What a log reduction is
Disinfection, sterilization and water treatment remove microbes by factors of ten. A log reduction counts those factors:
Log reduction = log10(N0 ÷ N)
where N0 is the number of microorganisms before treatment and N the number after, usually in colony-forming units (CFU). A 1-log reduction divides the count by 10, a 2-log reduction by 100, and so on.
| Log reduction | Percentage removed | Survivors out of 1,000,000 |
|---|---|---|
| 1 | 90% | 100,000 |
| 2 | 99% | 10,000 |
| 3 | 99.9% | 1,000 |
| 4 | 99.99% | 100 |
| 5 | 99.999% | 10 |
| 6 | 99.9999% | 1 |
Example
A surface carries 1,000,000 CFU before cleaning and 10 CFU after. The log reduction is log10(1,000,000 ÷ 10) = 5, or 99.999%.
Why “99.9% of germs” is less impressive than it sounds
A 3-log reduction still leaves 1,000 survivors out of a million. For pathogens that cause infection at low doses, such as norovirus, the remaining 0.1% can matter. That is why standards set targets of 4, 5 or 6 logs depending on the use.
Where the targets come from
- Disinfectants: EPA test methods and European standards such as EN 1276 (a 5-log reduction for bacteria) and EN 14476 (a 4-log reduction for viruses).
- Sterilization: a sterility assurance level of 10⁻⁶, a one-in-a-million chance that a single viable microbe remains, is the usual target for medical devices.
- Drinking water: US rules require at least 3-log removal or inactivation of Giardia and 4-log of viruses for treated surface water.
- Food: juice processors must achieve a 5-log reduction of the target pathogen.
Reading a result
Log reductions are only meaningful if the starting count is high enough. If you start with 1,000 CFU, you cannot demonstrate more than a 3-log reduction, because you cannot count fewer than one survivor. Tests therefore use high inocula, often 10⁶ to 10⁸ CFU.
Contact time matters
Every log reduction claim is tied to a concentration, a temperature and a contact time. A disinfectant that achieves 5 logs in 5 minutes may achieve much less in 30 seconds, so follow the label.