Log Reduction: What does it mean?

by Erica Mitchell | December 1 2021 | Research, Microbiology | 0 Comments

Log_reduction-01.jpgDiscussion of the reduction of microorganisms in healthcare settings will often include the data as “log reductions.” To those of us more accustomed to percentages, this can be confusing. Today's post will explain how to interpret these numbers and, we hope, help our readers better understand how they are used in scientific literature.

Many scientists deal with numbers at enormous scales. Astronomers deal with vast distances and speeds, while microbiologists deal with vast quantities of microscopic organisms. To help manage these enormous numbers more easily, these scientists use scientific notation, ways to abbreviate numbers and make them easier to work with. The most common use of scientific notation is to express a large number not in full, but rather as a factor of 10:

               1,234,567,890 as 1.2 x 109

Another use of scientific notation is the use of a logarithmic scale, or log scale. Microbiologists and others who work with microorganisms use a log scale to better describe changes (mostly reductions) in numbers of organisms.

Quick Historical Context: Logarithms (Greek logos ‘ratio’ + arithmos ‘number’), or logarithmic tables, were first developed in the 1400s to help scientists more easily work with the increasing amount of data that they were manipulating. Tedious and time-consuming calculations, specifically multiplication, presented a problem for these first modern scientists. The result was logarithms: Making multiplication problems into quicker addition problems. Today we continue to use log scales despite having computers to speedily manipulate our data since it creates a more efficient way to notate changes in huge numbers. A great explanation is here.

The log scale uses factors of 10, making each step a change by a factor of 10, thereby increasing the accuracy of the number without the need for excessively long decimal numbers. For example, if a number has already been decreased by 99.99%, if you decrease that number even further, you have to keep adding numbers, resulting in 99.999%, 99.9999%, etc. Instead, a log reduction would simply progress from 1 log, 2 log, 3 log, etc.

Here are a few ways of describing a reduction of 100 to 10.

A reduction of 1 log

A 90% reduction

A reduction by a factor of 10

A reduction to 1/10 of the original value

The result is 10 times smaller than the original value

To look at this in terms of percentage reduction alone, which is how many regulatory bodies describe reductions in microorganisms, here is a quick cheat sheet:

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

You will find descriptions of log reductions in much of the literature related to the infection control and microorganisms, including regulations regarding kill times and efficacy for various products. We hope this quick introduction helps you more readily understand these numbers when you encounter them.

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 Editor's Note: This post was originally published in June 2016 and has been updated for freshness, accuracy and comprehensiveness.