ARTICLE: Log Reductions Explained



How does Log 6 equal 99.9999% effective and why does this matter?

As someone responsible for assessing the effectiveness of different disinfectants you will be concerned with what percentage of a pathogen is killed by a particular disinfectant or process. But, if like many during the pandemic, choosing a disinfectant, to perhaps be fogged frequently to make your workplace safe, you may still be wondering why it is vital to choose the highest possible Log rating –usually expressed as a percentage between 90 and 99.9999% - and what this actually means 

The following provides an overview of how log reductions are calculated and what the different ratings on the scale mean for infection control. But, in short the higher the Log reduction or percentage efficacy quoted, the fewer colony forming units (CFUs) of the pathogen are left alive to continue on their infectious journey. 

In the case of MicroSafe Log6 / 99.9999%, the colony is reduced to 1 from an original 1,000,000 making this the highest Log reduction usually quoted, so the higher it is, the more effective the disinfectant. 

Although to most of us, 99.9% kill rate sounds impressive, the log reduction chart shows how, at Log3 there remain still 1000 CFUs to continue to infect and spread the virus. 

The maths and science

The  mathematical term for a power by which a number can be raised is known as a Logarithm. Log reduction stands for a 10-fold (or one decimal point) reduction in the pathogen being tested; that is the disinfectant reduces the number of live bacteria or viruses for example by 90 percent for every log reduction. A 6-log kill reduces the colony to 1 bacterium / virus after a 99.9999% reduction after the disinfectant has been applied to the control and the number of colony forming units (CFUs) has been counted.

So, a log reduction of 1 is equivalent to a 10-fold reduction or, a move down one decimal place, 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%.

Generally speaking, for each additional Log reduction number you add a 9 to the percentage reduction – so a log reduction of 3, is a 99.9% reduction compared with a log reduction of 6 which is equivalent to a 99.9999% reduction.

The table below presents summary of log reduction values with 1,000,000 CFUs as the original number of CFUs

Log reduction

Number of CFUs

Percentage reduction

Times smaller

0 log (Log0)

1 000 000



1 log (Log1)

100 000


x 10

2 log (Log2)

10 000


x 100

3 log (Log3)

1 000


x 1 000

4 log (Log4)



x 10 000

5 log (Log5)



x 100 000

6 log (Log6)



x 1 000 000

 As you can see from the graph the higher the Log reduction, the more effective the disinfectant so if you want to make sure that 

So to sum up unless a disinfectant is has a log reduction of 6 it leaves pathogens that can continue to infect and multiply. According to this highly informative article by Caroline Kochelec at infectioncontrol.tipsNot all products in the realm of environmental disinfection meet the EPA guidelines. When evaluating products for high-level disinfection, their log reduction capability is paramount.”