An Ethylene oxide (EO or EtO) gas is commonly used to sterilize objects sensitive to temperatures greater than 60 °C such as plastics, optics and electrics. Ethylene oxide treatment is generally carried out between 30 °C and 60 °C with relative humidity above 30% and a gas concentration between 200 and 800 mg/L for at least three hours. Ethylene oxide penetrates well, moving through paper, cloth, and some plastic films and is highly effective. Ethylene oxide sterilizers are used to process sensitive instruments which cannot be adequately sterilized by other methods. EtO can kill all known viruses, bacteria and fungi, including bacterial spores and is satisfactory for most medical materials, even with repeated use. However, it is highly flammable, and requires a longer time to sterilize than any heat treatment. The process also requires a period of post-sterilization aeration to remove toxic residues. Ethylene oxide is the most common sterilization method, used for over 70% of total sterilizations, and for 50% of all disposable medical devices. 
The two most important ethylene oxide sterilization methods are: (1) the gas chamber method and (2) the micro-dose method. To benefit from economies of scale, EtO has traditionally been delivered by flooding a large chamber with a combination of EtO and other gases used as dilutants (usually CFCs or carbon dioxide). This method has drawbacks inherent to the use of large amounts of sterilant being released into a large space, including air contamination produced by CFCs and/or large amounts of EtO residuals, flammability and storage issues calling for special handling and storage, operator exposure risk and training costs
Ethylene oxide is still widely used by medical device manufacturers for larger scale sterilization (e.g. by the pallet), but while still used, EtO is becoming less popular in hospitals. Since Eto is explosive from its lower explosive limit of 3% all the way to 100%, EtO was traditionally supplied with an inert carrier gas such as a CFC or halogenated hydrocarbon. The use of CFCs as the carrier gas was banned because of concerns of ozone depletion and halogenated hydrocarbons are being replaced by so-called 100% EtO systems because of the much greater cost of the blends. In hospitals, most EtO sterilizers use single use cartridges (e.g. 3M's Steri-Vac line, or Steris Corporation's Stericert sterilizers) because of the convenience and ease of use compared to the former plumbed gas cylinders of EtO blends. Another 100% method is the so-called micro-dose sterilization method, developed in the late 1950s, using a specially designed bag to eliminate the need to flood a larger chamber with EtO. This method is also known as gas diffusion sterilization, or bag sterilization. This method minimizes the use of gas.
Another reason for the decrease in use of EtO are the well known health effects. In addition to being a primary irritant, EtO is now classified by the IARC as a known human carcinogen. The US OSHA has set the permissible exposure limit (PEL) at 1 ppm calculated as an eight hour time weighted average (TWA) [29 CFR 1910.1047] and 5 ppm as a 15 minute TWA. The NIOSH Immediately dangerous to life and health limit for EtO is 800 ppm. The odor threshold is around 500 ppm and so EtO is imperceptible until concentrations well above the OSHA PEL. Therefore, OSHA recommends that some kind of continuous gas monitoring system be used to protect workers using EtO for sterilization. While the hazards of EtO are generally well known, it should be noted that all chemical sterilants are designed to kill a broad spectrum of organisms, by exposing them to high concentrations of reactive chemicals. Therefore, it is no surprise that all the common chemical gas sterilants are toxic and adequate protective measures must be taken to protect workers using these materials.
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