CRCST Exam PrepThe short answer: Low-temperature sterilization is for items that cannot withstand steam: heat- or moisture-sensitive devices such as electronics, certain plastics, cameras, and powered instruments. The three methods are ethylene oxide (EtO), which is slow, toxic, and requires hours of aeration; hydrogen peroxide gas plasma, which is fast with no aeration but cannot process cellulose, liquids, or powders; and ozone, a newer option with similar material limits. Steam remains the preferred method whenever the item can tolerate it, and the device IFU decides which method each item can withstand.
Why low-temperature methods exist
Steam is the preferred sterilization method whenever the device can tolerate it. It is faster, cheaper, less toxic, and more penetrating than the alternatives. The problem is that some devices cannot tolerate it: electronics melt or short-circuit, certain plastics deform, lenses and cameras are damaged by heat and moisture, and powered instruments have components that steam ruins.
Low-temperature methods solve this by killing microorganisms without high heat. The tradeoff is time, cost, toxicity, or material restrictions, sometimes all four. On the exam, the first decision is always method selection: if the question describes an item that can withstand steam, the answer is steam. Low-temperature methods enter the picture only when steam would damage the device. Our Spaulding classification guide covers why the item needs sterilization at all; this guide covers how to sterilize the ones steam would destroy.
Ethylene oxide (EtO): the slow, thorough workhorse
Ethylene oxide is a gas sterilant that kills microorganisms by alkylating proteins and DNA. It penetrates extremely well, including into long lumens and complex device geometries, which makes it the go-to method for intricate heat-sensitive instruments. But it is slow, toxic, flammable, and a known carcinogen, so it comes with the strictest safety requirements of any method on the exam.
The four cycle phases
- Preconditioning: the load is warmed and humidified. EtO needs controlled humidity to work, and the items must reach the cycle temperature before gas is introduced.
- Sterilant exposure: the chamber is charged with EtO at the specified gas concentration, and that concentration is held for the exposure time.
- Evacuation and flushing: the gas is pulled out of the chamber and the chamber is flushed, often multiple times, to remove residual sterilant.
- Aeration: items sit in a mechanical aerator (or at ambient temperature) until the EtO absorbed into materials dissipates to safe levels. Aeration commonly takes 8 to 12 hours in a mechanical aerator at elevated temperature, and longer at ambient temperature.
Total cycle time including aeration can exceed 10 to 12 hours. That is the number one disadvantage of EtO: turnaround is measured in hours, not minutes. Departments keep EtO for items that have no faster validated option.
The dry rule
Safety and exposure limits
Because EtO is toxic and carcinogenic, staff protection is a major exam topic. The OSHA permissible exposure limit (PEL) is 1 ppm as an 8-hour time-weighted average, with an action level of 0.5 ppm that triggers increased monitoring and medical surveillance. In practice, closed-system sterilizers, dedicated ventilation, continuous room air monitoring, and proper aeration keep exposures below these limits. The trend in the industry is toward replacing EtO with gas plasma or ozone wherever the device IFU allows it, precisely because of these toxicity concerns.
The biological indicator for EtO uses Bacillus atrophaeus (formerly Bacillus subtilis) spores. Our indicators guide covers the full monitoring framework; just remember that each method has its own test organism.
Hydrogen peroxide gas plasma: fast, but picky about materials
Hydrogen peroxide gas plasma systems (the best-known brand name is STERRAD) vaporize concentrated hydrogen peroxide and then use a plasma phase to break down residual peroxide into water and oxygen. The result is a low-temperature cycle with no toxic residue and no aeration step, which is why departments love it for routine heat-sensitive items.
Cycle times run roughly 45 to 75 minutes depending on the system and the cycle selected, far shorter than EtO. But gas plasma is picky about what it accepts, and the restrictions are heavily tested:
- No cellulose: paper, cotton, linen, and cellulose-based packaging absorb hydrogen peroxide and prevent sterilization. Only validated non-cellulose packaging (such as polypropylene wraps and pouches) may be used.
- No liquids or powders: liquids and powders cannot be processed; the cycle is for solid, dry, non-absorbent items only.
- Lumen limits: long, narrow channels beyond the validated length-to-diameter limits cannot be reliably sterilized. Check the sterilizer's cleared lumen claims against the device.
- Must be dry: moisture causes the cycle to cancel or abort. Items must be completely dry before loading, just like with EtO.
The consumable is a sealed cassette or cartridge of concentrated hydrogen peroxide, and staff should handle it per the SDS since concentrated peroxide is an oxidizer and a skin and eye irritant. The biological indicator organism for gas plasma is Geobacillus stearothermophilus, the same organism used for steam.
Ozone: the newer option
Ozone sterilizers generate ozone gas from medical-grade oxygen and water, sterilize at low temperature, and then convert the ozone back to oxygen at the end of the cycle, so there is no toxic residue and no aeration step. Material restrictions resemble gas plasma: items must be dry, and the device IFU and the sterilizer's cleared claims determine what may be processed. Ozone appears on the exam mainly as a recognition item: know that it exists, that it is a low-temperature method, and that it shares gas plasma's advantages of no residue and no aeration.
Comparing the low-temperature methods
| Method | Cycle time | Aeration | Key restrictions | BI organism |
|---|---|---|---|---|
| Ethylene oxide (EtO) | 10+ hours including aeration | 8 to 12 hours in mechanical aerator | Toxic and carcinogenic; items must be dry (moisture forms toxic ethylene glycol); OSHA PEL 1 ppm | Bacillus atrophaeus |
| Hydrogen peroxide gas plasma | ~45 to 75 minutes | None needed | No cellulose (paper, cotton, linen), no liquids or powders, strict lumen limits, items must be dry | Geobacillus stearothermophilus |
| Ozone | Under an hour typically | None needed | Similar material limits to gas plasma; items must be dry | Geobacillus stearothermophilus |
Notice the pattern the exam exploits: EtO is the thorough but slow and toxic option with the unique dry rule and a unique BI organism, while gas plasma and ozone are the fast, residue-free options with strict material restrictions. When a question lists cellulose packaging as an answer choice for gas plasma, it is wrong.
How the exam asks you to choose a method
Method-selection questions follow a decision tree. Work it in this order:
- Can the item withstand steam? If yes, steam is the answer. It is the preferred method: fastest, cheapest, most reliable.
- Is the item heat- or moisture-sensitive? If yes, you need a low-temperature method, and the device IFU narrows it further.
- Apply the restrictions: cellulose packaging or absorbent materials rule out gas plasma; lumens beyond validated limits rule out gas plasma; when nothing else is validated, EtO is the fallback because of its penetration.
- The IFU wins ties: as with steam parameters, the device manufacturer's instructions for use override generic rules.