Four Basic Factors to Microbial Kill

EO Concentration

• Commonly 400 and 700 mg/l
• As EO concentration increases at a given temp and RH,
  –microbial inactivation (kill) rate increases
• Killing concentration required at the site where the microbes (BI’s) are located

Water Vapor (Humidity)

Required for EO to react with the critical cell molecules
• Generally measured as Relative Humidity

Temperature

• Kill rate increases with temperature
  –D-value decreases (time) with temperature
  –Exponential function
• For each 10°C (18°F) rise in temperature, the spore inactivation rate will generally double

Time

• Amount of kill increases with exposure (EO gas dwell) time
• 90 percent of surviving microbes are killed for each D-value time in gas dwell

D Value ?

• Determined to prove predictable logarithmic death kinetics of challenge microorganisms (BIs) or natural bioburden
• “Decimal” Reduction Time - Measure of the biological organism’s resistance to the sterilant
• Time in minutes necessary to reduce (kill) a microbial population by one logarithm or 90%
• A Simple D-Value Calculation
  Time

-----------------------------

Log of Starting Population – Log of Final Population