Steam sterilization (autoclaving) is the most widely used method to sterilize medical equipment, but it’s one of several options. The right method depends on the device itself: what it’s made of, how it will be used, and whether it can withstand high heat. Hospitals, clinics, and manufacturers use a combination of steam, chemical gases, liquid chemicals, radiation, and dry heat to ensure instruments and devices are free of all living microorganisms.
Why Different Equipment Needs Different Methods
Not every piece of medical equipment requires the same level of treatment. The CDC classifies devices into three risk categories based on how they contact the body. Surgical instruments, implants, and catheters that enter sterile tissue or the bloodstream are considered critical items and must be fully sterilized. Equipment that touches mucous membranes or broken skin, like endoscopes and respiratory therapy devices, needs at minimum high-level disinfection. Items that only contact intact skin, such as blood pressure cuffs and bedpans, require only basic cleaning and low-level disinfection.
This classification drives every decision about which sterilization method to use. A stainless steel scalpel can handle the intense heat of a steam autoclave. A flexible plastic catheter or an electronic sensor cannot. Matching the method to the material is just as important as killing the microorganisms.
Steam Sterilization (Autoclaving)
Steam under pressure is the gold standard. Autoclaves work by exposing equipment to saturated steam at temperatures high enough to destroy bacteria, viruses, fungi, and bacterial spores. The two standard settings are 121°C (250°F) and 132°C (270°F). At the lower temperature, wrapped instruments need at least 30 minutes of exposure in a gravity displacement sterilizer. At the higher temperature, a prevacuum sterilizer can finish in as little as 4 minutes.
The pressure inside an autoclave isn’t what kills microorganisms directly. It’s what allows steam to reach temperatures far above the normal boiling point of water, and it’s that concentrated heat combined with moisture that destroys cellular structures. Steam sterilization is fast, reliable, nontoxic, and inexpensive, which is why it remains the first choice for any instrument that can tolerate heat and moisture. Stainless steel surgical tools, metal clamps, and many glass items are routinely autoclaved between uses.
Ethylene Oxide Gas
Many modern medical devices contain plastics, resins, layered packaging, or electronics that would melt or warp in an autoclave. For these heat-sensitive items, ethylene oxide (EtO) gas is often the only option that sterilizes effectively without damaging the device. Catheters, certain implants, and devices with hard-to-reach internal channels are commonly sterilized this way, especially during manufacturing.
The tradeoff is safety and time. Ethylene oxide is toxic to humans. OSHA limits workplace exposure to no more than 1 part per million over an 8-hour shift, with a ceiling of 5 ppm over any 15-minute window. After a sterilization cycle, devices must go through an aeration period to off-gas residual EtO before they’re safe to handle or use. Sterilizers equipped with purge cycles can speed this up, but units without them require the door to remain cracked open for at least 15 minutes, then fully open for another 15 minutes or more, with air monitoring to confirm safe levels. The FDA is actively encouraging the development of alternative methods that could reduce reliance on ethylene oxide.
Hydrogen Peroxide Gas Plasma
This newer low-temperature method fills a sealed chamber with hydrogen peroxide vapor, then applies a radio-frequency electrical field to create a plasma, a cloud of highly reactive molecules called free radicals. These free radicals tear apart the essential proteins and genetic material of any microorganisms on the device surfaces.
The process runs at just 37 to 44°C, making it safe for plastics, electrical components, and corrosion-prone metal alloys. It’s compatible with over 95% of medical devices and materials tested. Cycle times have improved significantly over successive generations of the technology: early systems took about 75 minutes, while current versions complete a cycle in 28 to 38 minutes. Unlike ethylene oxide, hydrogen peroxide gas plasma leaves no toxic residue. The byproducts are just water vapor and oxygen, so devices are ready to use as soon as the cycle ends.
Liquid Chemical Sterilants
Some semi-critical devices, particularly endoscopes and other instruments with narrow channels, are sterilized or high-level disinfected by soaking them in chemical solutions. The two most common agents are glutaraldehyde and ortho-phthalaldehyde (OPA). A 2% glutaraldehyde solution requires a minimum contact time of 20 minutes, while 0.55% OPA achieves the same level of disinfection in 10 minutes. Both are equally effective at eliminating bacteria, viruses, and mycobacteria, but OPA’s shorter soak time makes it more practical in busy clinical settings.
Peracetic acid and hydrogen peroxide solutions are also used for liquid chemical sterilization. These methods are typically reserved for devices that can’t be processed by steam or gas methods, since immersion sterilization requires careful rinsing afterward and doesn’t keep items sterile during storage the way sealed packaging does.
Dry Heat Sterilization
Dry heat ovens work on the same principle as steam sterilization but without moisture, which means they need higher temperatures and longer exposure times to achieve the same result. The standard combinations are 170°C for 60 minutes, 160°C for 120 minutes, or 150°C for 150 minutes. Dry heat is best suited for items that can tolerate extreme temperatures but might be damaged by moisture, such as powders, oils, and certain metal instruments. It’s less common in hospitals than autoclaving but fills an important niche for specific materials.
Radiation Sterilization
Gamma radiation is used primarily in manufacturing to sterilize single-use medical products in bulk, such as syringes, gloves, sutures, and wound dressings, while they’re still sealed in their packaging. The international standard (ISO 11137) specifies doses of either 15 or 25 kiloGray depending on the product’s bioburden, which is the number of microorganisms present before sterilization. Higher contamination levels require higher doses. Gamma rays penetrate packaging and products uniformly, making this method efficient for sterilizing large volumes at once. It’s not practical for reprocessing instruments in a hospital, but it’s how much of the disposable equipment you encounter in a clinical setting was sterilized before it reached you.
How Sterilization Is Verified
Sterilization isn’t just assumed to work. Every method is held to a sterility assurance level (SAL) of 10⁻⁶, meaning the probability of a single surviving microorganism must be no greater than one in a million processed items. To confirm this, facilities use biological indicators: small test vials containing highly resistant bacterial spores. For steam and hydrogen peroxide sterilization, the spores used are from a heat-resistant species called Geobacillus stearothermophilus. Ethylene oxide cycles use a different set of spore types suited to that process.
These biological indicators are placed inside sterilization loads in the hardest-to-reach spots. After the cycle, the vials are incubated. If the spores have been killed, no growth appears, confirming the cycle was effective. Modern incubators detect this within hours by measuring whether a specific enzyme from the spores is still active. A fluorescent signal means spores survived and the load failed. No fluorescence means everything in that load is sterile. Facilities run these tests regularly alongside chemical indicator strips and physical readouts of temperature and pressure to create multiple layers of verification.