--- title: 'Ethylene Oxide (EtO) Sterilization: Process, Uses, and Safety' author: Tuttnauer canonical_url: 'https://tuttnauer.com/knowledge-center/sterile-processing/sterilization-methods/ethylene-oxide-eto-sterilization' --- # Ethylene Oxide (EtO) Sterilization: Process, Uses, and Safety **Ethylene oxide (EtO) sterilization** uses a colorless, flammable gas to alkylate microbial DNA and proteins at 25–55 °C. It is the dominant low-temperature method used by medical device manufacturers because it penetrates long lumens, complex assemblies, and sealed packaging that other low-temperature methods cannot reach. EtO is also carcinogenic, flammable, and requires aeration cycles measured in days—so its use is heavily regulated and increasingly under scrutiny in favor of alternatives. This article explains the alkylation mechanism, the three stages of an EtO cycle, aeration and residual limits, and the ISO 11135 validation framework. ## [](#content-how-eto-sterilization-works "Permalink")How EtO Sterilization Works EtO (CH₂CH₂O) is a small, reactive epoxide molecule. It kills microorganisms through **alkylation**: it transfers an alkyl group to nucleophilic sites on microbial DNA, RNA, proteins, and enzymes, disrupting replication and metabolism. The reaction is non-selective, which is why EtO is broadly effective against bacteria, viruses, fungi, and bacterial spores—and why it is also toxic to humans. Three conditions must be controlled simultaneously for effective sterilization: - **EtO concentration** — typically 450–1200 mg/L of chamber volume - **Temperature** — 25–55 °C (lower temperatures require longer exposure) - **Relative humidity** — 30–80 % (moisture is required; too dry, and EtO cannot penetrate microbial cells) Pressure (sub-atmospheric vacuum or above-atmospheric) varies with cycle design. ## [](#content-the-eto-sterilization-process "Permalink")The EtO Sterilization Process A complete EtO cycle has three stages and typically runs longer than 14 hours, with aeration extending the total turnaround to days for some loads. ### [](#content-1-preconditioning "Permalink")1. Preconditioning The load is brought to target temperature and humidity before EtO admission. The chamber is evacuated to remove air, then steam is injected to humidify the load, and the jacket is held at temperature (often 24/7 to minimize fluctuations). Some industrial processes precondition loads in a separate chamber for hours or days before transferring into the sterilizer. ### [](#content-2-sterilization-exposure "Permalink")2. Sterilization (Exposure) EtO is admitted by evaporation, mixed with steam to maintain humidity. EtO is absorbed by polymers (PVC, polyurethane, silicone), so concentration in the chamber must be maintained—often by pulsing additional EtO into the chamber during the hold. Exposure time depends on concentration, temperature, and load resistance, typically 1–6 hours. ### [](#content-3-aeration-degassing "Permalink")3. Aeration (Degassing) The longest and most critical stage. Residual EtO and its reaction products (ethylene chlorohydrin, ethylene glycol) must be removed before the load contacts patients or operators. Aeration is performed at 30–50 °C with HEPA-filtered air and can take **24–48 hours or more** depending on material and lumen geometry. Industrial facilities use dedicated aeration rooms; hospital-scale EtO sterilizers integrate aeration into the cycle. > **Important:** Inadequate aeration is the most common cause of EtO injury. Residual EtO on devices contacting patient tissue can cause chemical burns, hemolysis, and allergic reactions. Validate aeration to the residual limits in [ISO 10993-7](https://www.iso.org/standard/76617.html). ## [](#content-what-is-sterilized-with-eto "Permalink")What Is Sterilized with EtO EtO is the standard method for devices that cannot tolerate steam or hydrogen peroxide and for sealed, packaged products at industrial scale: - **Long-lumen flexible endoscopes** and accessories - **Implantable medical devices** (pacemakers, orthopedic implants, drug-eluting stents in some cases) - **Single-use disposables** in their final packaging — sutures, surgical gowns, drapes, gauze, IV sets, syringes, dressings - **Catheters, tubing**, and other long, narrow polymer assemblies - **Pharmaceutical packaging components** In the US, roughly half of all medical devices that require sterilization are sterilized with EtO. Industrial EtO chambers can be very large (up to ~7 m³). ## [](#content-eto-cycle-parameters "Permalink")EtO Cycle Parameters ParameterTypical rangeNotesTemperature25–55 °CLower temperature requires longer exposureEtO concentration450–1200 mg/LHigher concentration shortens cycleRelative humidity30–80 %Required for microbial cell penetrationPressureSub-atmospheric or super-atmosphericCycle design specificExposure time1–6 hoursPlus preconditioning + aerationTotal cycle time>14 hoursExcluding aerationAeration time24–48+ hoursMaterial-dependent## [](#content-aeration-requirements-and-residual-limits "Permalink")Aeration Requirements and Residual Limits EtO residuals on sterilized devices are regulated under **[ISO 10993-7](https://www.iso.org/standard/76617.html)**, which sets limits based on the device's intended contact and duration: Contact categoryEtO limit (per device, average daily dose)Limited exposure (≤24 h)4 mgProlonged exposure (24 h–30 days)60 mg over 30 days, max 4 mg/dayPermanent contact (>30 days)2.5 g over lifetime, max 4 mg/day initialEthylene chlorohydrin (ECH) and ethylene glycol residuals are also controlled. Aeration must be validated for each device family and packaging configuration—lumens, dense