(Ethyne; welding gas)

CAS 74-86-2





Physical Properties

Colorless gas

bp -84 °C (sublimes), mp - 82 °C

Slightly soluble in water (0.106 g/100 mL)



Odorless, although garlic-like or ''gassy" odor often detectable because of trace impurities

Vapor Density

0.91 (air = 1.0)


Vapor Pressure

3.04 X 104 mmHg (~40 atmospheres) at 16.8 °C


Flash Point

-18 °C


Autoignition Temperature

305 °C


Toxicity Data

LC50 inhal (rat)

simple asphyxiant (>500,000 ppm)



simple asphyxiant

Major Hazards

Extremely flammable gas; simple asphyxiant.


Acetylene is relatively nontoxic and has been used as an anesthetic. Inhalation of acetylene can be hazardous because of its action as a simple asphyxiant. Concentrations of about 10% in air cause slight intoxication, and levels of 20% in air may produce headaches and labored breathing. At higher concentrations (33% and above), acetylene acts as a narcotic, causing unconsciousness in 7 min or less, with rapid and full recovery normally seen on removal from exposure of less than several hours. Concentrations of acetylene above 50% in air can cause death by asphyxiation within 5 min. Commercially available acetylene may contain highly toxic impurities, including phosphine, arsine, and hydrogen sulfide; the presence of these impurities must be considered in setting acceptable exposure levels to acetylene. For example, the concentration of acetylene containing 95 ppm of phosphine impurity (which has a TLV of 0.3 ppm) should not exceed 3160 ppm to stay within the TLV for phosphine.

There is no evidence that acetylene is a human carcinogen or reproductive toxin.

Flammability and Explosibility

Acetylene is a highly flammable gas and forms explosive mixtures with air over an unusually wide range of concentrations (2 to 80%). Acetylene can polymerize exothermically, leading to deflagration. With a very high positive free energy of formation, acetylene

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement