What is Ethylene Oxide?
Ethylene oxide (EtO) is colorless, flammable, and explosive gas. It was first used as a chemical sterilant in the healthcare field during the 1950’s for medical equipment that was sensitive to heat, moisture, and/or radiation.
How Common is the Use of Ethylene Oxide to Sterilize Medical Devices?
Currently, approximately 50% of all medical devices sterilized in the U.S. rely on ethylene oxide.
How are Medical Devices Sterilized with Ethylene Oxide?
There are 5 stages to ethylene oxide sterilization:
- Preconditioning/humidification
- Gas introduction
- Exposure
- Gas evacuation
- Air washes
- Aeration
Mixture of Ethylene Oxide with Other Gasses
Ethylene oxide was used in a mixture with chlorofluorocarbons (CFCs) until the mid-1990’s when they were banned due to their contribution to depleting stratospheric ozone. Nowadays there are primarily 2 mixtures to replace CFCs: EtO-carbon dioxide (less expensive, but higher pressure is required) and EtO-hydrochlorofluorocarbons (found to be 50 times less damaging to the ozone layer than CFCs). Exact guidance to the sterilization process can be found with ANSI/AAMI/ISO 11135 and ANSI/AAMI/ISO 10993-7, along with the EPA who enforce ethylene oxide emission regulations.
How Does Ethylene Oxide Kill Microorganisms?
The microbicidal activity of ethylene oxide is attributed to its alkylation of proteins, DNA (typically 7-guanine), and RNA. Alkylation is the removal of a hydrogen from an alkane molecule (e.g., methyl or hydroxyl group). In the case of ethylene oxide, alkylation causes point mutations in DNA (can result in chromosomal damage) and prevents normal cellular metabolism/replication. B. atrophaeus is a popular choice as a biological indicator for sterilization validation due to its resistance to ethylene oxide.
Is Ethylene Oxide Safe?
Ethylene oxide is only safe when used in a controlled, contained environment such as a sterilization chamber. The biological interactions ethylene oxide has with cells can cause a multitude of health problems in humans depending on the concentration and exposure time. Since March 10, 2010, the EPA requires hospitals and other healthcare facilities to use a single chamber process that combines sterilization and aeration in order to limit personnel exposure.
Ethylene Oxide Lawsuits
Several contract medical device sterilization companies have faced lawsuits related to occupational exposure to ethylene oxide gas, as well as from people living downwind of contract sterilization facilities that may have exhausted ethylene oxide into the environment without pre-treatment. Recently Sotera Heath, a large contract sterilizer, settled many of those lawsuits.
Emerging Alternatives to Ethylene Oxide
Medical devices are commonly sterilized using gamma irradiation and, to a lesser extent, e-beam irradiation. Alternative chemical vapors are also used, but rarely, including hydrogen peroxide gas plasma and chlorine dioxide.
Currently hydrogen peroxide gas plasma (HPGP) is being used as an alternative to ethylene oxide sterilization. HPGP is considered to be more environmentally safe and has a much shorter cycle time (~75 minutes). Its byproducts are non-toxic (water and oxygen) which eliminates the aeration step.
Chlorine dioxide is in the early stages of validation and development, but it is a well known germicide with a reliable, broad spectrum of activity and known activity via the vapor phase.
How is Medical Device Sterilization by Ethylene Oxide Tested?
Laboratories use methods such as USP <71> Sterility Test to determine the success of ethylene oxide sterilization. In addition to testing the sterility of finished medical devices, labs use process control devices and biological indicators to assess the effectiveness of ethylene oxide sterilization regimens.
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