Using High-Temperature Formaldehyde Sterilization as a Model for Studying Gaseous Sterilization

Abstract

This study uses the high-temperature formaldehyde sterilization system provided by the Harvey Chemiclave, manufactured by Barnstead Thermolyne Corporation (Dubuque, IA), as a model to investigate certain phenomena associated with gaseous chemical sterilization systems. Although formaldehyde sterilization presents some unique and complex system attributes, the current studies provide helpful insights into general sterilization methods by chemicals in the gaseous state. Both population recovery and fraction negative (FN) techniques were used to assay surviving populations from biological indicators of the organism Geobacillus stearothermophilus following exposure to incremental Chemiclave cycles. Models 5500 and 6000 of the Barnstead/Thermolyne Chemiclave were used in the study. Reusable instruments such as scalers, explorers, and various hinged pieces were tested in minimum versus maximum load studies. Population recovery study results demonstrated that lethality rates increase with time throughout the Chemiclave sterilization process and that there are significant variations in lethality according to load location. The population recovery data in conjunction with the FN studies and temperature data confirm that one-half the full-cycle time is not a good estimator of one-half the full-cycle lethality because lethality curves are concave downward and lethality varies by load location. This conclusion can also be applied to other types of gaseous, chemical sterilization such as ethylene oxide. The work outlined in this study was a result of investigations into the parameters affecting formaldehyde chemical vapor sterilization with the Harvey Chemiclave sterilizer. During these studies, it became apparent that results clearly depicted the effects of continued acceleration of the rate of microbial lethality, as well as variations in delivered lethality as a function of position in the sterilizer load. This publication focuses on these observations because they are important considerations for understanding general concepts of sterilization efficacy in process applications. Erroneous conclusions can be drawn when one evaluates sterilization without a thorough understanding of affecting variables.