Hydrogen Cyanide (HCN)

CAS Number: 74-90-8
Hydrogen cyanide (HCN) is an extremely toxic colorless gas or liquid with a faint almond-like odor, though a significant portion of the population cannot detect it by smell. It inhibits cytochrome c oxidase, effectively shutting down cellular respiration and causing rapid asphyxiation at high doses. HCN is generated during combustion of nitrogen-containing materials, in metal plating operations, chemical synthesis, fumigation, and certain mining processes (gold and silver extraction via cyanidation). Acute exposure at even moderate concentrations can be rapidly fatal, making real-time personal monitoring and emergency response planning critical in any workplace where HCN may be present.

Hydrogen cyanide (HCN) is an extremely toxic gas that can cause rapid, life-threatening effects in industrial settings and during fire and incident response. Rent HCN detection equipment from RAECO Rents to support confined space screening, area monitoring, and emergency response where cyanide hazards may be present. Our rental instruments are well maintained and backed by responsive support to help teams monitor safely in the field.

Regulatory Exposure Limits

Updated on March 09, 2026

OSHA PEL
TWA: 10 ppm (11 mg/m³) [skin]
STEL: N/A
C: N/A
NIOSH REL
TWA: N/A
STEL: 4.7 ppm (5 mg/m³) [skin]
C: N/A
ACGIH TLV
TWA: N/A
STEL: N/A
C: 4.7 ppm (5 mg/m³)
Frequently Asked Questions
Common questions about
Can I rely on smell to warn me about hydrogen cyanide (HCN)?
Assuming odor will warn you. Many people can't detect HCN reliably, and dangerous levels can occur without clear sensory cues—use an alarming monitor and treat any alarm as urgent.
Should I use a dedicated HCN monitor or a multi-gas monitor with HCN capability?
Use a dedicated HCN monitor when cyanide exposure is the primary hazard (fumigation, electroplating, certain chemical manufacturing), and you need the best sensitivity and fastest response to HCN specifically. Use a multi-gas monitor with an HCN sensor when you also need to simultaneously track other gases (O₂, CO, LEL). For confined space entry where HCN may be present alongside other hazards, the multi-gas approach is more practical. For applications with HCN as the sole concern, a dedicated instrument typically offers better detection limits.
When is continuous monitoring especially important for hydrogen cyanide (HCN)?
HCN is especially dangerous because its warning properties are unreliable—it can cause rapid incapacitation at concentrations that may not be immediately detectable by smell, especially in people who are desensitized. Continuous monitoring is critical in any enclosed space where HCN may be released (fumigation, fires, specific industrial processes), whenever workers are present in areas where HCN generation is possible, and during re-entry after fumigation or chemical incidents. Spot checks are not a substitute for continuous monitoring when HCN is a credible hazard.
What should I know before renting HCN monitoring equipment?
Know the exposure limit you're working to (OSHA PEL is 10 ppm ceiling), whether you need personal or area monitoring, and whether the job involves intermittent or continuous potential exposure. HCN sensors have a limited life and must be bump-tested before use.
What jobs require hydrogen cyanide (HCN) monitoring?
Electroplating and metal finishing, chemical manufacturing, fumigation operations, and fire or combustion environments involving nitrogen-containing materials. The practical trigger is any scenario where rapid toxicity from HCN is a credible risk.
Where should HCN monitors be worn or placed?
Wear personal monitors in the breathing zone. For area coverage, place monitors near likely release points and where people work — avoid relying on a single monitor near the entrance.
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