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Short answer: a log reduction is a drop in the number of organisms, measured on a base-10 logarithmic scale. One log is a 90% reduction, two logs 99%, three logs 99.9%. Heat sterilization uses the same idea through the D-value (the time for a 1-log drop) and the F0 value (lethality in equivalent minutes at 121 °C). General information only, not medical or procedural advice.
Definition and scope
Log reduction (also written log10 reduction, LRV or “N-log kill”) compares a count before treatment with a count after it: log10(count before) − log10(count after). Each whole log removes 90% of what was left. The scale is useful because microbial counts span many orders of magnitude, so a “6-log” claim is easier to read than a percentage with six nines.
The CDC/HICPAC Guideline for Disinfection and Sterilization in Healthcare Facilities uses the same convention when it defines the D-value as the “time to reduce the surviving population by 90% or 1 log10“. This page covers the general arithmetic and how it links to sterilization terms. It does not give cycle settings or procedures.
Log reduction to percent: the conversion
The table below is straight arithmetic from the definition (percent reduced = (1 − 10−L) × 100). It is not a performance claim for any product or process.
| Log reduction (L) | Percent reduced | Organisms left, from 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 |
Non-integer logs convert the same way: percent = (1 − 10−L) × 100, and L = −log10(1 − percent/100). A 2.5-log reduction is therefore 99.68%, not 99.5%. Going the other way, 99.5% is about 2.3 logs.
How log reduction is used in practice
Cleaning, disinfection and sterilization all get log numbers
The CDC guideline reports log reductions for several steps in reprocessing, so the number alone does not show whether a step is “cleaning” or “killing”:
- It states that cleaning reduces the level of microbial contamination by 4–6 log10, and that manual and mechanical cleaning of endoscopes achieves approximately a 4-log10 reduction.
- For washer-disinfectors, it cites a study in which cleaning, measured as a 5–6 log10 reduction, was achieved on surfaces that had adequate contact with the water flow.
Worked example (arithmetic only). The guideline gives an average bronchoscope load of 6.4×104 CFU/mL before cleaning. A 4-log10 reduction of that figure leaves about 6.4 CFU/mL; a 6-log10 reduction leaves about 0.064 CFU/mL. The same “cleaning” step can differ by two logs, a hundredfold gap in what remains.
D-value, z-value and F0 in heat sterilization
For heat, the log scale becomes a time scale:
- D-value. The CDC glossary defines it as the time or radiation dose required to inactivate 90% of a population of the test microorganism under stated exposure conditions. A subscript marks the temperature (D121C). The CDC reports D121C values of 1 to 2 minutes for Geobacillus stearothermophilus, the organism used to monitor steam sterilization.
- z-value. The EMA guideline on sterilisation defines it as the change in temperature required to alter the D-value by a factor of 10.
- F0. For saturated steam, the EMA defines F0 as the lethality expressed as equivalent minutes at 121 °C, with reference to micro-organisms with a theoretical z-value of 10.
The links between them are arithmetic. If z = 10, a process that holds 15 minutes at 121 °C delivers F0 = 15. Because z = 10 means a 10 °C drop makes the D-value ten times longer, 10 minutes at 111 °C is equivalent to 1 minute at 121 °C, an F0 of 1.
Related concepts at a glance
Each row below is taken from the CDC and EMA documents listed under Sources.
| Term | What it states | Source |
|---|---|---|
| D-value | Time or dose for a 90% (1-log) reduction under stated conditions; “only of significance under precisely defined experimental conditions” | CDC glossary; EMA glossary |
| z-value | Temperature change that alters the D-value by a factor of 10 | EMA glossary |
| F0 | Equivalent minutes at 121 °C, referenced to z = 10; steam processes require F0 of at least 8 minutes and a hold temperature of at least 110 °C | EMA glossary and section 4.1.1 |
| Overkill sterilisation | A process with F0BIO above 12 minutes, for example at least a 12-log reduction of biological indicator organisms with a minimum D-value of 1 minute | EMA glossary |
| SAL | Probability of a surviving organism; an SAL of 10−6 is a 1 in 1,000,000 chance that a single viable organism is present on a sterilized item | CDC glossary |
The SAL argument that connects 12 logs of kill to 10−6 is covered in sterility assurance level (SAL), so it is not repeated here.
