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:

• Tr (°C) (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 (°C): 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.
• F0 Target: the target of lethality result that you want to obtain from your process expressed in minutes and seconds
• Start Let. Cal. (°C) (Start Lethality Calculation): the minimum temperature from which to start the Lethality calculation. All contributions to the final result recorded with temperatures lower than the one set in the field will not be taken into consideration.

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)
• FT: where T 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