Ethylene oxide (EO) is the most commonly used agent in chemical sterilization. Because it is a gas, it rapidly penetrates packaging and items to be sterilized at temperatures tolerated by almost all materials. However, its use is limited by the size of the equipment, the time requirement, and concerns about toxicity. It is recommended for use only for items unsuitable for steam sterilization,including laparoscopes, light cables, and camera heads.In fact, because of environmental concerns, ethylene oxide sterilizers are now required by law to be retrofitted with abaters that reduce more of the exhausts to water vapor. Despite the use of abaters, the Environmental Protection Agency has outlawed the use of EO sterilizers altogether in some areas. Gas plasma sterilizers are a logical replacement choice.

Ethylene oxide is an alkylating agent that kills microorganisms by inactivation of proteins, DNA, and RNA, and it is effective against vegetative bacteria, fungi, viruses, and spores.It is supplied as a gas mixed with a carrier agent (Freon or CO2) to reduce flammability.Mixed with air or oxygen, EO is explosive and flammable.Carbon dioxide is the preferred diluent because of environmental concerns about fluorinated hydrocarbon (Freon) release, although a tendency to stratify from carbon dioxide in storage containers could affect sterilization.

Sterilization by ethylene oxide is influenced by gas concentration, temperature, humidity, and exposure time. The more sophisticated equipment for EO sterilization includes methods for temperature elevation to shorten sterilization times.Spores require time for humidification to allow optimal killing by ethylene oxide.The humidity should not be raised by wetting the materials to be sterilized, because ethylene oxide forms condensation products with water that may damage rubber and plastic surfaces. Also, the effectiveness of ethylene oxide sterilization may be reduced below the lethal point by moisture left in needles and tubing.Instruments need to be cleaned as described for steam sterilization.