Medical tests show that a concentration of hydrogen cyanide gas in an amount of 300 ppm in air is rapidly fatal. Generally, for execution purposes a concentration of 3200 ppm is used to ensure rapid death. This is a weight/volume of some 120 to 150 grams/ 2 cubic feet of gas, depending on temperature and pressure. Some 100 ppm of HCN is fatal within half an hour. Toxic effects are skin irritation and rashes, eye irritation, blurring of vision and permanent eye damage; non-specific nausea; headache; dizziness; vomiting and weakness; rapid respiration, lowered blood pressure, unconsciousness, convulsions and death; symptoms of asphyxia, dyspnea, ataxia, tremors, coma and death through a disruption of the oxidative metabolism.

Hydrocyanic acid does not have to be breathed to be fatal. In concentrations of over 50 ppm, the user must wear a chemical suit to completely protect his body and breathe bottled air. Gas masks are generally ineffective and should never be utilized. Specialized first aid kits and medical supplies are available and should be present in all areas where a person may contact the gas.