Thursday, April 16, 2009

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


  • 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


  • 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


Log of Starting Population – Log of Final Population

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