Several common materials should never go into an autoclave: most plastics, sealed containers, flammable or corrosive chemicals, paper products, non-stainless steel metals, and certain biological substances. Autoclaves work by exposing items to pressurized steam at 121°C (250°F) for at least 30 minutes, and anything that melts, explodes, releases toxic fumes, or degrades under those conditions is off the list.
If you landed here from a study guide or practice exam, this breakdown covers every category of prohibited material you’re likely to see as an answer choice.
Plastics That Cannot Be Autoclaved
This is the category that trips people up most, because some plastics handle autoclaving just fine while others warp or melt into a dangerous mess. The following plastic types are not autoclavable under any conditions:
- Polystyrene (PS): common in disposable petri dishes, cell culture plates, and certain test tubes
- Polyvinyl chloride (PVC): found in tubing and some labware
- Low-density polyethylene (LDPE) and high-density polyethylene (HDPE): used in wash bottles, plastic bags, and many disposable containers
- Nylon: sometimes used in filters and fittings
- Acrylic: found in some shields, containers, and display cases
- Polyurethane: common in flexible tubing
These plastics have melting or softening points well below 121°C. When they deform inside the autoclave, they can fuse to chamber surfaces, block drain lines, and create a cleanup nightmare. Polypropylene (PP) and polycarbonate (PC), by contrast, are heat-resistant enough to survive standard autoclave cycles, which is why you’ll see those resin codes stamped on reusable lab bottles and centrifuge tubes.
Sealed Containers and Liquids
Any liquid in a sealed, airtight container is a serious explosion hazard. As the autoclave heats up, pressure builds inside the sealed vessel far beyond what the container walls can handle. Even worse, bottles that survive the heating phase can explode during the cooling stage. When a bottle sitting at 80 to 105°C or higher is exposed to a sudden temperature change (like opening the autoclave door too early), the thermal shock can shatter the glass instantly.
The fix is simple: always loosen caps before loading liquids. Never seal a container with a cork or stopper. And always place liquid containers on a solid-bottom, heat-resistant tray to catch any spills or boil-overs.
Flammable, Corrosive, and Toxic Chemicals
This is the most dangerous category. The following should never enter an autoclave:
- Flammable solvents: ethanol, methanol, and similar organic solvents can ignite or produce explosive vapors under heat and pressure
- Acids and bases: these corrode the stainless steel chamber and release harmful fumes
- Household bleach (sodium hypochlorite): breaks down into chlorine gas at high temperatures, which is toxic and also damages the autoclave
- Chlorides and sulphates: cause pitting corrosion on stainless steel surfaces
- Phenol: produces toxic vapors when heated
- Radioactive materials: require specialized decontamination procedures, not steam sterilization
Bleach is a particularly common wrong choice on exams because people associate it with sterilization. It is a disinfectant, but it should never be placed inside an autoclave. The combination of heat and pressure turns it into a corrosive, toxic problem.
Paper and Combustible Materials
Paper is combustible and should not be placed directly inside an autoclave. At 121°C under pressurized steam, paper can char, disintegrate, or in some configurations pose a fire risk during the drying phase. This includes cardboard, paper towels placed loosely in the chamber, and similar cellulose products. If paper-wrapped items need to go through an autoclave (as with some surgical instrument packs), they require specific autoclave-grade wrapping material designed for that purpose.
Non-Stainless Steel Metals
Stainless steel instruments are built for repeated autoclaving. Other metals are not. Carbon steel, iron, and aluminum alloys can corrode, rust, or pit when exposed to repeated cycles of high-temperature steam. Seawater and salt solutions make this worse by accelerating corrosion, which is why those are also listed as incompatible. If you’re autoclaving metal instruments, check that they’re surgical-grade stainless steel and remove any attached plastic components that might melt.
Heat-Sensitive Biological Materials
Certain lab materials survive autoclaving chemically intact, but others are destroyed by the heat. Antibiotics, amino acids, vitamins, and many proteins are inactivated at 121°C. In microbiology, the standard practice is to autoclave the base growth medium first, let it cool below 50°C, and then add heat-sensitive components that were sterilized separately by passing them through a 0.2 micrometer filter.
Paraffin-embedded tissue samples are another explicitly prohibited item. The paraffin melts and can coat the autoclave’s interior, clogging drains and damaging the unit.
Electronics and Precision Instruments
Electronic components, circuit boards, battery-powered devices, and delicate optical instruments cannot survive the moisture and heat of an autoclave. Steam penetrates housings and destroys circuitry, corrodes fine mechanisms, and fogs lenses. For medical devices that can’t tolerate steam, hospitals typically use ethylene oxide gas sterilization instead. The FDA notes that for many devices made from certain polymers, or those with complex designs and hard-to-reach internal spaces, ethylene oxide may be the only sterilization method that works without damaging the device.
How to Tell if Something Is Autoclave-Safe
Look for the autoclave symbol on labware: a small icon showing a pressure vessel, sometimes accompanied by a temperature rating. Reusable plastic items rated for autoclaving will typically be made of polypropylene (PP, recycling code 5) or polycarbonate. When in doubt, check the manufacturer’s specifications. Glass items are generally safe as long as they’re borosilicate (lab-grade) rather than soda-lime glass, and as long as they aren’t sealed.
After running a cycle, chemical indicator tape on the outside of wrapped packs changes color to confirm exposure to steam. For higher assurance, biological indicators containing heat-resistant bacterial spores can verify that conditions inside the chamber actually reached sterilization levels. These spores require exposure to 121°C saturated steam for at least 20 minutes to be killed, so if the indicator comes back negative, you know the cycle worked.