Hydrogen cyanide

(Hydrocyanic acid; prussic acid)

CAS 74-90-8





Physical Properties

Colorless or pale blue liquid or gas

bp 26 °C, mp -13 °C

Miscible in water in all proportions



Bitter almond odor detectable at 1 to 5 ppm; however, 20 to 60% of the population are reported to be unable to detect the odor of HCN

Vapor Pressure

750 mmHg at 25 °C

Flash Point

-18 °C


Autoignition Temperature

538 °C


Toxicity Data

Approx LD oral (rat)

10 mg/kg


Approx LD skin

-1500 mg/kg





LC50 inhal (rat)

63 ppm (40 min)



10 ppm (11 mg/m3)—skin



Ceiling 10 ppm (11 mg/m3)—skin

Major Hazards

High acute toxicity; inhalation, ingestion, or skin contact may be rapidly fatal.


The acute toxicity of hydrogen cyanide is high, and exposure by inhalation, ingestion, or eye or skin contact can be rapidly fatal. Symptoms observed at low levels of exposure (e.g., inhalation of 18 to 36 ppm for several hours) include weakness, headache, confusion, nausea, and vomiting. Inhalation of 270 ppm can cause immediate death, and 100 to 200 ppm can be fatal in 30 to 60 min. Aqueous solutions of HCN are readily absorbed through the skin and eyes, and absorption of 50 mg can be fatal. In humans, ingestion of 50 to 100 mg of HCN can be fatal. Because there is wide variation in the ability of different individuals to detect the odor of HCN, this substance is regarded as having poor warning properties.

Effects of chronic exposure to hydrogen cyanide are nonspecific and rare.

Flammability and Explosibility

Hydrogen cyanide is a highly flammable liquid. Liquid HCN contains a stabilizer (usually phosphoric acid), and old samples may explode if the acid stabilizer is not maintained at a sufficient concentration.

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement