Small Clinics
Types of Sterilization: Methods, Classification and Uses
Learn the types of sterilization, from autoclaving and dry heat to ethylene oxide and radiation, with a classification chart, uses and how sterility is checked.
Sterilization is the process of destroying or removing all forms of microbial life, including bacteria, viruses, fungi and highly resistant bacterial spores, from an object or substance. The main types of sterilization are physical methods, such as heat, radiation and filtration, and chemical methods, such as ethylene oxide gas and hydrogen peroxide.
In hospitals, clinics, laboratories and the pharmaceutical industry, sterilization prevents instruments, devices and products from carrying infection. This guide explains how sterilization methods are classified, how each works, where each is used and how sterility is confirmed.
What Is Sterilization?
Sterilization means making an item free of all living microorganisms, including spores. An item is either sterile or it is not; there is no "partly sterile".
In practice, sterility is expressed as a probability. For medical devices, sterilization processes are usually designed so that the chance of a single surviving microorganism on an item is no more than one in a million.
In everyday language, "sterilization" can also refer to a surgical procedure that prevents pregnancy. This article covers only the sterilization of instruments, materials and products.
Sterilization vs Disinfection
Sterilization and disinfection are often confused, but they are not the same. Disinfection reduces harmful microorganisms to a safe level but does not reliably kill bacterial spores. Many disinfectants are used on surfaces and non-critical equipment, while sterilization is needed for anything that enters sterile body tissue.
| Sterilization | Disinfection | |
| What it removes | All microorganisms, including spores | Most harmful microorganisms, not reliably spores |
| Typical methods | Autoclave, hot air oven, ethylene oxide, hydrogen peroxide plasma, radiation | Chemical disinfectants, boiling, pasteurisation, UV light |
| Used for | Surgical instruments, implants, needles, culture media | Surfaces, floors, some reusable non-critical items |
| Result | Sterile | Safe for use, but not sterile |
Classification of Sterilization
Sterilization methods are usually classified into two main groups, physical and chemical. Some textbooks list filtration separately as a mechanical method, which gives three groups.
| Group | Type | Methods |
| Physical | Moist heat | Autoclaving (steam under pressure), tyndallisation, inspissation |
| Physical | Dry heat | Hot air oven, flaming, red heat, incineration |
| Physical | Ionising radiation | Gamma rays, electron beam |
| Physical | Non-ionising radiation | Ultraviolet (disinfection only) |
| Physical (mechanical) | Filtration | Membrane filters, HEPA filters |
| Chemical | Gases | Ethylene oxide, formaldehyde, hydrogen peroxide gas plasma |
| Chemical | Liquids | Glutaraldehyde, peracetic acid |
Physical Methods of Sterilization
Physical methods use heat, radiation or filtration. They are the most widely used because they are reliable, well understood and leave no chemical residue.
Moist Heat: Autoclaving, Tyndallisation and Inspissation
Moist heat kills microorganisms by denaturing their proteins. Steam transfers heat far more efficiently than dry air, so moist heat works at lower temperatures and in shorter times than dry heat.
Autoclaving uses steam under pressure and is the most common sterilization method in hospitals and laboratories. Autoclaves are used for surgical instruments, linen, glassware, culture media and many liquids. Instruments are usually packed in sterilization reels and pouches that let steam in and keep the contents sterile afterwards. Autoclaving is not suitable for heat- or moisture-sensitive items, powders or oils.
| Method | Typical conditions | Used for |
| Autoclave (gravity cycle) | 121°C, about 15 psi, 15 to 30 minutes depending on load | Instruments, linen, glassware, media |
| Autoclave (pre-vacuum cycle) | 134°C, about 30 psi, 3 to 4 minutes | Wrapped instrument sets, porous loads |
| Tyndallisation (fractional sterilization) | 100°C for 20 to 30 minutes on three successive days | Media that cannot withstand autoclaving |
| Inspissation | 80 to 85°C for about 30 minutes on three successive days | Egg- and serum-based culture media |
The correct cycle always depends on the sterilizer, the load and the manufacturer's validated settings.
Boiling at 100°C and pasteurisation also use moist heat, but they do not reliably kill spores, so they count as disinfection rather than sterilization.
Dry Heat: Hot Air Oven, Flaming and Incineration
Dry heat kills microorganisms by oxidation. It needs higher temperatures and longer times than steam, but it suits items that moisture would damage.
