Log reduction is a measure of how thoroughly a decontamination process reduces the concentration of a contaminant. It is defined as the common logarithm of the ratio of the levels of contamination before and after the process, so an increment of 1 corresponds to a reduction in concentration by a factor of 10.In general, an -log reduction means that the concentration of remaining contaminants is only times that of the original. So for example, a 0-log reduction is no reduction at all, while a 1-log reduction corresponds to a reduction of 90 percent from the original concentration, and a 2-log reduction corresponds to a reduction of 99 percent from the original concentration.[1]
Let and be the numerical values of the concentrations of a given contaminant, respectively before and after treatment, following a defined process.It is irrelevant in what units these concentrations are given, provided that both use the same units.
Then an -log reduction is achieved, where
R=log10{cb
For the purpose of presentation, the value of is rounded down to a desired precision, usually to a whole number.
R=-log10{\left(
| 0.725 | |
| 580 |
\right)}=-log10{0.00125} ≈ 2.903
Rounded down, is 2, so a 2-log reduction is achieved.
Conversely, an -log reduction means that a reduction by a factor of has been achieved.
Reduction is often expressed as a percentage. The closer it is to 100%, the better.Letting and be as before, a reduction by % is achieved, where
P=100~ x ~
| cb-ca | |
| cb |
.
| P~=~100~ x ~ | 580-0.725 |
| 580 |
~=~100~ x ~0.99875~=~99.875.
The following table summarizes the most common cases.
| Log reduction | Percentage | |
|---|---|---|
| 1-log reduction | 90% | |
| 2-log reduction | 99% | |
| 3-log reduction | 99.9% | |
| 4-log reduction | 99.99% | |
| 5-log reduction | 99.999% |
In general, if is a whole number, an -log reduction corresponds to a percentage reduction with leading digits "9" in the percentage (provided that it is at least 10%).