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The Log Kill Reduction


To keep the individual numbers manageable, microbiologists usually express them using scientific notation.

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