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General product and process information from public regulator and standards sources. It is not medical advice and not instructions for operating a sterilizer.
Short answer
Ethylene oxide (EtO) sterilization is a low-temperature gas process in which a load is conditioned and humidified, exposed to EtO, evacuated and air-washed, and then aerated so the gas dissipates from the product. CDC describes a basic healthcare cycle as five stages taking about 2.5 hours excluding aeration; FDA recognizes ISO 11135 for developing, validating and controlling the process and ISO 10993-7 for the residual EtO and ethylene chlorohydrin left on a device.
Definition and scope
EtO sterilization is used for medical devices and supplies that cannot tolerate heat or moisture. CDC’s healthcare guideline says EtO “should be considered a known human carcinogen”, and notes chronic inhalation has been linked to cataracts. This page covers how the cycle is organized and why residuals matter. Occupational exposure limits are covered separately in ethylene oxide exposure, and the method family in sterilization technology.
How an EtO cycle runs: the five stages
The CDC guideline (page dated November 28, 2023) describes the basic cycle as “five stages (i.e., preconditioning and humidification, gas introduction, exposure, evacuation, and air washes)”. The cycle takes about 2.5 hours excluding aeration. Four parameters govern it:
- Gas concentration: 450 to 1200 mg/l
- Temperature: 37 to 63°C
- Relative humidity: 40 to 80%
- Exposure time: 1 to 6 hours
Humidity matters because, per CDC, water molecules carry EtO to reactive sites. CDC also states that in healthcare EtO sterilizers the gas concentration and humidity “cannot be monitored”; mechanical monitors cover time, temperature and pressure, which is why biological indicators carry weight. CDC names B. atrophaeus spores as the recommended biological indicator.
Aeration and ethylene oxide residuals
Aeration
Aeration is the stage after the cycle in which residual EtO leaves the load. CDC gives two reference conditions: mechanical aeration for 8 to 12 hours at 50 to 60°C, and ambient room aeration for 7 days at 20°C. The same load can therefore need very different waiting times depending on the aeration method. Industrial processors describe aeration time as dependent on product material composition (STERIS AST technical note, a vendor source).
Residuals
FDA’s sterilization page (current as of 05/14/2025) recognizes two voluntary consensus standards, ANSI AAMI ISO 11135:2014 and ANSI AAMI ISO 10993-7:2008(R)2012, that “describe how to develop, validate, and control ethylene oxide sterilization processes for medical devices”. The second standard sets the acceptable levels of residual ethylene oxide and ethylene chlorohydrin left on a device. CDC adds the clinical reason: “Injuries (e.g., tissue burns) to patients have been associated with ETO residues in implants used in surgical procedures.”
The chain is: cycle validated to ISO 11135, aeration long enough for the product, residuals checked against ISO 10993-7 limits. Related reading on how a sterility claim is expressed: sterility assurance level and sterilization validation.
Healthcare and industrial sequences compared
| Setting | Stages as the source names them | Source |
|---|---|---|
| Healthcare-facility sterilizer | Preconditioning and humidification; gas introduction; exposure; evacuation; air washes (aeration follows separately) | CDC guideline |
| Contract (industrial) sterilizer | Environmental preconditioning; initial evacuation; humidification; gas injections and dwell; post-exposure purge and air inbleed; heated aeration | STERIS AST technical note (vendor) |
The two sequences are not identical, so a clinic’s sterilizer cycle is not a description of how packaged supplies were sterilized before they arrived.
Where it matters for labs, clinics and supply decisions
- Documentation: an EtO-labelled sterile product’s records are tied to the process standard (ISO 11135) and the residuals standard (ISO 10993-7) that FDA recognizes.
- Packaging: CDC lists foil and PVC as unacceptable packaging for EtO, and treats aeration (allowing EtO to dissipate) as part of wrap quality.
- Monitoring: because gas concentration and humidity cannot be monitored in healthcare units, indicators and records matter more than the gauges.
- Regulatory timeline: FDA’s page lists the November 25, 2019 EtO Sterilization Master File Pilot Program for PMA holders and a November 26, 2024 guidance on transitional enforcement for EtO sterilization facility changes for Class III devices.
For choosing between EtO and other low-temperature methods, see low-temperature sterilization method selection.
Common misunderstandings
- “Aeration is optional once the cycle ends.” CDC links patient tissue burns to EtO residues, and gives aeration conditions for removing them.
- “Aeration time is a fixed number.” CDC’s own figures range from 8 to 12 hours mechanical to 7 days ambient.
- “A hospital EtO cycle and an industrial one are the same.” The stage sequences differ, as the table shows.
- “A cycle can be verified by reading gas concentration.” In healthcare sterilizers CDC states it cannot be monitored.
Frequently asked questions
What are the stages of ethylene oxide sterilization?
CDC lists five: preconditioning and humidification, gas introduction, exposure, evacuation and air washes, with aeration afterward.
How long does ethylene oxide aeration take?
CDC gives 8 to 12 hours at 50 to 60°C with mechanical aeration, or 7 days at 20°C in ambient room aeration.
What are ethylene oxide residuals and what limits them?
They are residual ethylene oxide and ethylene chlorohydrin left on a device. FDA recognizes ISO 10993-7 as setting the acceptable levels.
Which standard covers validating an EtO process?
FDA recognizes ANSI AAMI ISO 11135:2014 for developing, validating and controlling EtO sterilization processes.
Sources
- CDC, Ethylene Oxide “Gas” Sterilization, Guideline for Disinfection and Sterilization in Healthcare Facilities, page dated November 28, 2023; checked 2026-10-30.
- CDC, Sterilizing Practices, page dated November 28, 2023; checked 2026-10-30.
- FDA, Sterilization for Medical Devices, current as of 05/14/2025; checked 2026-10-30.
- STERIS AST, Anatomy of the ethylene oxide sterilization process, vendor technical note; checked 2026-10-30.
