6: DRY HEAT STERILIZATION

by - Pauline W. Fallis,

Dry heat sterilization is probably the least used and most misunderstood method of sterilization.
It is usually used for heat safe items that can not be disassembled or may rust. Sterilizing by dry heat is accomplished by conduction. The heat is absorbed by the outside surface of the item, then passes towards the centre of the item, layer by layer. The entire item will eventually reach the temperature required for sterilization to take place. Death of microorganisms occurs in dry heat by the process called oxidation. In other words, the slow process of coagulating the protein of the cell. The sterilization process is accomplished in HOT AIR OVENS, and it is longer than in steam sterilizers, due to the lack of moisture.
There are two types of hot air ovens used in dry heat sterilization. Those that use gravity convection and those that use mechanical convection. Convection is the term used for the circulation of the heated air within the chamber of the oven.
In Gravity Convention Ovens, air is heated and rises. As you know, heated air expands and possesses less density, thus less weight than cooler air. Therefore heated air rises. The cooler air falls as it is displaced by the rising heated air. Due to this rising and falling of heated and cool air the temperature tends to be uneven in the chamber. Therefore, sterilizing in a gravity convection oven is sometimes difficult, because you cannot be sure that you will uniformly achieve the required minimum temperature for the required minimum time. Due to this temperature variation, monitoring a gravity convection oven is often difficult. Examples of gravity convection ovens are a regular kitchen oven, or a toaster oven.
Mechanical Convection Ovens, are the most effective type. The oven contains a fan or blower which continually circulates the heated air to maintain a uniform temperature throughout the chamber. Most commercially available dry heat sterilizers are of this type. An example is the home convection oven.
Like other methods of sterilization there are both advantages and disadvantages for using dry heat as a method of sterilization. Advantages:

• Dry heat can sterilize items that can not be sterilized in steam or chemical sterilizers, such as powders and oils, or those that are prone to rust.
• Dry heat can be used for glassware, as it will not score or erode the surface as, steam might do.
• Dry heat will not corrode or rust instruments or needles.
• Dry heat will sterilize instruments containing many parts that can not be disassembled. Disadvantages:
• Dry heat penetrates slowly and unevenly.
• Dry heat requires long exposure times to effectively achieve sterility.
• Dry heat requires higher temperatures that many items cannot be safely exposed to.
• Dry heat requires specialized packaging materials that can sustain integrity under high heat conditions.
• Dry heat may require different temperature and exposure times, depending on the type of item being sterilized.

As in all methods of sterilization, all items must be clean and free of all types of visible soil. Due to its high temperature, specific packaging materials must be used. Only four types of packaging materials may be used in hot air ovens. They are glass such as petri dishes, test tubes and small jars, stainless steel trays or pans with lids. Cotton wrappers or aluminum foil may be used, if the temperature within the chamber does not exceed 204°C. , (or 400°F.) In order to sterilize effectively, the hot air must circulate freely throughout the chamber. Packages must be kept away from the walls, from each other, and should be of a similar nature, size and thickness. Never overload the oven. The manufacturer's recommendations as to the preparation, packaging and loading of the sterilizer must always be followed. Exposure times will vary depending on the how the items are packaged, the temperature, types of items being sterilized, and depth of substances in the container. The most common temperatures used are 162.8°C (325°F) for 90 minutes, and 160°C (320°F) for 120 minutes. When monitoring biologically, a spore test must be done at least weekly. When a gravity convection oven is used, it is recommended that a spore test be done with each load. The spore used for testing dry heat sterilizers is Bacillus Subtilus. As in all spore testing, a negative result means that the sterilization cycle has been successful. Periodic inspection, cleaning and maintenance should be done according to the manufacturer's recommendations. Cleaning and preventative maintenance reduces possible equipment malfunctions that could lead to sterilization failures. However, there is usually very little preventative maintenance required for hot air ovens. Monitoring the accuracy of the thermostats, the motor for the fan, the electrical cords and plugs, is really all that is required. Dry heat sterilization can be an effective method of sterilization, using gravity or mechanical convection ovens. As in all sterilizers, correct time, temperature, packaging, routine monitoring, cleaning and preventative maintenance, will assure that the items that you sterilize, will be sterile. References AMMI, Table top dry Heat (heated Air) Sterilization and Sterility Assurance in Dental and Medical Facilities (March 1992 draft), The Association for the Advancement of Medical Instrumentation, Arlington VA, 1992 Dornette, W.H.L. ed, Central Service Technical Manual, International Association of Hospital Central Service Management, Chicago, Illinois, 1981 Fallis, P.W., Handbook on Infection Control in Office-based Health Care and Allied Services, Canadian Standards Association, PLUS 1112, Toronto, 1994 Mahan, J.L., Ebersole, M.E., Infection Control Standards for the Practice of Electrology, The American Association of Electrology, USA, 1991 Whipple, L., Helgeson, J., Infection Control Practices and Sterilization Standards, The Society of Clinical and Medical Electrologists, Inc., Massachusetts 1993