Sterilization should be used for instruments, surgical gloves and other items that come in direct contact with the blood stream or normally sterile tissues (Spaulding 1939). It can be achieved by high-pressure steam (autoclave), dry heat (oven), chemical sterilants (glutaraldehydes or formaldehyde solutions) or physical agents (radiation). Because sterilization is a process, not a single event, all components must be carried out correctly for sterilization to occur.
To be effective, sterilization requires time, contact, temperature and, with steam sterilization, high pressure. The effectiveness of any method of sterilization is also dependent upon four other factors:
1. The type of microorganism present. Some microorganisms are very difficult to kill. Others die easily.
2. The number of microorganisms present. It is much easier to kill one organism than many.
3. The amount and type of organic material that protects the microorganisms. Blood or tissue remaining on poorly cleaned instruments acts as a shield to microorganisms during the sterilization process.
4. The number of cracks and crevices on an instrument that might harbor microorganisms. Microorganisms collect in, and are protected by, scratches, cracks and crevices such as the serrated jaws of tissue forceps.
Finally, without thorough cleaning, which removes any organic matter remaining on the instruments that could protect microorganisms during the sterilization process, sterilization cannot be assured, even with longer sterilization times.
1. Methods of heat sterilization
High-pressure, saturated steam using an autoclave, or dry heat using an oven, are the most common and readily available methods used for sterilization.
a. High-pressure steam sterilization is an effective method of sterilization but is the most difficult to do correctly (Gruendemann and Mangum 2001). It is generally the method of choice for sterilizing instruments and other items used in healthcare facilities. Where electricity is a problem, instruments can be sterilized in a nonelectric steam sterilizer using kerosene or other fuel as a heat source.
b. Dry-heat sterilizers (ovens) are good in humid climates but need a continuous supply of electricity, making them impractical in many remote (rural) areas. Furthermore, dry-heat sterilization, which requires use of higher temperatures, can be used only with glass or metal objects—it will melt other substances.
Standard Conditions for Heat Sterilization
Steam sterilization (Gravity): Temperature should be 121oC (250oF); pressure should be 106 kPa (15 lbs/in2); 20 minutes for unwrapped items; 30 minutes for wrapped items. Or at a higher temperature of 132oC (270oF), pressure should be 30lbs/in2; 15 minutes for wrapped items. Allow all items to dry before removing them from the sterilizer. Pressure settings (kPa or lbs/in2) may vary slightly depending on the sterilizer used. When possible, follow manufacturers’ recommendations.
• 1700C (3400F) for 1 hour (total cycle time—placing instruments in oven, heating to 170oC, timing for 1 hour, and then cooling—is from 2–
1.5 hours), or
• 1600C (3200F) for 2 hours (total cycle time is from 3–3.5 hours).
Sterile instruments and other items should be used immediately unless they:
• were wrapped in a double layer of muslin, paper or other appropriate material prior to sterilization; or
• can be stored in a dry, sterile container with a tight-fitting lid.
The material used for wrapping instruments and other items must be porous enough to let steam through but tightly woven enough to protect against dust particles and microorganisms. Wrapped sterile packs should remain sterile until some event causes the package or container to become contaminated. An event can be a tear or worn area in the wrapping, the package becoming wet or anything else that will allow microorganisms to enter the package or container.
2. Sterilization By Steam
General Principles Steam is an effective sterilant for two reasons. First, saturated steam is an extremely effective “carrier” of thermal energy. It is many times more effective in conveying this type of energy to the item than is hot (dry) air. In a kitchen, potatoes can be cooked in a few minutes in a steam pressure cooker while cooking may take an hour or more in a hot-air oven, even though the oven is operated at a much higher temperature. Steam, especially under pressure, carries thermal energy to the potatoes very quickly, while hot air does so very slowly. Second, steam is an effective sterilant because any resistant, protective outer layer of the microorganisms can be softened by the steam, allowing coagulation (similar to cooking an egg white) of the sensitive inner portions of the microorganism. Certain types of contaminants, however, especially greasy or oily materials, can protect microorganisms against the effects of steam, thus hindering the process of sterilization. This reemphasizes the need for thorough cleaning of objects before sterilization.
