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 n-log reduction means that the concentration of remaining contaminants is only 10−n times that of the original.
[1] Let cb and ca be the numerical values of the concentrations of a given contaminant, respectively before and after treatment, following a defined process.
Then an R-log reduction is achieved, where For the purpose of presentation, the value of R is rounded down to a desired precision, usually to a whole number.