- Hot air oven: commonly 160°C for 2 hours or 170°C for 1 hour. Used for glassware, metal instruments, powders and oils. It is also the traditional choice for sharp instruments, as moisture can blunt or rust cutting edges. Read more about the role of the hot air oven in sterilization, or see an example such as the Remi Dry Hot Air Oven.
- Red heat and flaming: holding inoculating loops, needles or forceps tips in a flame until red hot. Used in laboratories for small metal items.
- Incineration: burning contaminated waste, such as dressings and laboratory waste, to ash. It destroys the items, so it is a disposal method as well.
Radiation
Ionising radiation, such as gamma rays (from cobalt-60) and electron beams, penetrates packaging and sterilizes products without heat. It is widely used in industry for single-use items such as syringes, catheters, sutures and gloves, which are sterilized in their final packaging. It needs specialised facilities, and some plastics can discolour or weaken.
Ultraviolet (UV) light is non-ionising. It has poor penetration and works only on exposed surfaces and air, so it is used for disinfection of rooms, cabinets and water rather than for sterilizing instruments.
Filtration (Mechanical Method)
Filtration removes microorganisms rather than killing them. Liquids are passed through membrane filters with a pore size of about 0.22 micrometres, which hold back bacteria. It is used for heat-sensitive solutions, such as some injectable drugs, antibiotic solutions and serum. HEPA filters remove particles, including microorganisms, from air in operating theatres and clean rooms.
Filtration does not reliably remove viruses, so it is often combined with other controls in pharmaceutical production. Some textbooks classify filtration as a mechanical method of sterilization.
Chemical Methods of Sterilization
Chemical methods use gases or liquids and are mainly used for items that heat would damage, such as plastics, electronics and endoscopes. Because they work at low temperatures, they are sometimes called cold sterilization. They need careful handling, and items must usually be aerated or rinsed to remove residues.
| Chemical agent | How it is used | Typically used for | Key cautions |
| Ethylene oxide (EtO) gas | Low-temperature gas cycle followed by aeration | Heat-sensitive devices, plastics, catheters | Toxic, flammable; needs long aeration to remove residue |
| Hydrogen peroxide gas plasma | Low-temperature vapour cycle | Heat-sensitive instruments, electronics | Not for linen, paper, powders or liquids; limited penetration into long narrow lumens |
| Formaldehyde | Gas or low-temperature steam with formaldehyde; formalin fumigation | Some heat-sensitive items; room and cabinet fumigation | Irritant and toxic; fumigation is usually disinfection. See formalin chambers |
| Glutaraldehyde (for example, 2%) | Liquid immersion | Endoscopes and heat-sensitive instruments | High-level disinfectant; sterilization needs much longer contact; irritant |
| Peracetic acid | Liquid, often in automated systems | Endoscopes and immersible instruments | Corrosive in concentrated form; items used soon after processing |
Advantages and Disadvantages of Sterilization Methods
Each method involves trade-offs between speed, cost, material compatibility and safety.
| Sterilization method | Advantages | Disadvantages |
| Steam (autoclave) | Fast, reliable, non-toxic, economical; penetrates wrapped packs and liquids; easy to monitor | Not for heat- or moisture-sensitive items, powders or oils; can corrode some metals; burn risk |
| Dry heat (hot air oven) | No moisture, so no rusting or blunting; suitable for powders, oils and glassware; simple equipment | Slow (hours, not minutes); high temperatures damage many materials; cannot be used for liquids or plastics |
| Ethylene oxide | Low temperature; compatible with most materials; penetrates packaging and complex devices | Toxic, flammable, suspected carcinogen; long cycle including aeration |
| Hydrogen peroxide plasma | Low temperature; short cycles; breaks down into water and oxygen | Not for cellulose (paper, linen), liquids or powders; limited penetration; higher equipment cost |
| Radiation (gamma or electron beam) | Low temperature, suits heat-sensitive single-use items; sterilizes products in final packaging; no chemical residue | Needs industrial facilities; some plastics discolour or weaken; not practical in hospitals |
| Filtration | Suits heat-sensitive liquids; fast; no heat or chemicals | Does not reliably remove viruses; only for liquids and air; filters can clog |
Choosing the Right Sterilization Method
There is no single best method of sterilization. The right method depends on what the item is made of and how it is used.