Requirements Steam sterilization requires four conditions: adequate contact, sufficiently high temperature, correct time and sufficient moisture. Although all are necessary for sterilization to take place, sterilization failures in clinics and hospitals are most often caused by lack of steam contact or failure to attain adequate temperature. All four conditions are discussed, in order of their importance in ensuring complete sterilization by steam, appendix also contains detailed instructions for operating steam sterilizers as well as instructions for wrapping and packing the items for sterilization.
• Most commonly used, effective method of sterilization.
• Sterilization cycle time is shorter than with dry heat or chemical sterilants.
• Requires a continuous source of heat (wood fuel, kerosene or electricity).
• Requires equipment (steam sterilizer), which must be expertly maintained to keep it in working condition.
• Requires strict adherence to time, temperature and pressure settings.
• Difficult to produce dry packs because breaks in procedure are common (e.g., not allowing items to dry before removing, especially in hot, humid climates).
• Repeated sterilization cycles can cause pitting and dulling of cutting edges of instruments (i.e., scissors).
• Plastic items cannot withstand high temperatures.
Ideally, a steam sterilizer log should be kept, noting time:
• heat begun,
• correct temperature and pressure achieved,
• heat turned down, and
• heat turned off.
Keeping a log can help ensure that the required amount of time will be
observed, even when multiple, new or hurried workers are responsible for
3. Sterilization By Dry Heat
When available, dry heat is a practical way to sterilize needles and other instruments. A convection oven with an insulated stainless steel chamber and perforated shelving to allow the circulation of hot air is recommended, but dry-heat sterilization can be achieved with a simple oven as long as a thermometer is used to verify the temperature inside the oven.
Effectiveness Dry-heat sterilization is accomplished by thermal (heat) conduction. Initially, heat is absorbed by the exterior surface of an item and then passed to the next layer. Eventually, the entire object reaches the temperature needed for sterilization. Death of microorganisms occurs with dry heat by a process of slow destruction of protein. Dry-heat sterilization takes longer than steam sterilization, because the moisture in the steam sterilization process significantly speeds up the penetration of heat and shortens the time needed to kill microorganisms.
• Effective method, as dry heat by conduction reaches all surfaces of instruments, even for instruments that cannot be disassembled.
• Protective of sharps or instruments with a cutting edge (fewer problems with dulling of cutting edges).
• Leaves no chemical residue.
• Eliminates “wet pack” problems in humid climates.
• Plastic and rubber items cannot be dry-heat sterilized because temperatures used (160–170C) are too high for these materials.
• Dry heat penetrates materials slowly and unevenly.
• Requires oven and continuous source of electricity.
4. Chemical Sterilization
An alternative to high-pressure steam or dry-heat sterilization is chemical sterilization (often called “cold sterilization”). If objects need to be sterilized, but using high-pressure steam or dry-heat sterilization would damage them or equipment is not available (or operational), they can be chemically sterilized.
Some high-level disinfectants will kill endospores after prolonged (10–24 hour) exposure. Common disinfectants that can be used for chemical sterilization include glutaraldehydes and formaldehyde. Sterilization takes place by soaking for at least 10 hours in 2–4% glutaraldehyde solution or at least 24 hours in 8% formaldehyde. Glutaraldehydes, such as Cidex, are often in short supply and very expensive, but they are the only practical sterilants for some instruments, such as laparoscopes, which cannot be heated. Both glutaraldehydes and formaldehyde require special handling and leave a residue on treated instruments; therefore, rinsing with sterile water is essential if the item must be kept sterile. Also, if not rinsed off, this residue can interfere (cause sticking) with the sliding parts of the laparoscope and cloud the lens.
Although formaldehyde is less expensive than glutaraldehydes, it is also more irritating to the skin, eyes and respiratory tract and is classified as a potential carcinogen (Rutala 1996). When using either glutaraldehydes or formaldehyde, wear gloves to avoid skin contact, wear eyewear to protect from splashes, limit exposure time and use both chemicals only in wellventilated areas (Clark 1983). As items are unwrapped after chemical sterilization, they should be transported and stored in a covered, sterile container.
• Glutaraldehydes and formaldehyde solutions are not readily inactivated by organic materials.
• Both can be used for items that will not tolerate heat sterilization such as laparoscopes.
• Formaldehyde solutions can be used for up to 14 days (replace sooner if cloudy); some glutaraldehydes can be used for up to 28 days.