Where it matters for labs, clinics and supply decisions
A log number is only as informative as the conditions attached to it. Before relying on a stated log reduction, a lab or clinic reader can check which of these the claim states, each point taken from the CDC and EMA text above:
- The condition. The EMA says a D-value has meaning only under precisely defined experimental conditions; the CDC glossary likewise ties it to “stated exposure conditions”. A bare “5-log” with no time, temperature or dose is incomplete.
- The test organism. D121C for G. stearothermophilus is 1–2 minutes, while heat-resistant non-spore-forming bacteria, yeasts and fungi have D121C values too low to measure experimentally. A figure for one organism does not transfer to another.
- What was counted. Cleaning alone is reported at 4–6 log10 by the CDC, so a claim should say whether the drop reflects removal, inactivation, or both.
- The direction of the scale. Log reduction describes the drop, while SAL describes the probability that an organism survives. Mixing the two produces nonsense such as “SAL 6-log”.
The same 3-log figure appears elsewhere on CASRAI as 99.9%: a 3-log10 bactericidal threshold in time-kill assays, and an EPA 3-log10 reduction in one hour in antimicrobial surfaces. For how biological and chemical indicators relate to these numbers, see autoclave validation with biological and chemical indicators and chemical indicators for sterilization.
Common misunderstandings
- “2.5 log is 99.5%.” It is 99.68%. Logs are not linear in percent.
- “A higher log is always a better product.” A log number compares a before and after count under one set of conditions. It cannot be compared across different organisms, times, temperatures or doses.
- “Only sterilization has a log reduction.” The CDC reports log reductions for cleaning, disinfection and sterilization alike.
- “A D-value is a property of the organism alone.” The EMA ties it to precisely defined experimental conditions; the CDC notes the temperature subscript (D121C).
- “F0 of 8 means 8 minutes of cycle time.” F0 is lethality in equivalent minutes at 121 °C, so it can be reached at other temperatures and hold times.
FAQ
What is a D-value in sterilization?
It is the time (or radiation dose) needed to inactivate 90% of a test microorganism population, a 1-log reduction, under stated exposure conditions. The temperature is written as a subscript, for example D121C.
What is an F0 value?
F0 is the lethality of a saturated steam process expressed as the equivalent number of minutes at 121 °C, referenced to a theoretical z-value of 10 (EMA definition). The EMA guideline requires a minimum F0 of 8 minutes for steam sterilisation processes, with a minimum hold temperature of 110 °C.
What does 3-log reduction mean?
It means the count fell by a factor of 1,000, which is a 99.9% reduction. From 1,000,000 organisms, 1,000 remain.
How do I convert log reduction to percent?
Percent reduced = (1 − 10−L) × 100. To go back, L = −log10(1 − percent/100).
What is the relationship between D-value and F0?
The D-value is the time for a 1-log drop at one temperature; the z-value says how that time changes with temperature; F0 adds up the lethality delivered at different temperatures as equivalent minutes at 121 °C.
Sources
- CDC / HICPAC, Guideline for Disinfection and Sterilization in Healthcare Facilities, 2008 (last update June 2024): https://www.cdc.gov/infection-control/hcp/disinfection-and-sterilization/index.html. Date checked: 2026-10-30.
- European Medicines Agency, Guideline on the sterilisation of the medicinal product, active substance, excipient and primary container, EMA/CHMP/CVMP/QWP/850374/2015 (adopted November 2018, in effect 1 October 2019): https://www.ema.europa.eu/en/documents/scientific-guideline/guideline-sterilisation-medicinal-product-active-substance-excipient-and-primary-container_en.pdf. Date checked: 2026-10-30.
- Percent and survivor-count figures above are arithmetic from the log definition, not data from either source.
