Expert: Paul J. Sordellini - 9/17/2004

Question
As defined by ISO 11135:1994, EO exposure time is the time for which the sterilizer chamber is maintained within the specified range for temperature, sterilant concentration, pressure and humidity. During Method B execution can EO exposure time be defined as : time begins when ethylene oxide injection begins to enter into the chamber, and ends with the completion of the evacuation of the sterilant from the chamber.

Answer
The definition according to 11135 is correct for routine processing of medical supplies in a validated ethylene oxide cycle. If you validate an exposure time, at a certain range of temperature and gas concentration, then you must run every process at the same exposure time and under the same conditions. A Method A or B validation study involves fractional cycles. Since you are looking for partial (incremental) lethality of the indicator organism, then you must consider the contributory lethality that occurs during EO injection and during the post-dwell evacuations. While "exposure time" is confined to the dwell period, in a Method a and B validation you need to consider "exposure" as including gas contact during injection and evacuations. If gas injection is too slow, and the post evacuations are too slow, instead of fractional lethality you may end up with total lethality that wold defeat the purpose of the validation (lethality curve generation).
I hope this helps.
Paul