What does "99.9%" actually mean?
When disinfectant companies claim they kill "99.9% of germs" they are referring to something called logarithmic (log) reduction.63
Log reduction in infection prevention represents a reduction of live pathogens. In other words, it tells us how thoroughly a decontamination process has reduced a contaminant (viruses, bacteria, fungi, etc.). Log reduction is either expressed in a log format or a percentage.
The performance standard of log reduction is measured using colony forming units (CFUs). CFUs measure the number of viable bacteria or fungi that can grow and spread in a sample.
So, if you had a sample with 1,000,000 colony forming units (CFUs), a 1-log reduction would result in 100,000 CFUs remaining.
The table below shows each log reduction's corresponding percentage reduction in bacteria and remaining CFUs:
Log Reduction | Percentage Reduction | Number of CFUs remaining |
0-log | 0% | 1,000,000 |
1-log | 90% | 100,000 |
2-log | 99% | 10,000 |
3-log | 99.9% | 1,000 |
4-log | 99.99% | 100 |
5-log | 99.999% | 10 |
6-log | 99.9999% | 1 |
Regulatory Definitions
Disinfection, decontamination and sterilization have specific definitions that are determined by the EPA and have public health/safety implications. These definitions are relied on by infection preventionists, nurses, environmental services and more to ensure their protocols meet federal regulations.
The UV industry uses these terms in their marketing even though they have not submitted efficacy data to the EPA. Definitions include:62
General Disinfection: Must reach at least a 6-log reduction of “Staphylococcus aureus” and “Salmonella enterica” (gram-negative and gram-positive bacteria) in less than 10 minutes
Hospital Disinfection: Must reach at least a 6-log reduction of “Staphylococcus aureus” and “Pseudomonas aeruginosa” (gram-negative and gram-positive bacteria) in less than 10 minutes
Disinfectant with Fungicidal Claims: Must reach at least a 6-log reduction of “Trichophyton mentagrophytes”
Sterilization: Required that all microorganisms on a surface are destroyed, allowing for no growth to occur.
UV and Clostridioides difficile (C. diff)
Studies show that UV light treatment may only achieve a 1 to 3 log reduction in organisms like C. diff.18 This results in a significant amount of untreated colony forming units (cfu).
There are 3.8 million colony forming units of C. diff in 1 gram of stool. On average, patients infected with C. diff have 200 grams per stool event, with events occurring 8-10 times a day.61
If using UV light to disinfect a room with a C. diff patient this means:
- 1 log reduction = 7,600,000,000 untreated cfu
- 2 log reduction = 760,000,000 untreated cfu
- 3 log reduction = 76,000,000 untreated cfu
That’s up to 76 billion cfu of untreated C. diff released into the environment every day per patient.
