What is log reduction?
Log reduction is a mathematical term used to express the relative number of living microbes eliminated by a disinfection or sterilization process. It represents a 10-fold (one decimal log) decrease in bacteria. For example, a 1-log reduction means the number of bacteria is 10 times smaller; a 2-log reduction means it is 100 times smaller.
Log reduction vs percentage reduction
While percentage reduction is easier to conceptualize, log reduction is the industry standard for expressing high levels of microbial kill.
- 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
Formula for log reduction
The simple formulas used to calculate both log and percentage reduction are:
Log Reduction = log₁₀(N₀ / N)
% Reduction = [(N₀ - N) / N₀] × 100
- N₀: Initial microbe count (before treatment)
- N: Final microbe count (after treatment)