The short answer: Steam under pressure is the preferred sterilization method for anything that can withstand heat and moisture. Heat-sensitive instruments go to low-temperature methods: ethylene oxide, hydrogen peroxide gas plasma, or ozone. Dry heat covers moisture-sensitive items like powders and oils. The device IFU always decides which method is validated for a given instrument.
The master comparison table
Memorize this table and the sterilization questions get easy. Each row is one method, with the temperature and time the exam expects you to know cold, what it sterilizes, and the limitation that makes each method the wrong answer in certain scenarios.
| Method | Temperature | Exposure time | What it sterilizes | Key limitations |
|---|---|---|---|---|
| Steam, gravity displacement | 121 C (250 F) at 15 psi 132 C (270 F) |
30 minutes 15 minutes |
Heat- and moisture-tolerant items: metal instruments, wrapped trays, textiles, some plastics | Longest steam cycles; air removal is passive and slow, so loading must allow steam contact |
| Steam, dynamic air removal (prevacuum) | 132 C (270 F) | 4 minutes | Same as gravity, plus porous loads and lumened instruments more reliably | Needs a vacuum pump; requires a daily Bowie-Dick air-removal test |
| Steam, immediate-use (IUSS) | 132 C (270 F) | 3 minutes | Unwrapped metal instruments for genuine emergencies with no sterile alternative | Emergencies only, never a routine shortcut; unwrapped items must be used immediately |
| Ethylene oxide (EtO) | Low temperature, gas | 10+ hours total including aeration (8 to 12 hours in mechanical aerator) | Heat-sensitive and complex instruments with long lumens; excellent penetration | Slow, toxic, flammable, known carcinogen; items must be dry; OSHA PEL 1 ppm (8-hour TWA), action level 0.5 ppm |
| Hydrogen peroxide gas plasma | Low temperature | About 45 to 75 minutes | Routine heat-sensitive items validated in the IFU; no toxic residue, no aeration | No cellulose (paper, cotton, linen), no liquids or powders, strict lumen limits; items must be completely dry |
| Ozone | Low temperature | Under an hour typically | Heat-sensitive items per the sterilizer's cleared claims; no toxic residue, no aeration | Material limits similar to gas plasma; items must be dry |
| Dry heat | 160 C (320 F) or 170 C (340 F) | 2 hours at 160 C; 1 hour at 170 C | Items that cannot tolerate moisture: powders, oils, glassware, sharp instruments (less dulling) | Long cycles and poor penetration; hot dry air penetrates far less efficiently than steam, so it is rarely the first choice |
Steam: the preferred method
Steam sterilization uses moist heat under pressure to denature microbial proteins. It is the preferred method whenever the item can withstand it because it is fast, effective, economical, and leaves no toxic residue. The exam tests the time-temperature pairs relentlessly: 121 goes with 30, 132 goes with 15 on gravity, 132 goes with 4 on prevacuum, and 132 goes with 3 for IUSS. For the full breakdown, see our steam sterilization parameters guide.
Low-temperature methods: for heat-sensitive items
Ethylene oxide is the slow, thorough workhorse: it penetrates long lumens and complex geometries better than anything else, but it is toxic, flammable, and a known carcinogen, so aeration and exposure monitoring are mandatory. Hydrogen peroxide gas plasma is the department favorite for routine heat-sensitive items: fast, with no toxic residue and no aeration, but picky about materials. Ozone shares gas plasma's advantages and limitations. Our low-temperature sterilization guide covers all three in depth.
How to read exam questions about methods
The exam rarely asks "what temperature does X run at" in isolation. It gives you an instrument and a situation and asks which method is appropriate, or which statement about a method is true. Work backward from the limitation: a heat-sensitive laparoscope with a long lumen points to EtO, a routine heat-sensitive instrument points to gas plasma, anything that survives heat and moisture points to steam. Try our sterilization method picker to practice choosing between them.
Quick-memorize cards
Gravity steam pairs: 121 C for 30 minutes at 15 psi, 132 C for 15 minutes.
Prevacuum steam is 132 C for 4 minutes; IUSS is 132 C for 3 minutes, emergencies only.
10+ hours with aeration; OSHA PEL 1 ppm; never on wet items.
45 to 75 minutes, no liquids, no paper or linen, no powders.
Turn the table into a score
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