UN 3521 — Silicon tetrafluoride, adsorbed
Placard: Toxic Gas. ERG Guide 173. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
UN 3521 is Silicon tetrafluoride, adsorbed, a toxic corrosive adsorbed gas assigned to ERG Guide 173. Moisture can form hydrofluoric acid and silicic acid fumes.
Hazard overview: TOXIC and CORROSIVE silicon tetrafluoride gas adsorbed on a porous solid; inhalation may be fatal. Reacts vigorously with moisture to form hydrofluoric acid and silicic acid fumes. HF exposure can cause severe respiratory injury, deep tissue burns and systemic toxicity.
Response guidance: For UN 3521, 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 3521 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: Silicon tetrafluoride, 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: Silicon tetrafluoride, 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 3521 Quick Details
Common Hazards of UN 3521
- TOXIC and CORROSIVE silicon tetrafluoride gas adsorbed on a porous solid; inhalation may be fatal.
- Reacts vigorously with moisture to form hydrofluoric acid and silicic acid fumes.
- HF exposure can cause severe respiratory injury, deep tissue burns and systemic toxicity.
- Released gas is heavier than air and may collect in low/confined spaces.
- Non-flammable gas, but heat or fire can release toxic/corrosive products.
- Runoff or vapor-control water may be fluoride-contaminated and corrosive.
- Fuming in moist air may indicate dangerous hydrolysis products.
Chemical Identity & Physical Properties
Colorless gas with a pungent, suffocating odor. Fumes strongly in moist air forming a white cloud of silicic acid and hydrofluoric acid vapors.
| Also known as | Silicon fluorideTetrafluorosilaneSiF4Silicon(IV) fluorideSilane, tetrafluoro- |
| CAS Number | 7783-61-1 |
| Appearance | Colorless gas with a pungent, suffocating odor. Fumes strongly in moist air forming a white cloud of silicic acid and hydrofluoric acid vapors. |
| Flash Point | Not applicable (non-flammable gas) |
| Boiling Point | -86C (-123F) |
| Vapor Density | 3.6 (heavier than air) |
| Water Reactivity | Reacts vigorously with water forming hydrofluoric acid and silicic acid; highly corrosive reaction |
Fireground Response Guidance — UN 3521
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 3521 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.
- 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 3521 — Silicon tetrafluoride, 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.