What Is Ethylene Oxide (ETO) Sterilization?
Ethylene oxide (ETO or EtO) sterilization is a low-temperature chemical sterilization method used for medical devices that cannot tolerate the heat and moisture of steam sterilization. It's one of the oldest and most widely used low-temperature sterilization methods in healthcare, and it's a heavily tested topic in the Sterilization Process domain of the CRCST exam — which accounts for 21% of the exam.
How ETO Sterilization Works
Ethylene oxide is a colorless gas that kills microorganisms through alkylation — it disrupts the DNA and proteins of bacteria, viruses, and spores, preventing them from reproducing or functioning. Because it works as a gas, it can penetrate packaging, lumens, and complex device geometries that other low-temperature methods can't reach.
ETO Sterilization Cycle Parameters
| Parameter | Typical Range |
|---|---|
| Gas concentration | 450–1200 mg/L |
| Temperature | Room temperature to 60°C (140°F) |
| Relative humidity | 40–80% |
| Exposure time | 1–6 hours (varies by cycle type) |
| Aeration time | 8–12 hours (mechanical aeration) |
Humidity matters more with ETO than with almost any other sterilization method — microorganisms must be adequately hydrated for the gas to penetrate and alkylate effectively. Too little humidity and the cycle can fail even though every other parameter looks correct.
What Can Be Sterilized with ETO
ETO is the go-to method for items that are:
- Heat-sensitive (many polymers and plastics)
- Moisture-sensitive
- Have long, narrow lumens
- Electronic components with heat-sensitive circuitry
- Complex devices with hard-to-reach internal surfaces
Common examples: certain endoscopic equipment, powered instruments, some implants, and single-use devices manufactured with ETO at the industrial level.
What CANNOT Be Sterilized with ETO
- Items that absorb and retain ETO residue unsafely (without proper aeration)
- Devices whose manufacturer IFU does not list ETO as a validated method
- Items requiring immediate turnaround — ETO's long cycle and aeration time make it unsuitable for urgent same-day use
Why Aeration Is Non-Negotiable
ETO is toxic, carcinogenic, and flammable, and residual gas remains absorbed in porous materials after the sterilization cycle. Aeration removes this residue to safe levels before an item can be handled or used on a patient.
- Mechanical aeration cabinets: typically 8–12 hours at controlled temperature
- Ambient (room) aeration: can take days if mechanical aeration isn't available
- Skipping or shortening aeration risks chemical burns and toxic exposure to patients and staff
ETO Safety Concerns for Sterile Processing Staff
Because ETO is a known human carcinogen, departments that use it must follow strict safety protocols:
- OSHA-regulated permissible exposure limits (PELs) for ETO in the workplace
- Dedicated, ventilated ETO rooms — never used in open work areas
- Environmental and personal ETO gas monitoring
- Proper PPE and emergency spill procedures
- Regular equipment maintenance and leak testing
Biological and Chemical Indicators for ETO
- Biological indicator organism: Bacillus atrophaeus spores — chosen because they are highly resistant to ETO
- Chemical indicators: Class-specific ETO indicators change color when exposed to the correct combination of gas concentration, humidity, temperature, and time
- A BI must be run with every ETO load — this is a frequently tested exam fact
ETO vs. Hydrogen Peroxide Gas Plasma: Quick Comparison
| Factor | ETO | Hydrogen Peroxide Plasma (STERRAD) |
|---|---|---|
| Cycle time | Hours + 8–12 hr aeration | ~28–75 minutes, no aeration needed |
| Toxicity | Toxic, carcinogenic | No toxic residue |
| Lumen penetration | Excellent (long, narrow lumens) | Limited (cannot process long narrow lumens) |
| Materials | Broad compatibility | Cannot process linens, liquids, or cellulose/paper |
| Turnaround | Slow | Fast |
Common CRCST Exam Questions on ETO
Q: What biological indicator organism is used to monitor ETO sterilization?
A: Bacillus atrophaeus
Q: Why is humidity critical in the ETO sterilization process?
A: Microorganisms must be adequately hydrated for ethylene oxide gas to penetrate and alkylate their DNA and proteins effectively.
Q: Why is aeration required after an ETO cycle?
A: To remove toxic, carcinogenic ETO gas residue from processed items to a safe level before handling or patient use.
Q: What type of items is ETO best suited for?
A: Heat- and moisture-sensitive items, and devices with long, narrow lumens.
Practice every sterilization method tested on the CRCST exam with our free practice questions — including ETO, steam, hydrogen peroxide plasma, and dry heat.
Official Resources
- OSHA — Ethylene Oxide Standard — Occupational exposure limits and safety requirements
- CDC Guideline for Disinfection and Sterilization in Healthcare Facilities — Sterilization method reference including ETO
- FDA — Ethylene Oxide Sterilization for Medical Devices — Regulatory and safety overview