4.2 Low-Temperature Sterilization Methods

Not all devices survive the heat and moisture of steam. Low-temperature sterilization processes heat-sensitive and moisture-sensitive items such as flexible endoscopes, cameras, fiber-optic light cables, and battery-powered instruments. Selecting the method always begins with the device manufacturer's Instructions for Use (IFU).

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1. Ethylene Oxide (EtO)

EtO is a colorless gas that kills by alkylation — it adds alkyl groups to DNA and proteins, blocking reproduction.

Advantages: penetrates packaging, complex geometries, and long lumens; compatible with nearly all materials.

Disadvantages: toxic and carcinogenic (OSHA permissible exposure limit 1 ppm as an 8-hour TWA, 5 ppm 15-minute excursion limit), flammable in pure form, and harmful to the environment. The long aeration step makes total turnaround 12-24+ hours, so EtO is never used for urgent items. Aeration removes residual gas that could otherwise cause chemical burns or allergic reactions.

Biological indicator: Bacillus atrophaeus (formerly B. subtilis var. niger).

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2. Hydrogen Peroxide Gas Plasma (STERRAD)

Vaporized hydrogen peroxide is energized by radiofrequency energy into a plasma, breaking down into water and oxygen — leaving no toxic residuals.

Advantages: fast, no aeration, items usable immediately, environmentally benign byproducts.

Limitations: cannot process cellulose (paper, cotton, linen, cardboard) because cellulose absorbs the peroxide; cannot process liquids or powders; has lumen length/diameter restrictions (verify against STERRAD compatibility charts and device IFU); requires Tyvek or polypropylene packaging, never woven wraps.

Biological indicator: Geobacillus stearothermophilus.

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3. Vaporized Hydrogen Peroxide (VHP / V-PRO)

Uses hydrogen peroxide in the vapor phase without a plasma stage.

Same cellulose, liquid, and powder restrictions as gas plasma; lumen capability varies by model (some offer extended-lumen cycles). BI: Geobacillus stearothermophilus.

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4. Ozone Sterilization

Generates ozone (O3) from medical oxygen and water at very low temperature (85-112°F / 30-44°C), cycle 60-290 minutes. No chemical storage and no aeration, but limited material-compatibility data and longer cycles limit adoption. Natural rubber and some plastics can be damaged. BI: Geobacillus stearothermophilus.

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5. Peracetic Acid (Liquid Chemical Sterilization)

Peracetic acid (PAA) systems immerse devices in heated 0.2% PAA at 122-131°F (50-56°C) for 25-30 minutes. The defining limitation: items are processed at point of use and cannot be stored — they must go directly to the sterile field and be used immediately. Used mainly for immersible flexible endoscopes.

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Method Comparison

Exam trap: the BI organism flips to B. atrophaeus ONLY for EtO; every other method on this chart uses G. stearothermophilus.

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Choosing the Right Method: IFU First

The most important rule in low-temperature processing is that the device manufacturer's IFU dictates the method, never the technician's preference or the department's convenience. A flexible ureteroscope, a rigid camera head, and a battery handpiece may each list different validated processes, and a method that is safe for one can damage or fail to penetrate another. When two devices in the same set list incompatible methods, they must be processed separately.

Cellulose is the classic failure mode: a single paper towel or cotton-tipped applicator left in a hydrogen peroxide tray can absorb enough sterilant to abort the cycle and leave the whole load unsterile.

A worked scenario: a surgeon needs a heat-sensitive flexible scope cleaned, high-level disinfected or sterilized, and returned for a same-day add-on case. EtO is ruled out immediately because its 8-12 hour aeration makes turnaround 12-24+ hours. Hydrogen peroxide gas plasma or VHP becomes the practical choice if the scope's lumen length and diameter fall within the system's validated limits; if the lumen is too long or narrow for any low-temperature gas process, the device may require a liquid peracetic acid system with point-of-use, use-immediately handling. This is exactly the kind of method-selection judgment the CRCST exam tests.

Safety and Residual Concerns

EtO demands the strictest occupational controls of any method: dedicated ventilation, continuous area and personal gas monitoring against the OSHA limits, and emergency procedures because the gas is both toxic and flammable. Hydrogen peroxide processes are far safer for staff because the byproducts are water and oxygen, but concentrated peroxide cartridges are still corrosive and require careful handling.

Across all low-temperature methods, the unifying safety theme is verifying that no harmful residual remains on a device before it touches a patient — which for EtO means completing the full aeration, and for the peroxide and ozone methods means simply completing the validated cycle.