Achieving a Sterility Assurance Level (SAL) is paramount in healthcare and laboratory environments. For methods like steam sterilization (autoclaving), precise control over cycle parameters is necessary for inactivating microbial life. The timing of when the exposure timer begins is fundamental, as an incorrect start time can lead to process failure and an under-processed load. Understanding the specific point at which the clock starts ensures items are sterile and safe for use.
The Sequential Phases of Sterilization
The sterilization process is not a single, instantaneous event but a carefully controlled sequence of distinct phases within the sterilizer chamber. A typical steam cycle begins with the Conditioning phase, where air is actively removed from the chamber and the load, often using a series of vacuum and steam pulses. The presence of residual air is detrimental because it prevents saturated steam from reaching all surfaces, which would result in a failed sterilization.
Following air removal, the Heat-Up phase begins, during which steam is continually introduced to raise the temperature of the chamber and the items inside. This period of temperature increase is often referred to as the “come-up time,” which prepares the load for the actual microbicidal action. The goal of these initial phases is to create a condition of saturated steam at the correct pressure and temperature throughout the entire sterilization vessel.
The Exposure phase, also known as the Hold Time, is the period where the required microbicidal action takes place. During this phase, the temperature and pressure are maintained at a fixed set point for a specified duration, ensuring the destruction of microorganisms. This time interval is governed by industry standards and is determined by the type of item being sterilized and the required Sterility Assurance Level.
Once the exposure time is complete, the cycle moves into the Exhaust and Drying phases. During the exhaust phase, steam is released from the chamber, and the pressure is gradually lowered. The final drying stage uses a vacuum to remove residual moisture from the load and packaging, which prevents recontamination of the items when they are removed from the sterilizer.
Criteria for Initiating the Exposure Timer
The initiation of the exposure timer must only begin when two specific, simultaneous criteria have been met. First, the chamber must reach the minimum required sterilization temperature, typically 250°F (121°C) or 270°F (132°C), depending on the cycle. This temperature must be accurately measured by sensors within the sterilizer.
The second criterion is that the entire sterilization load must have achieved temperature equilibration. This means the required temperature has stabilized not just at the chamber drain, but at the coldest, most difficult-to-reach location within the load. The time it takes for the entire load to reach the target temperature after the chamber has done so is called the equilibration time.
The timer starts only after the temperature at the most challenging point in the load has equalized with the set sterilization temperature. Starting the timer prematurely, before the entire load is saturated with steam and at the correct temperature, results in under-processing. This failure means the required lethality is not delivered, jeopardizing the sterility of the instruments.
Modern sterilizers manage this by monitoring the corresponding pressure, as the specific temperature of saturated steam directly correlates to a specific pressure level. If the pressure or temperature drops below the set point at any time during the exposure phase, the timer will automatically stop and reset. This safeguard ensures that the entire load receives the full duration of microbicidal action necessary for sterilization once the parameters are restored.
Documentation and Verification of Start Time
Confirmation that the exposure timer was initiated and maintained correctly is accomplished through physical monitoring of the cycle. Sterilizers generate a permanent record, typically a printed chart or digital log, that displays the time, temperature, and pressure throughout the entire cycle. This representation allows personnel to confirm exactly when the temperature plateau was reached and how long it was sustained.
This physical documentation must show a continuous, unbroken line or log of the required temperature and pressure for the full duration of the programmed exposure time. Any dip in temperature or pressure during the hold time is immediately visible on the printout, signaling an interruption and a potential cycle failure. The printout therefore verifies the precise moment the exposure timer began and how the conditions were maintained.
In addition to physical monitors, chemical indicators play a role in verifying that the sterilization conditions were met during the hold time. These indicators are sensitive chemical strips or devices that change color or form when exposed to the correct combination of temperature, time, and steam presence. While not confirming the exact start time, they provide an independent, item-level verification that the conditions necessary for the exposure phase were successfully achieved.