To keep the individual numbers manageable, microbiologists usually express them using scientific notation.
Similarly, when calculating the magnitute 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 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
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.
So, if you get a study report from our lab indicating a 2.5 log reduction, then you know it corresponds to a percent reduction somewhere between 99% and 99.9%. However, because of the way log scales work, a 2.5 log reduction does not equal a 99.5% reduction.