About log reduction
Log reduction (also called the log reduction value, or LRV) measures how much a treatment lowers the number of viable microorganisms — bacteria, viruses, spores, or other cells — in a sample. It is the workhorse metric of disinfection, sterilization, water treatment, and antimicrobial testing because microbial populations span many orders of magnitude, and a base-10 logarithm compresses those huge numbers into a small, comparable scale.
The formula
Log reduction is the base-10 logarithm of the ratio of the initial count to the final count:
- Log reduction = log₁₀(N₀ / N), where N₀ is the count before treatment and N is the count after treatment.
- Equivalently, log reduction = log₁₀(N₀) − log₁₀(N).
- To convert to percent removed: percent reduction = (1 − 10−LR) × 100.
Counts are usually expressed as colony-forming units per millilitre (CFU/mL) for bacteria or plaque-forming units (PFU) for viruses, but any consistent unit works because the formula uses their ratio.
Log values as fold and percent reductions
Every whole log is a tenfold drop in survivors, so the percentages climb quickly:
- 1-log = 10× fewer = 90% reduction
- 2-log = 100× fewer = 99% reduction
- 3-log = 1,000× fewer = 99.9% reduction
- 4-log = 10,000× fewer = 99.99% reduction
- 5-log = 100,000× fewer = 99.999% reduction
- 6-log = 1,000,000× fewer = 99.9999% reduction
Where the benchmarks come from
Regulatory and industry standards set target log reductions for specific claims. The U.S. EPA generally requires a 6-log (99.9999%) reduction of a bacterial test organism for a product to be labeled a "sterilizer," and a 3-log reduction for a "sanitizer" on hard non-food surfaces. Drinking-water rules under the Surface Water Treatment Rule call for at least 3-log (99.9%) removal or inactivation of Giardia and 4-log (99.99%) of viruses. Hand-hygiene and hard-surface disinfectant claims are often stated in log reductions as well.