INTRODUCTION

Sterilization can be accomplished using sporicidal agents suspended in air (i.e., vapor). Sterilizing agents that operate in this fashion include hydrogen peroxide (H₂O₂), peracetic acid (CH₃CO₃CH), formaldehyde (CH₂O), and glutaraldehyde [CH₂(CH₂CHO)₂] in aqueous solution. At room temperature these are liquids or solids that can be vaporized for introduction into a vessel or chamber. They differ from sterilizing gases and liquids in that there are multiple phases within the vessel during sterilization. Vapor sterilization systems are well suited for heat-sensitive materials and surface sterilization. Items exposed to the process should have their surfaces exposed to the greatest extent possible. Vapor sterilization processes require appropriate sterilant concentration, temperature, and relative humidity, all of which may be variable during the exposure period. Because the agent is ordinarily supplied as an aqueous solution, moisture is introduced with the agent. The consequences of variation in these parameters may be localized differences in relative humidity, agent concentration, and condensation rates on the surfaces to be treated, resulting in variations in process lethality. The parameters to be established include sterilant amount (usually derived from injection quantities), relative humidity, and temperature. There is no demonstrated correlation between gas phase conditions, surface conditions, and microbial kill. For this reason, online monitoring of vapor phase concentration is not widely utilized as a control parameter. Efforts to develop a standardized biological indicator for vapor systems have been hampered by the multiphasic nature of these sterilants. Selection of the appropriate biological indicator (BI) and resistance should be based on experimentation within the user's system. Only under well-defined, specific conditions (e.g., agent concentration, humidity level, temperature, substrate, and phase) can a reliable D-value be established (for a definition of D-value, see Sterilization of Compendial Articles 〈1229〉).