Log Reductions – a Beginner’s Guide

LOG REDUCTIONS

What are ‘Log Reductions’ and why are they important.

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In terms of infection control, ‘Log Reductions’ convey how effective a product is at reducing pathogens. The greater the log reduction the more effective the product is at killing bacteria and other pathogens that can cause infections.

‘Log’ is short for logarithm, a mathematical term for a power to which a number can be raised. For example, if using 10 as a given number, a Log 3 increase can be shown as 103 or 10 x 10 x 10 = 1,000.

A log reduction takes the power in the opposite direction. For example, a log reduction of 1 is equivalent to a 10-fold reduction or, to put it another way, moving down one decimal place or a 90% reduction.

During product efficacy testing, the microbiology laboratories count the number of colony forming units (CFUs) of the given pathogen present at the start of the test.  They then apply the disinfection product being tested, alongside a control product and wait the required test time before recounting the number of CFUs present.

The result of the difference between the control and the test product is then expressed as a Log reduction. For example, if the number of CFUs in the control was found to be 1,000,000 (or 106) and the end result using the product was only 1,000 (103), that would be a Log reduction of 3 or a reduction of 99.9%.

LogReductionPetriDish

As a basic rule of thumb, for every additional Log reduction number you add a 9 to the percentage reduction – so a log reduction of 3, as illustrated above, is a 99.9% reduction compared with a log reduction of 6 which is equivalent to a 99.9999% reduction.

A summary of log reduction values using a starting point of 1,000,000 CFUs is outlined below:

Log reduction

Number of CFUs

Percentage reduction

Times smaller

0 log (Log0)

1 000 000

0%

N/A

1 log (Log1)

100 000

90%

x 10

2 log (Log2)

10 000

99%

x 100

3 log (Log3)

1 000

99.9%

x 1 000

4 log (Log4)

100

99.99%

x 10 000

5 log (Log5)

10

99.999%

x 100 000

6 log (Log6)

1

99.9999%

x 1 000 000

 

Log reduction

 

Using one of our test results as a working example, Enduro Hand Sanitiser was tested according to EN 13697 against Escherichia coli using a contact test time of 3 minutes. The control of bacteria was given as 4.02 x 106 (4,020,000 CFUs). The number of CFUs remaining after 5 minutes using Enduro Hand Sanitiser was <10 and a log reduction given of >5.60, i.e. >99.9999% reduction.

We hope this information helps with your understanding of log reductions. Should have any questions, or you’d like to see the log reductions our high performance, water-based products can offer, please don’t hesitate to get in touch.

Reviewed: 18/09/2018