polymers, and absorbent materials retain EtO longer. ## [](#content-safety-considerations-and-regulatory-status "Permalink")Safety Considerations and Regulatory Status EtO is classified by the IARC as a **Group 1 human carcinogen** and is acutely toxic, flammable, and explosive at concentrations above ~3 % in air. This drives a heavy compliance burden: - **OSHA permissible exposure limit (PEL):** 1 ppm time-weighted average over 8 hours; 5 ppm short-term excursion limit (15 minutes). - **EPA NESHAP** and state-level emissions regulations restrict EtO emissions from sterilization facilities. Several US facilities have closed or reduced output following community air-quality concerns since 2019. - **EU REACH** restricts EtO to specific industrial uses and prohibits consumer applications. - **FDA** has been actively encouraging adoption of alternative sterilization methods (vaporized hydrogen peroxide, nitrogen dioxide, supercritical CO₂, X-ray) to reduce US dependence on EtO. EtO sterilizers require dedicated ventilation (separate from facility HVAC), gas-leak detection, emergency exhaust scrubbers or catalytic converters, and trained operators. ## [](#content-eto-vs-alternative-low-temperature-methods "Permalink")EtO vs Alternative Low-Temperature Methods FactorEtOH₂O₂ PlasmaOzoneFormaldehyde (LTSF)Temperature25–55 °C45–55 °C~30–35 °C60–80 °CCycle time>14 h + aeration35–75 min~4 h3–5 hAeration requiredDaysNoneNoneMinimalLong lumen capabilityExcellentLimitedLimitedModerateMaterial compatibilityVery broadNo cellulose, restricted lumensLimited polymer setModerateCarcinogenicYes (Group 1)NoNoYes (Group 1)ResidualsYes — regulatedWater + oxygenOxygenFormaldehyde residue possibleFDA cleared (US)YesYesYes (since 2003)NoFor routine reprocessing in modern hospitals, [hydrogen peroxide plasma](hydrogen-peroxide-plasma-sterilization) has displaced most EtO use. EtO retains a strong industrial role for packaged single-use devices and long-lumen reusables that other methods cannot validate. ## [](#content-regulatory-standards "Permalink")Regulatory Standards - **[ISO 11135](https://www.iso.org/standard/56137.html)** — Sterilization of health-care products: Ethylene oxide. Requirements for development, validation, and routine control. - **[ISO 10993-7](https://www.iso.org/standard/76617.html)** — Biological evaluation of medical devices: Ethylene oxide sterilization residuals. - **ANSI/AAMI ST41** — Ethylene oxide sterilization in health care facilities: Safety and effectiveness. - **OSHA 29 CFR 1910.1047** — Ethylene oxide occupational exposure standard. - **EPA NESHAP Subpart O** — National emission standards for ethylene oxide commercial sterilization facilities. ## [](#content-faq "Permalink")FAQ ### [](#content-what-is-ethylene-oxide-sterilization-used-for "Permalink")What is ethylene oxide sterilization used for? EtO sterilization is used for heat- and moisture-sensitive medical devices, particularly those with long lumens, complex geometries, or sealed packaging — including endoscopes, implants, catheters, and packaged single-use disposables such as gowns, sutures, and IV sets. ### [](#content-how-long-does-eto-sterilization-take "Permalink")How long does EtO sterilization take? The exposure phase typically runs 1–6 hours, but the full cycle exceeds 14 hours and aeration adds 24–48 hours or more. Industrial facilities often quote total turnaround in days, not hours. ### [](#content-is-ethylene-oxide-sterilization-safe "Permalink")Is ethylene oxide sterilization safe? EtO is a Group 1 human carcinogen; the gas itself is not safe. Sterilized devices are safe once aeration has reduced residuals below ISO 10993-7 limits. Operator and community safety depend on facility engineering, OSHA compliance, and emissions controls. ### [](#content-why-is-eto-being-phased-out "Permalink")Why is EtO being phased out? Carcinogenicity, community air-quality concerns, EPA emission standards, and the availability of safer alternatives have pushed regulators and industry toward methods such as hydrogen peroxide plasma, vaporized hydrogen peroxide, nitrogen dioxide, X-ray, and supercritical CO₂. EtO remains in use because no single alternative covers its full material and lumen range. ### [](#content-what-are-eto-residuals "Permalink")What are EtO residuals? EtO residuals are the unreacted EtO, ethylene chlorohydrin, and ethylene glycol left in or on a device after sterilization. They are regulated under ISO 10993-7 by the device's patient-contact category and duration. ### [](#content-what-is-the-difference-between-eto-and-100--eto-sterilization "Permalink")What is the difference between EtO and 100 % EtO sterilization? Older EtO cycles used EtO mixed with a diluent gas (CO₂ or HCFC) to reduce flammability. Modern industrial cycles use 100 % EtO under controlled vacuum to eliminate the diluent and reduce gas volume. Both achieve the same sterilization but differ in cycle engineering and emissions profile. ## [](#content-conclusion "Permalink")Conclusion EtO sterilization is irreplaceable for some industrial and clinical applications because of its broad material compatibility and ability to penetrate long lumens and packaged loads. It is also one of the most heavily regulated sterilization methods, with carcinogenicity, residual, and emissions requirements that no facility can ignore. For most healthcare reprocessing, [hydrogen peroxide plasma](hydrogen-peroxide-plasma-sterilization) is the lower-risk alternative; for packaged disposables, [gamma irradiation](gamma-irradiation-sterilization) is the main industrial competitor. 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