At a glance
- Growth between 25 and 45 °C
- Die-off from 60 °C, growth inhibited above 55 °C
- Hot water ≥ 60 °C, circulation ≥ 55 °C in line with DVGW W 551
- Stagnation favours growth; regular water exchange counteracts it
- Biofilm as a refuge and protection against brief heating
The temperature range
| Temperature | Behaviour of the Legionella |
|---|---|
| below 20 °C | virtually no growth |
| 25 to 45 °C | optimal growth |
| 45 to 55 °C | growth slowed |
| above 55 °C | growth inhibited |
| above 60 °C | die-off within minutes |
| above 70 °C | die-off within seconds |
This is why the technical standards require hot water to be stored at a minimum of 60 °C and kept from falling below 55 °C in the circulation loop, while cold water should stay permanently below 25 °C (in line with DVGW W 551). The transition zones are the critical ones: uninsulated cold water pipes running next to warm risers, or hot water pipes that cool down between two uses. This is exactly where water sits in the ideal growth range for hours.
Why stagnation is a key factor
When water flows regularly, it is exchanged before any significant populations can build up. If it sits in the pipe for days, it takes on the ambient temperature and almost inevitably ends up in the critical range. That is why the empty room, not the occupied one, is a key risk factor.
The role of the biofilm
A biofilm of microorganisms forms on the inner wall of every pipe. Legionella live within it, protected, and multiply inside amoebae that colonise the biofilm. The biofilm is therefore less a by-product than a precondition: it makes Legionella resistant to brief temperature spikes and to disinfection.
That explains why a single heat treatment rarely solves the problem for good. Thermal disinfection lowers the bacterial count in the short term, but as long as the biofilm stays intact and the water keeps stagnating, the population builds back up within a few weeks. Only the combination of the right temperature and regular water exchange has a lasting effect.
Where Legionella sit in a building
Not every point in a drinking water system is equally susceptible. Growth is particularly favoured wherever water stands still or turns lukewarm:
- Rarely used outlets such as showers in empty rooms or guest bathrooms.
- Dead legs and dead-end pipes, meaning pipe sections without regular flow, often the remnants of decommissioned connections.
- Hot water storage below 60 °C or circulation loops that do not hold the temperature everywhere.
- Oversized or uninsulated pipes, in which water moves or warms up slowly.
- Shower heads, hoses and aerators, where deposits and biofilm collect.
In practice, a single neglected shower is enough to cause elevated readings in an otherwise well-maintained system.
How transmission happens
Legionella do not spread through drinking. They spread through aerosols, the fine water droplets a few micrometres across that you breathe straight in. Only these reach deep into the lungs when inhaled, where the bacteria can trigger pneumonia. This is why the shower is the most critical point in a building, because it atomises the water into droplets of exactly this size. Taps with aerators, whirlpools or air conditioning systems with evaporative cooling also produce such aerosols.
Far more people fall ill than the figures suggest. The Robert Koch Institute reports a notified incidence of around 1.7 cases per 100,000 inhabitants (2018), but on the basis of studies assumes an actual figure of 18 to 36 per 100,000. The majority of cases therefore go undetected.
Symptoms and course of the illness