixture |
Fireground Response Guidance — UN 3520
Extinguishing Media
PPE Requirements
Use Level A fully encapsulating chemical protective clothing with positive-pressure SCBA for entry. Zone A toxic gas requires maximum respiratory and skin protection.
Isolation & Evacuation
First Actions for a UN 3520 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 avoid low or poorly ventilated areas unless monitoring shows they are safe.
- Treat the release as a life-threatening inhalation hazard until the gas is identified and monitored.
- Keep fuels, oils, greases, combustibles and reducing agents away from oxidizing gas.
- Do not handle leaking cylinders, adsorbed-gas packages or damaged gas articles without proper training and PPE.
- Ventilate only after monitoring and only if properly trained and equipped.
- Use ERG Guide 173, shipping papers, markings and air monitoring to set isolation, evacuation and entry decisions.
📋 Copy & Share Field Card
UN 3520 — Chlorine, adsorbedUse 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.