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Infection Prevention
A Primer on Dry Heat Sterilization
Dan Mayworm
Publish Date: June 9, 2008   |  Tags:   Infection Prevention
Most devices in health care facilities can be sterilized with steam, ethylene oxide, plasma, or peracedic acid. But certain items and materials cannot tolerate moisture and others are impervious to steam. Some examples include:
  • non-aqueous liquids or semi-solids such as glycerin, oils, petroleum jelly, and waxes;
  • powders such as talc and sulphonamides; glassware;
  • stainless steel, particularly instruments that have facing surfaces that are difficult to reach with steam and instruments that dull and/or stain when subjected to steam and its impurities; and,
  • water and aqueous solutions, like IVs.

For these items and materials, there is another choice-dry heat sterilization. But before using dry heat, it's important to understand its limitations. Here are three of the most important ones:
  • Use dry heat only when the use of steam would be impractical and/or ineffective. Steam must be able to contact all surfaces of the item being sterilized.
  • Use it only with materials that can withstand temperatures at least as high as 170o C (338o F). Temperatures above 170o C are required to kill endotoxins (pyrogens) as well as some bacterial spores. Some of the new dry heat sterilizers can attain temperatures up to 210o C (410o F).
  • Dry heat sterilize water and aqueous solutions like IVs only if they are in glass bottles or plastic pouches designed for this purpose.

For a primer on dry heat sterilization, read on.

How it works
The three mechanisms of heat transfer in dry heat sterilization are conduction, convection, and infrared radiation. Heat penetration of materials via conduction is dependent in part upon how much heat is in the surrounding medium, which is air. Air has poor thermal conductivity properties. At any given temperature, it takes about six times as much air as steam to deliver a given amount of heat to an object. An exposure time of 60 minutes at 160o C (320oF) for dry heat is approximately the equivalent of 15 minutes at 120o C (248o F) for steam. Due to this low heat capacity and heat transfer rate of air, convection oven sterilizers have many serious drawbacks. Heating is slow, long sterilization times at very high temperatures are required, maintaining reproducible cycles is difficult, and charring of materials can occur. Today, these devices are mostly used in dentists' offices.

Mechanical convection hot air sterilizers improve heat transfer by employing forced air convection and mechanically moving the air stream. New, recently introduced units operate at very high temperatures (up to 210o C [410o F]) with forced air and convective heating. These units have comparatively fast cycle times and are ideal for microsurgical instrumentation, talc, and petroleum jelly impregnated gauze/dressings. Two notes about these devices:
  • Always check cycle parameters against the manufacturer's instructions for the particular load configuration and items being sterilized.
  • Ensure that the packaging is appropriate and that the device itself can be safely sterilized using dry heat. Items not designed for these very high temperatures can be seriously damaged.

Preparation and packaging
Attention to detail is paramount when preparing items for dry heat sterilization. Wherever possible, only sterilize what is required for single-use applications of liquids or powders. Also, try to clean, decontaminate, and dry reusable items that you plan to dry heat sterilize as soon as possible after use.

Packaging for dry heat sterilization should:
  • allow for adequate heat penetration of all surfaces of the item being sterilized,
  • provide an effective barrier to recontamination,
  • be free of toxic ingredients and non-fast dyes,
  • be able to withstand the planned sterilization temperatures without distortion or melting.

The two main packages for dry heat sterilization are pouches that can be made either out of paper or nylon film (either as preformed pouches or as seamless tubing roll stock)1 and metal cassettes or containers. Paper or film pouches are normally used only for single instruments, small amounts of talc, or small treated dressings. Do not use the common paper/plastic pouch because the plastic side will melt at dry heat sterilization temperatures. Only use all-paper pouches specifically designed as sterilization pouches or all-nylon pouches.

The design of cassette and container systems varies widely, so it is imperative to follow the manufacturer's processing recommendations, particularly as it applies to dry heat sterilization. Do not confuse steam heat recommendations with dry heat recommendations.

For sterilization to be effective, the items being sterilized should be thin and flat and have a maximum exposed surface. For example, if you are sterilizing talc, use a large pouch and spread out the talc as thinly as possible. If you are sterilizing a dressing, lay it flat instead of rolling it. Refer to Fig. 1 to see the time required to heat specific quantities of powder or Vaseline from room temperature to 320o F in a hot air sterilizer.

