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Decontamination Process Validation
Understanding Thermal Disinfection

Lethality (F0) in Thermal Sterilisation


The Need for Accuracy and Specialist Equipment

A thermal steriliser supplies a predetermined quantity and type of heat energy in a controlled environment aiming to keep the degradation of the material at a minimal, to ensure a suitable eradication of micro-organisms.

To understand lethality in thermal sterilisation, it is important to know what sterile is. A product is considered sterile when it is free from viable micro-organisms. Sterilisation is the physical or chemical process which destroys all life forms to achieve a sterile product, this is when the chance of survival of any micro-organisms is reduced to 10-6  irrespective of the number and resistance of the micro-organisms.


What is meant by the term F0 or Equivalent value of energy?

To achieve a lower rate of survival of the micro-organisms a lethal agent is required. The time of exposure to a lethal agent required to cause a selected reduction in the survivorship of micro-organisms is known as lethality (F0). The energy value is calculated and describe in terms of F0.

Penetration test can be used to calculate the lethality for containers studied using the normal production cycle. It is worth noting that no one autoclave runs an exactly similar cycle to another. The amount of energy accrued at the start of the cycle as the autoclave heats up to the holding time temperature along with the precise temperature fluctuations in the chamber can mean varied energy transposed from one similar cycle to another.

If a cycle is 1 minute at 121.1C with saturated steam, then the energy accumulated is equivalent to the energy value of 1 F0. This would mean a cycle of 20 minutes at 121C would have an energy value of 20 F0.

F0 is a calculated variable where the measured temperature inaccuracies have an exponential effect.

F0= The Accumulated Lethality Over the Data Collection Period

F0 Calculation


∆t = 60 Sec. = 1min. - The time interval between temperature data

10 = The z value usual for steam sterilisation

T = The actual temperature for sterilisation

121 = The usual base temperature for steam sterilisation

Z = 10°C