HCN toxicity is caused by covalent bonding to heme iron.
A hydrogen cyanide concentration of 300 mg/m³ in air will kill a human within 10–60 min.
The toxicity is caused by the cyanide ion, which binds irreversibly to the iron atoms in the heme subunit of cytochrome c oxidase. This is an electron transfer enzyme found in the energy-producing mitochondria of a cell.
The enzyme is essential to life because it catalyzes the final stage of glucose oxidation. When it is blocked, the source of energy within the body quickly dries up. This immediately affects the heart and the central nervous system.
Within minutes the victim becomes unconscious but continues breathing. Slowly the heart gives out, and the victim dies.