Dry heat sterilization is a process utilized for heat-stable items (glass, stainless steel, nonaqueous liquids, powders, etc.) that are unsuited for steam sterilization because of either an absence of water (nonaqueous liquids and powders) or requirements for absolute dryness following processing (product contact parts for nonaqueous products). Because dry heat relies on air for the transfer of heat to and from the load items, the process takes longer than a steam process for a comparable size item or load. Lengthy heating and cooling periods require that the load items be unaffected by heat over a long period of time and also require the use of the overkill method for cycle development and validation.

Dry heat sterilization is typically performed in the range of 160–190 where the objective is sterilization rather than depyrogenation. (Depyrogenation will be covered separately in Dry Heat Depyrogenation 1228.1). In dry heat sterilization, hot air is in direct contact with the load items (whether wrapped or unwrapped) and transfers some of its thermal energy. Unlike steam sterilization, in dry heat sterilization there is no phase change of the heating medium, and thus heat transfer is less efficient. The items can be stainless steel, glass, ceramic, or other heat-stable materials and may be wrapped or covered with aluminum foil to protect them during pre- and postprocess handling. Dry heat sterilization is commonly used for heat-stable materials (e.g., petrolatum or powders).

The limited heat transfer capacity of air requires that items in the oven be placed in locations that were confirmed to be acceptable during the validation effort. Manufacturers should exercise caution with varying load sizes because in some instances (resulting from system design and control probe positioning) minimum load sizes may present a worst case.

Process equipment for dry heat sterilization is controlled by calibrated temperature sensors. During the exposure portions of the cycle, attainment of a minimum dwell time at a predefined temperature is used to document process lethality. Cycle efficacy for dry heat sterilization customarily is measured using FH, which typically is defined as the amount of time the load receives the equivalent of exposure at 170. The FH approach is used to compare sterilization processes that operate at varying temperature conditions to a single standard. The process lethality at temperatures other than 170 can be calculated to determine lethality equivalent to that provided at 170. Sterilizer control systems must deliver conditions within a predefined time–temperature or FH range. Simple mathematics can be used to calculate the total lethality over the course of the process. For the specific reference temperature of 170 and a z-value (for definitions see Steam Sterilization by Direct Contact 1229.1) of 20, the FH calculation can be determined by the following equation:

Accumulation of the lethality (FH) for the sterilization process across the entire cycle (heat-up and cool-down segments included) includes the contribution of those segments and allows the cycle to be defined by a targeted lethality rather than by a time at a defined minimum temperature.

Because dry air has limited heat capacity and dry heat conditions are more variable than those encountered with other thermal sterilization methods, analysts routinely validate their dry heat sterilization procedures using the overkill method as defined in Sterilization of Compendial Articles 1229.

Overkill sterilization can be defined as a method in which the destruction of a high concentration of a resistant microorganism supports the destruction of reasonably anticipated bioburden that could be present during routine processing. That objective can be demonstrated by attaining any of the following: a defined minimum lethality; a defined set of method conditions; or confirmation of minimum log reduction of a resistant biological indicator.

The validation requirements for the overkill method are less onerous than those of the other sterilization approaches. Because the load items can withstand substantial amounts of heat without adverse consequence, the greater lethality provided by the overkill method clearly is justifiable.

As with all sterilization processes, after the dry heat sterilization process has been validated, it must be subject to formalized controls that keep it in a validated state over time. General chapter Sterilization of Compendial Articles 1229 details the general practices that are appropriate for all sterilization systems. This is accomplished by a number of related practices that are essential for the continued use of the process over an extended period of time. The essential practices to maintain validated status include calibration, physical measurements, physical integrators and indicators, ongoing process control, change control, preventive maintenance, and periodic reassessment and training.