Fh value Calculation in Dry heat sterilization

This article contains the Fh value calculation, Difference between F0 and Fh value, What is mean by Dry heat sterilization and Depyrogenation.

What is Mean by Fh value and its Calculation :

There are different types of sterilization process i.e. Moist Heat sterilization (Steam sterilization), Dry Heat Sterilization, Ethylene Oxide Sterilization, Gamma rays sterilization and Depyrogenation, etc.

Adopting right method for sterilization depends on the material being sterilized and the purpose of sterilization.

Steam sterilization is done under high temperature and high pressure in moist conditions. Hence it takes less time to sterilize.

Whereas, dry heat sterilization is done under high temperature and in dry condition, hence it takes more time to sterilize or remove/Reduction of all types of microorganisms.

Most of these techniques carry their own effectiveness values.

In F0 value article, we established an exposure equivalency for steam sterilization (compared the ideal and actual exposure times).thus, the F0 value is called Equivalent Exposure Time.

Here, we will discuss about Fh value calculator which is in analogy with the above. But in terms of dry heat, the Fh value is called Heat Penetration Factor.

Though the F0, Fh, Fd values are similar in structure, their application is different.

Simply put:

Value Values used in
F0 Moist Heat Sterilization/Steam Sterilization
Fh Dry Heat Sterilization
Fd Depyrogenation (discussed further…)

Logically speaking, sterilization is successful when the product contact parts are free of viable microorganisms.

Now coming to the z-value. if you know, We assumed it as 10°C for the F0 value in Moist heat Sterilization or steam sterilization when the exact microorganism was unknown.

Similarly, in the Fh concept, the z-value should be assumed as 20°C.

When it comes to moisture-sensitive items, steam sterilization is not suitable and may damage the accessories or affect on drug products/Drug substances.

In such cases, Dry Heat Sterilization is the more appropriate option given that the materials withstand higher temperatures than steam sterilization conditions.

Formula For Fh Value calculation :

The following formula is used to calculate the Fh value in dry heat sterilization.

Fh Value Calculator
Formula: Fh Value (Heat Penetration Factor)

Δt – Sterilization Hold Time in Minutes
T – Average Temperature of all probes during a sterile hold in °C
z – Temperature Coefficient in °C

The term next to Δt in the above equation is lethality rate.

For steam sterilization, the term right to “T” was 121.1°C whereas, in dry heat sterilization, it is 170°C and the z-value should be assumed as 20°C for dry heat Sterilization.


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