| Item | Method commonly used |
| Stainless steel surgical instruments | Autoclave |
| Sharp cutting instruments | Hot air oven traditionally; autoclave with suitable packaging is also used |
| Glassware, powders, oils | Hot air oven |
| Linen, gauze, cotton | Autoclave |
| Heat-sensitive plastics and devices | Ethylene oxide or hydrogen peroxide plasma |
| Endoscopes | High-level disinfection or liquid chemical sterilization, as per manufacturer |
| Heat-sensitive liquids | Filtration |
| Single-use disposables (manufacturing) | Gamma radiation or ethylene oxide |
| Culture media | Autoclave; inspissation or tyndallisation for special media |
| Dental instruments | Autoclave, often in pouches; see dental sterilization equipment |
Always follow the instrument or device manufacturer's reprocessing instructions, as they state which methods the item can withstand.
Sterilization in Hospitals and CSSD
Most hospitals process reusable instruments in a Central Sterile Services Department (CSSD), using autoclaves as the main method and low-temperature methods for heat-sensitive devices. CSSD equipment typically includes horizontal and vertical autoclaves, washers and packing stations.
Which Items Need Sterilization (Spaulding Classification)
Hospitals decide whether an item needs sterilization or disinfection based on the Spaulding classification.
| Category | Contact with the body | Examples | Minimum processing |
| Critical | Enters sterile tissue or the bloodstream | Surgical instruments, implants, needles | Sterilization |
| Semi-critical | Contacts mucous membranes or non-intact skin | Endoscopes, laryngoscope blades | High-level disinfection at minimum; sterilization where possible |
| Non-critical | Contacts intact skin only | BP cuffs, stethoscopes, bedpans | Low-level disinfection |
Steps of the Sterilization Process
- Collection and decontamination: used instruments are collected safely and kept moist to stop debris drying.
- Cleaning: instruments are cleaned manually or in a washer. Sterilization cannot work reliably on dirty items.
- Inspection and drying: each item is checked for damage and remaining dirt.
- Packing: items are assembled into sets and wrapped or placed in pouches such as Avue sterilization reels, with an indicator.
- Sterilization: the load is processed using the correct validated cycle.
- Cooling, storage and distribution: packs are cooled, stored in clean, dry conditions and issued for use.
- Records: cycle data and indicator results are documented for each load.
Applications of Sterilization
Sterilization prevents healthcare-associated infections and protects patients, staff and product quality across several fields.
| Field | What is sterilized |
| Hospitals and clinics | Surgical instruments, implants, linen, dressings |
| Dental practice | Handpieces, forceps, burs and other reusable instruments |
| Pharmaceutical industry | Injectable drugs, vaccines, containers, manufacturing equipment |
| Laboratories and microbiology | Culture media, glassware, waste |
| Medical device manufacturing | Syringes, catheters, sutures, gloves |
| Food industry | Canned foods and packaging |
How Sterilization Is Monitored and Validated
Because microorganisms cannot be seen, facilities use a combination of checks to confirm that each load has been sterilized.
- Physical monitoring: the sterilizer's time, temperature and pressure readings are checked and recorded for every cycle. Digital controllers with alarms make this more reliable.
- Chemical indicators: these change colour when certain conditions are reached. Under ISO 11140-1 they are grouped into six types, from Type 1 process indicators (such as indicator tape) and Type 2 specific tests (such as the Bowie-Dick test for pre-vacuum autoclaves) to Type 5 integrating and Type 6 emulating indicators placed inside packs.
- Biological indicators: these contain highly resistant bacterial spores, for example Geobacillus stearothermophilus for steam and Bacillus atrophaeus for ethylene oxide and dry heat. After the cycle they are incubated; no growth confirms the process killed the spores.
- Maintenance: sterilizers are serviced, cleaned and calibrated according to the manufacturer's schedule.
- Record keeping: each cycle's parameters, indicator results and operator are recorded so that any failed load can be traced and recalled.
Conclusion
The main types of sterilization are physical methods, such as autoclaving, dry heat, radiation and filtration, and chemical methods, such as ethylene oxide and hydrogen peroxide plasma. Each suits different materials, so the choice depends on what is being sterilized and how it will be used.
For hospitals and clinics, reliable sterilization depends as much on cleaning, packing, monitoring and records as on the sterilizer itself.
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