Legionella limit value: what 100 CFU per 100 ml means
Strictly speaking, there is no limit value for Legionella, only a technical action value: 100 colony-forming units (CFU) per 100 ml. This is set out in Annex 3 Part II of the Drinking Water Ordinance. Once the value is reached, the operator must act without delay: inform the health authority, establish the cause, draw up a risk assessment and take measures (Section 51 TrinkwV).
What is the technical action value?
The technical action value is a special indicator parameter for drinking water installation systems. It does not describe a limit beyond which the water may no longer be drunk, but the point at which the operator must take action. The Ordinance expressly takes account of the measurement uncertainty of the testing and sampling methods. In the Ordinance's table the value appears as "100/100 ml"; authorities and laboratories write "100 CFU/100 ml".
What applies at which result?
| Result in CFU per 100 ml | What applies | Source |
|---|---|---|
| below 100 | Action value not reached; continue testing at the prescribed interval | Section 31 TrinkwV |
| from 100 | notify without delay (unless the laboratory has already done so), establish the cause with a site inspection, written risk assessment, measures in line with recognised engineering standards, inform consumers | Sections 51, 52 TrinkwV |
| 100 to 1,000 | if the risk assessment shows a need for action, further remedial measures are required | LGL Bavaria |
| above 1,000 | short-term remediation measures | DVGW W 551, as reported by LGL Bavaria |
| above 10,000 | additional immediate hazard-prevention measures, for example a ban on showering or point-of-use filters | LGL Bavaria |
| High-risk areas | any detection of Legionella leads to a hygiene assessment and to restrictions on use | UBA recommendation of 13 October 2025 |
The levels above 100 CFU are reported by the Bavarian State Office for Health and Food Safety (LGL); it explicitly attributes the level above 1,000 CFU to DVGW code of practice W 551, and states the others without citing its own source. The current recommendation from the German Environment Agency (UBA) on risk assessment, dated 13 October 2025, does not work with fixed numerical bands but with risk categories such as low, medium and high, which are used to prioritise measures.
You can assess your own value with the result guide.
What does CFU mean?
CFU stands for colony-forming unit. In the laboratory, the water sample is applied to a growth medium; every viable germ cell or group of germs grows there into a visible colony, which is then counted. The result states how many such units were found in 100 millilitres of water.
Why is Legionella dangerous?
Legionella becomes dangerous when it is inhaled as a fine water mist, for example while showering. It can then trigger Legionnaires' disease, a severe form of pneumonia, or the milder Pontiac fever. This is why the testing obligation is linked to showers and other fixtures that create a fine mist.
Frequently asked questions about the Legionella limit value
Is 100 CFU per 100 ml a limit value?
Not formally. The Drinking Water Ordinance calls the value a technical action value. It triggers obligations: notification, establishing the cause, a risk assessment and measures under Section 51. In everyday usage it is often called the Legionella limit value.
Who must act on a result of 100 CFU per 100 ml or more?
The operator of the water supply system that contains the drinking water installation. The accredited laboratory must also notify the health authority of the result without delay under Section 53. The operator's own notification is only unnecessary if they hold proof that this notification has already been made (Section 51(1) No. 1).
What happens above 10,000 CFU per 100 ml?
Above this so-called hazard threshold, the Bavarian State Office for Health and Food Safety (LGL) states that additional immediate hazard-prevention measures are needed, for example a ban on showering or the use of point-of-use filters.
Do I have to inform tenants about a result?
Yes. Under Section 52(3) TrinkwV, the operator must inform the affected consumers without delay as soon as they have the risk assessment and details of any restrictions on use.
Does the value also apply to cold water?
The technical action value applies to the drinking water installation, and therefore to the samples taken there in line with the rules. Systemic testing focuses on the hot water, because Legionella multiplies most strongly at around 25 to 45 °C. If the cold water warms into this range, it can grow there too.
Sources
- Drinking Water Ordinance, Annex 3 Part II: technical action value
- Section 51 TrinkwV, Section 52 TrinkwV, Section 53 TrinkwV
- German Environment Agency (UBA): recommendation on risk assessment, dated 13 October 2025
- Bavarian State Office for Health and Food Safety: Legionella
- DVGW: energy savings in the hot water sector (2023), temperature range of multiplication
This page does not replace legal advice or information from the health authority. The version of the Drinking Water Ordinance in force at the time is authoritative.