Dry heat sterilization
Dry heat sterilization of an article is one of the earliest forms of sterilization practiced. Dry heat, as the name indicates, utilizes hot air that is either free from water vapor, or has very little of it, and where this moisture plays a minimal or no role in the process of sterilization.
The Dry-Heat sterilization process is accomplished by conduction; that is where heat is absorbed by the exterior surface of an item and then passed inward to the next layer. Eventually, the entire item reaches the proper temperature needed to achieve sterilization. The proper time and temperature for Dry-Heat sterilization is 160 °C (320 °F) for 2 hours or 170 °C (340 °F) for 1 hour. Instruments should be dry before sterilization since water will interfere with the process. Dry-heat destroys microorganisms by causing coagulation of proteins.
The presence of moisture, such as in steam sterilization, significantly speeds up heat penetration.
There are two types of Hot-Air convection (Convection refers to the circulation of heated air within the chamber of the oven) sterilizers:
- Gravity convection
- Mechanical convection
Gravity convection process
As air is heated, it expands and possesses less density (weight per unit volume) than cooler air. Therefore, the heated air rises and displaces the cooler air (the cooler air descends). The method of Dry-heat gravity convection produces inconsistent temperatures within the chamber and has a very slow turn over.
Mechanical convection process
A mechanical convection oven contains a blower that actively forces heated air throughout all areas of the chamber. The flow created by the blower ensures uniform temperatures and the equal transfer of heat throughout the load. For this reason, the mechanical convection oven is the more efficient of the two processes.
Instruments used for dry heat sterilization
Effect on microorganisms
Dry heat coagulates the proteins in any organism, causes oxidative free radical damage, causes drying of cells, and can even burn them to ashes, as in incineration.
- Ninemeier J. Central Service Technical Manual (6th ed.). International Association of Healthcare Central Service Materiel Management. p. 159.