What does it mean when a hand sanitiser has a 99.99% kill rate? How does that compare with one that can kill 99.99999% of bacteria? The numbers refer to ‘log’. ‘Log’ is short for logarithm, a mathematical term for a power to which a number can be raised.

Log reduction is a mathematical term used to show the relative number of live microbes eliminated from a surface by disinfecting or cleaning. For example, a “7-log reduction” means lowering the number of microorganisms by 10,000,000-fold, that is, if a surface has 10,000,000 pathogenic microbes on it, a 7-log reduction would reduce the number of microorganisms to one.
In terms of infection control, log reductions are used to 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.
During product efficacy testing, the microbiology laboratories determine the number of colony-forming units (CFUs) of a given pathogen present at the start of the test. They then apply the product to a test sample, alongside a control sample and wait the required test time before recounting the number of CFUs present.

The result of the difference between the control and the test samples is then expressed as a Log reduction. For example, if the number of CFUs in the control was found to be 10,000,000 (or 107) and the end result using the product was only 1,000 (103), that would be a Log reduction of 4 or a reduction of 99.99%.