A3.12 Understanding log₁₀ reductions

To keep large numbers manageable, these are often expressed using scientific notation. When calculating the magnitude of change in cell numbers, a logarithmic scale is often used. As the series demonstrates, if the log reduction is a whole integer, the numerical value equals the number of nines in the percent reduction figure.

• 1 log reduction = 90%

• 2 log reduction = 99%

• 3 log reduction = 99.9%

• 4 log reduction = 99.99%

• 5 log reduction = 99.999%

Similarly, the relationship can be expressed in terms of showing the number of organisms eliminated during the treatment process:

• 1 log reduction = 10 times smaller

• 2 log reduction = 100 times smaller

• 3 log reduction = 1 000 times smaller

• 4 log reduction = 10 000 times smaller

• 5 log reduction = 100 000 times smaller