Log Reduction

Similarly, when calculating the magnitude of the change in cell number, microbiologists often use a logarithmic scale (log scale). Simply speaking, taking the log value of a large number, such as the number of cells destroyed in a disinfectant test, transforms it into a smaller one that is easier to work with.

Understandably, this “scientist shorthand” often prompts questions concerning how to translate log reductions to percent reductions and vice versa.

A series of true mathematical statements showing a pattern is presented below. If you can identify the pattern, then you are well on your way to understanding the relationship between log reduction and percent reduction.

• • • 1 log reduction = 90% reduction
    • 2 log reduction = 99% reduction
    • 3 log reduction = 99.9% reduction
    • 4 log reduction = 99.99% reduction
    • 5 log reduction = 99.999% reduction
    • 6 log reduction = 99.9999% reduction  (Next best products on the market)
    • 9 log reduction = 99.9999999% reduction (One thousand times more effective than 6 log reduction)

As the series demonstrates, if a log reduction is a whole integer, then its numerical value equals the number of nines in the percent reduction figure.

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