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What is Lethality?

Lethality is a time and its value is used to evaluate a thermal process such as sterilisation or pasteurisation.
The variables that have to be set in the formula are:

  • Reference temperature: the theoretical temperature at which the process should stay constantly. Usually the given information are: “I sterilize at 121°C for 10 minutes”. 121°C is the reference temperature.
  • Z: this is also a temperature, indicating the increase of temperature to be applied to the Reference temperature to reduce the time to a tenth. This parameter is connected to a specific microorganism. For example, if it takes 10 minutes at 121°C to have a specific reduction of a specific microorganism and the Z value is 10°C for that specific microorganisms, it will take 1 minute at 131°C to obtain the same reduction.

Depending on the parameters used in the formula and the purpose of the thermal process it may have different names:

  • F0: standard sterilisation, when the Reference temperature is 121,11°C (250°F) and the Z value is 10 (botulinum)
  • FN: where N stand for a Reference temperature different from 121°C. It might be F134, F110 and so on. Z value might also be different from 10
  • PU: Pasteurisation Units, indicating a process with a Reference temperature usually below 100°C.
  • A080: used for washer disinfectors with a Reference temperature of 80°C
  • A090: used for washer disinfectors with a Reference temperature of 90°C
  • A0T: used for washer disinfectors with a Reference temperature that can be set by the user
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