Loading and unloading the sterilizer
Make sure the devices you are sterilizing are clean and dry before placing them in the sterilizer. The sterilizer's efficiency depends upon the unobstructed flow of hot air over the items being sterilized, so placement within the sterilizer is important. Place unpackaged items, such as single instruments, on a shallow aluminum tray to enhance the rate of heating through the heat-conducting tray and to facilitate removal from the sterilizer. Use heavy gloves or an insulated handle when removing these hot trays, containers or packages.

Sterilization cycles
It goes without saying that it is critical to follow the sterilizer manufacturer's instructions when sterilizing with dry heat. Also, consult with the manufacturer of any devices you intend to sterilize to make sure it is safe to use this method. A few notes about specific materials and instruments:
  • Cataract knives and keratomes can be dry heat sterilized in one hour at 320o F. Anything above 320o F could dull these instruments, so it is critical to have good temperature monitoring of the cycle.
  • If you have occasion to reuse needles, dry heat sterilization is the preferred method; steam can cause significant rusting. Simply wrap the needles in gauze and a single layer of muslin and sterilize for a minimum of an hour at 320oF.
  • If you use sterile talc in surgical procedures, you should know that dry heat is about the only safe and effective way to ensure that all the granules of talc are sterile. Place only the amount needed for a single procedure in a paper or nylon pouch so that the thickness of the talc, when the pouch is laid flat, is no more than 1/4 inch deep. Three hours at 320o F should be adequate.
  • You can dry heat sulphonamides as long as you keep the maximum temperature below 320o F. A safe margin would be 285 to 300o F for three hours.

Monitoring the sterilization cycle

The three types of indicators used to monitor dry heat sterilization are:

Chemical indicators: These indicators have very little value in dry heat sterilization. Most chemical indicators will change color far in advance of the conditions necessary for dry heat sterilization. You should have a suitable indicator on each package to at least tell you that the package has gone through the cycle.

Biological indicators: Use a biological test pack at least weekly for every sterilizer and with each cycle containing implantables. Since there is no commercially available test pack for dry heat sterilizers, I recommend that you design one that is representative of the type of dry heat sterilization you will be doing. Suppose the only items you dry heat sterilize are single-use portions of talc in nylon pouches. Once a week, insert a suitable biological indicator in the pouch with the talc and record the results. I know of only one type of biological indicator that is available for dry heat sterilization. These are filter paper strips impregnated with Bacillus subtilis var. niger. These strips come packaged in glassine envelopes. Their main disadvantages are that the paper and glassine become marginal at temperatures above 400o F, they require aseptic culturing techniques, and there is a relatively long period before the results are known.

Physical indicators: Measuring and recording the physical parameters of your desired cycle is extremely important in dry heat sterilization. Physical equipment monitoring is the most reliable and user-friendly indicator for dry heat sterilization. Modern table-top dry heat sterilizers have incorporated improved heat transfer methods using mechanical air circulation, high speed laminar flow, and higher process temperatures. Extensive validation testing required by the FDA, in addition to these improved manufacturing techniques, make the new sterilizers very reliable.

A final note: If you don't use dry heat sterilization very often, you may be able to use a steam-jacketed steam sterilizer. While it is true that the jacket temperature (and presumably the air immediately adjacent to the wall) are kept at approximately 250 to 255o F, there is no reliable means of checking temperature conditions at various locations within the chamber. As a result, this is an inefficient method and is best used with an overnight sterile cycle using a sterilizer that would otherwise be shut down. If you plan to sterilize petrolatum oils or jellies using this method, you must be extra vigilant. It is not uncommon for leakage to occur when you are sterilizing lubricated gauze with hot air. If this occurs you must carefully clean all sterilizer internal surfaces to remove all traces of the oil or grease. If you do not, the residual oil or grease can leave a thin coat on items during subsequent cycles that would inhibit contact with the steam. Even with all the problems associated with this method, it has been used successfully for many years, since it does provide a wide margin for error.

1. Commercially prepared scalpel blades and some sutures are packaged in aluminum foil packages and dry heat sterilized. Foil is not recommended for your use as it is not readily available and presents barrier properties that are uncommon to hospital sterilizers.

Dan Mayworm is the former publisher of the Journal of Healthcare Resource Management and Infection Control