Log reductions and Virus Control

When discussing infection control ‘log reductions’ refers to how effective a product is at reducing pathogens. The larger the reduction, the better the product is as reducing the number of pathogens that could be causing infection and sickness.

The terminology comes from the mathematical term logarithm, which is another way to denote numbers. For example, if 10 is the given number, a log 4 increase can be written as 10⁴ or 10 x 10 x 10 x10 – 10,000. A reduction will go in the opposite direction, meaning a log 1 reduction is a 10-fold reduction or, moving the decimal place one step down, or a 90% reduction.

 In a microbiology context, when counting pathogen colonies, you are counting very large numbers, applying and testing the efficiency of various disinfectant products, then recounting the colonies present. If the product was successful at killing the present pathogens, you would report it as the log reduction:

This highlights how important those extra decimal points are when shopping for cleaning products. When discussing viruses, bacteria and other pathogens, the numbers present on a surface is huge. If there are 1 billion bacteria on a dirty surface, and the disinfectant product was capable of a Log3 reduction, or 99.9%, that would mean there would still be 1,000,000 bacteria left on that surface, compared with 1,000 with a 99.9999% reduction. Using a product with a Log6 reduction will leave significantly fewer on your countertop than a product that is categorised as Log3, that 0.0999% will make a big difference in ensuring cleanliness and safety.