• Glutaraldehydes and formaldehyde are chemicals that cause skin irritation; therefore, all equipment soaked in either solution must be thoroughly rinsed with sterile water after soaking.
• Because glutaraldehydes work best at room temperature, chemical terilization cannot be assured in cold environments (temperatures less than 20oC/68oF), even with prolonged soaking.
• Glutaraldehydes are expensive.
• Vapors from formaldehyde (classified as a potential carcinogen), and to a lesser degree glutaraldehydes, are irritating to the skin, eyes and respiratory tract. Wear gloves and eyewear, limit exposure time and use both chemicals only in well-ventilated areas.
• Formaldehyde cannot be mixed with chlorine or chlorinated water because a dangerous gas (bis-chloromethyl-ether) is produced.
5. Other Sterilization Methods
a. Gas Sterilization
The use of formaldehyde gas for killing microorganisms was practiced before the turn of the century. One of the first uses of formaldehyde gas was to fumigate rooms, a practice long since shown to be ineffective and unnecessary (Schmidt 1899). There are, however, automatic, low temperature steam formaldehyde sterilizers that are effective and can be used to process heat-sensitive instruments and plastic items. As mentioned previously, because formaldehyde vapors are irritating to the skin, eyes and respiratory tract, the use of formaldehyde in this form should be limited.
In the United States and several other countries, ethylene oxide (ETO) gas is used for sterilization of heat- and moisture-sensitive surgical instruments, such as plastic devices and delicate instruments. Sterilization using ETO, however, is a more complicated (requires a 2-hour exposure time and a long aeration period) and expensive process than either steam or dry-heat sterilization.6 In addition, it requires sophisticated equipment and skilled staff specially trained for its safe use, making it impractical for use in many countries (Gruendemann and Mangum 2001).
ETO is hazardous to healthcare workers, patients and the environment. Because ETO is moderately toxic when inhaled, regular exposure to low levels (greater than 1 part per million) may produce harmful effects in humans. Moreover, the gas is irritating to the eyes and mucous membranes, and residual ETO on instruments can cause skin injuries and inflammatory reactions in patients. Finally, because ethylene oxide, a toxic product, is classified as a potential carcinogen as well as a mutagen, disposing of it is difficult (Gruendemann and Mangum 2001).
b. Ultraviolet Light Sterilization
Ultraviolet (UV) light has been used to help disinfect the air for more than 50 years (Morris 1972). For example, UV irradiation can interrupt transmission of airborne infections in enclosed indoor environments where living conditions are poor and people are crowded together. Because UV irradiation has very limited energy, UV light does not penetrate dust, mucous or water. Therefore, despite manufacturers= claims, it cannot be used to sterilize water. Although in theory intense UV light can be both bactericidal and viricidal, in practice only limited disinfection of instruments can be achieved. This is because the UV rays can kill only those microorganisms that are struck directly by UV light beams. For surfaces that cannot be reached by the UV rays (e.g., inside the barrel of a needle or laparoscope), any microorganisms present will not be killed (Gruendemann and Mangum 2001).
Other disadvantages of UV:
• It requires a reliable source of electricity.
• It is not effective in areas of high relative humidity.
• UV bulbs require frequent cleaning to remain effective.
• Exposure to UV rays can burn the skin and eyes.
As a consequence, UV irradiation is neither a practical nor effective method
in most situations (Riley and Nardell 1989).
6. Other Chemical Sterilants
a. Paracetic acid (peroxyacetic acid). The acid is rapidly effective against all microorganisms, organic matter does not diminish its activity and it decomposes into safe products. When diluted, it is very unstable and must be used with a specially designed automatic sterilizer (APIC 2002). It is usually used for sterilizing different types of endoscopes and other heat-sensitive instruments.
b. Paraformaldehyde. This solid polymer of formaldehyde may be vaporized by dry heat in an enclosed area to sterilize objects (Taylor, Barbeito and Gremillion 1969). This technique, called “self-sterilization” (Tulis 1973), may be well suited for sterilizing endoscopes and other heat-sensitive instruments.
c. Gas plasma sterilization (hydrogen peroxide based). This method can sterilize items in less than 1 hour and has no harmful by products. It does not penetrate well, however, and cannot be used on paper or linen. A specialized sterilizer is required for performing gas plasma sterilization (Taurasi 1997).