Logarithmic (log₁₀) levels

The effectiveness of measures aimed at reducing microorganisms is often expressed in so‑called logarithmic (log₁₀) reduction levels. One log₁₀ level corresponds to a tenfold reduction in microbial count. [1]

This means:

1 log₁₀ reduction = 90 % fewer microorganisms (factor of 10)

2 log₁₀ reduction = 99 % fewer microorganisms (factor of 100)

6 log₁₀ reduction = 99.9999 % fewer microorganisms (factor of 1,000,000)

This system is typically used to illustrate the differences between cleaning, disinfection, and sterilization: [2]

Cleaning generally achieves reductions of 1 to 2 log₁₀ levels (90 % to 99 % reduction).

Disinfection usually requires a reduction of around 3 log₁₀ levels (99.9 % reduction). In healthcare and medical technology, however, many applications require an acceptance criterion of at least 5 log₁₀ levels (99.999 % reduction), as defined in standards such as DIN EN 1040, DIN EN 13727, DIN EN 14561.

Sterilization means that all microorganisms (both living cells and highly resistant spores) are completely removed or destroyed. A sterilization process is considered successful when the number of spores is reduced by at least 6 log₁₀ levels (99.9999 %) within half the duration of a cycle of a sterilization procedure. [3]

Note on disinfection and sterilization:

In addition to the achieved log₁₀ reduction, the type of microorganisms targeted by the decontamination process also plays an important role. For example, during disinfection it is not necessarily ensured that microbial spores are inactivated, whereas this is mandatory in a sterilization process. If, for instance, a microbial reduction of more than 6 log₁₀ levels is achieved but spores survive the process, this is referred to as high‑level disinfection. [4]