Ethylene oxide (EtO) is a flammable, colorless, and reactive gas. It is a synthetic chemical created as a byproduct of human (i.e., anthropogenic) activities or generated as a natural metabolic product of ethylene by microbes, plants, and animals, including humans (i.e., biogenic).  

Ethylene Oxide Emissions (EtO-1)
Section 2 (Introduction)
Physical and Chemical Properties of EtO, Production and Use, Emission and Fate, and Health Effects of EtO.