Radiation and Filtration Sterilisation
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Direct answer
Radiation and filtration sterilisation destroy or remove microbes without heat, which suits heat-sensitive materials. Gamma rays and electron beams are ionising radiation that penetrate sealed packaging and sterilise prefabricated disposables, whereas UV light is non-ionising and acts only on exposed surfaces. Membrane filters with a pore size of 0.22 µm retain bacteria from heat-labile fluids, but most viruses pass through them.
What you must remember
- Ionising radiation: gamma rays from cobalt-60 and electron beams carry enough energy to break DNA, so they sterilise sealed packs.
- Radiation dose: gamma sterilisation of medical devices commonly uses about 25 kGy, a dose chosen to reach a high sterility assurance level.
- Non-ionising radiation: UV light at about 254 nm damages DNA on exposed surfaces and cannot penetrate packaging or shadowed areas.
- Sterilising membrane: a 0.22 µm pore size retains bacteria in heat-labile solutions such as sera, vitamins and antibiotics.
- Viruses: most viruses are 20 to 300 nm across and pass through 0.22 µm membranes, so they need nanofilters or ultrafiltration.
- HEPA filter: removes at least 99.97 per cent of particles of 0.3 µm diameter, used in operation theatres, isolation rooms and biosafety cabinets.
- Cold sterilisation: liquid chemicals such as 2 per cent alkaline glutaraldehyde treat heat-sensitive instruments and endoscopes, with contact times set by the manufacturer.
Matching the method to the material
Each non-heat method suits a different kind of item, and the choice depends on whether the item is sealed, solid or liquid. The table compares the methods most often asked in examinations. Radiation works well on sealed or dry items, while filtration treats only liquids and gases.
| Method | Mechanism | Penetration | Typical use |
|---|---|---|---|
| Gamma radiation | Ionising, breaks DNA | High, through sealed packs | Disposable syringes, catheters |
| Electron beam | Ionising, breaks DNA | Low to moderate | Thin or low-density devices |
| UV light | Non-ionising, forms DNA dimers | Exposed surface only | Biosafety cabinets, air and surfaces |
| Membrane filtration | Physical removal at 0.22 µm | Not applicable | Sera, antibiotics, vitamins |
| Liquid chemicals | Chemical, at room temperature | Surface contact | Endoscopes, heat-sensitive instruments |
Radiation and filtration both avoid heat, but neither works on every material. Gamma and electron-beam sterilisation can degrade some polymers, so the device material must be checked before irradiation. A filter that cannot pass the item is not an option for solids or powders.
Where the exam tries to catch you
Radiation and filtration questions usually test penetration, pore size and which microbes survive the method.
- Trap: UV light is used to sterilise instruments Correct: UV acts only on exposed surfaces and cannot penetrate packaging or shadows, so it is disinfection rather than sterilisation.
- Trap: a 0.22 µm filter removes all viruses Correct: viruses are often smaller than 0.22 µm and pass through, so viral removal needs nanofilters or ultrafiltration.
- Trap: membrane filtration is used for a heat-sensitive powder Correct: membrane filters handle liquids and gases, so powders and solid devices are sterilised by radiation or another method.
Frequently asked questions
Why is gamma radiation used to sterilise disposable syringes?
Gamma radiation is used because it penetrates sealed plastic packaging, so the syringe can be sterilised in its final pack without heat damage. Disposable syringes and catheters are commonly irradiated after packing for this reason.
What is the difference between ionising and non-ionising radiation in sterilisation?
Ionising radiation, such as gamma rays and electron beams, carries enough energy to ionise molecules and break DNA. Non-ionising UV light lacks this energy and mainly forms pyrimidine dimers on exposed surfaces, so it cannot sterilise shadowed or deep areas.
What pore size is used for a sterilising membrane filter?
A sterilising membrane filter has a pore size of 0.22 µm, which retains bacteria from heat-labile liquids. This size is the standard for filter sterilising sera, antibiotic solutions and similar heat-sensitive fluids.
Why can viruses pass through a 0.22 µm membrane filter?
Most viruses measure roughly 20 to 300 nm across, which is smaller than the 0.22 µm pore of a sterilising membrane. They pass through, so removing viruses from a fluid needs nanofilters or ultrafiltration.
What efficiency does a HEPA filter achieve?
A HEPA filter removes at least 99.97 per cent of airborne particles of 0.3 µm diameter. It is used in operation theatres, isolation rooms and biosafety cabinets, where microbes carried on particles are a risk.