5.15 : Methods of Sterilization I: Physical Methods

As used in a healthcare facility, sterilization destroys all microorganisms through physical or chemical methods. The physical method includes steam, dry heat, boiling water, and radiation.

Steam sterilization uses non-toxic, low-cost moist heat in the form of saturated steam under pressure, which is fast, microbicidal, and sporicidal, and quickly warms and penetrates fabrics. Autoclaves, or steam sterilizers, expose each item to direct steam contact for a predetermined time at the necessary temperature and pressure. Steam sterilization is influenced by four factors: steam, pressure, temperature, and time. The ideal steam for sterilization is dry saturated steam and entrained water with a dryness fraction of ≥97%. Pressure is used to generate the high temperatures needed to destroy microorganisms quickly. The presence of microbicidal activity requires a particular temperature to be reached. Standard steam sterilization temperatures range from 121°C (250 °F) to 132°C (270 °F). A minimum exposure time of 4 minutes at 132°C (270 °F) in a pre-vacuum sterilizer or 30 minutes at 121°C (250 °F) in a gravity displacement sterilizer is required to sterilize wrapped medical supplies. Moist heat destroys microorganisms through the irreversible coagulation and denaturation of enzymes and structural proteins. Items that can be steam-sterilized include all critical and semi-critical, heat- and moisture-resistant items.

Dry heat sterilization uses moisture-free heat to remove microorganisms and is only suitable for moisture-sensitive materials. It is non-toxic and has no adverse environmental effects; it penetrates materials and is non-corrosive to metal objects and sharps.

Boiling water is typically used in the home because it is easy and affordable and controls or kills most microorganisms. However, it does not eliminate spores and some viruses.

Sterilization by radiation exposes items or areas to different types of radiation, including non-ionizing radiation and ionizing radiation. Non-ionizing radiation uses ultraviolet radiation applied to the surface of aseptic work areas. Ionizing radiation is used for several medical products, including pharmaceuticals, and utilizes cobalt 60 gamma rays or electron accelerators to sterilize.