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6: DRY HEAT STERILIZATION |
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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
INFECTION CONTROL ARTICLES
- PREPARING FOR THE PATIENT/CLIENT
Spring 2001, Volume 8, Number 1
- PREVENTING INFECTION FOR ELECTROLYSIS
Fall 2000, Volume 7, Number 2
- INFLUENZA
Spring 2000, Volume 7, Number 1
- MICROORGANISMS AND INFECTIOUS DISEASE
Summer 1999, Volume 6, Number 2
- BLOOD BORNE INFECTIOUS DISEASES AND PERSONAL PROTECTION
Spring 1999, Volume 6, Number 1
- MICROORGANISMS & INFECTION
Fall 1998, Volume 5, Number 2.
- BIOLOGICAL TESTING OF YOUR STERILIZATION PROCESS
Spring 1998, Volume 5, Number 1
- STEAM STERILIZATION
Fall 1997, Volume 4, Number 2
- CHEMICAL STERILIZATION
Spring 1997, Volume 4, Number1
- DRY HEAT STERILIZATION
Fall 1996, Volume 3, Number 2
- STERILIZATION
Spring 1996, Volume 3, Number 1
- IS YOUR USE OF NEEDLES SAFE IN YOUR PRACTICE?
Fall 1995, Volume 2, Number 2
- PREPARING INSTRUMENTS FOR STERILIZATION
Spring 1995, Volume 2, Number 1
- WHY ARE GLASS BEAD STERILIZERS NOT RECOMMENDED FOR USE IN ELECTROLOGY?
Fall 1994, Volume 1, Number 2
- STERILIZATION: HOW, WHEN, AND WHAT WITH
Spring 1994, Volume 1, Number1
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