How many bacteria survive?
A log reduction is always described in terms of the microbe count being reduced by a factor of 10.
1 log reduction = microbe count reduced by 90% (1 in 10 survival rate)
2 log reduction = microbe count reduced by 99% (1 in 100 survival rate)
3 log reduction = microbe count reduced by 99.9% (1 in 1000 survival rate)
4 log reduction = microbe count reduced by 99.99% (1 in 10,000 survival rate)
5 log reduction = microbe count reduced by 99.999% (1 in 100,000 survival rate) = High-Level Disinfection
6 log reduction = microbe count reduced by 99.9999% (1 in 1,000,000 survival rate)
7 log reduction = microbe count reduced by 99.99999% (1 in 10,000,000 survival rate)
A 1-log reduction therefore means that at least 90% of a given number of microbes are killed, and that no more than 1 in 10 will survive. A 6-log reduction means that one in one million microbes survives, or in other words, that the microbe population is reduced by 99.9999%. A 5-log reduction is considered high-level disinfection. When all micro-organisms on an object are killed, it is called ‘sterilisation’.
Standard for disinfection
A standard has been developed for the disinfection of products. In Europe, this standard is known as the NEN-EN 14885.¹ This standard specifies the standards to which products have to conform to support the claims for microbicidal activity referred to in the European standard. It is applicable to products for which activity is claimed against the following micro-organisms:
- Bacteria Spores
- Fungal spores
- Tuberculin and
Among other things, these standards apply to the field of medicine, and thus to the disinfection of medical equipment. It is vital that you check what level of log reduction the various UV devices available on the market attain, and whether they meet the standards specified in the NEN-EN 14885.¹
If you’d like to learn more about UV-C light, be sure to read ‘What Is UV-C Light Used for?’
- NEN. (2018). Chemical disinfectants and antiseptics - Application of European Standards for chemical disinfectants and antiseptics. Retrieved from https://www.nen.nl/en/nen-en-14885-2018-en-253404