How long does legionella take to develop in water?
Updated 12 September 2026
Short answer
Legionella can grow from undetectable levels to a clinically significant population in a matter of days under the right conditions. Warm, stagnant water with a biofilm surface can see doubling every three to four hours in the 25 to 42 degrees Celsius range. The practical implication is that a system left unused for two to three weeks can develop a significant population even if it was clean when it was shut down.
The timescale matters for two reasons. First, it sets how long a shutdown can last before the system has to be treated as contaminated. Second, it explains why infrequently used outlets need weekly flushing rather than monthly.
Growth kinetics
In the laboratory, Legionella pneumophila doubles every three to six hours at 37 degrees Celsius, which is close to the optimum. Below 20 degrees it effectively stops growing. Above 55 degrees it starts to die. Between those points, growth rate increases with temperature up to the optimum and then falls away rapidly.
In a real water system, the picture is more complicated because legionella lives in biofilms rather than free-floating in the water column. Biofilms form on the inside of pipes, in the spaces under deposits of limescale, and in the washers and aerators of taps and shower heads. They provide protection from chlorine and from short periods of high temperature, and they support a much more stable population than free bacteria.
Implications for shutdown periods
HSG274 Part 2 treats any shutdown of more than a week as needing attention. For shutdowns of up to two weeks, a thorough flush of every outlet before reoccupation is usually enough. For shutdowns of a month or more, some systems will need thermal disinfection as well, and sampling may be warranted in higher-risk premises.
Schools returning from summer holidays are the classic worked example. Six weeks is long enough for biofilm to mature significantly, and the pattern of building use in September (every tap and every shower suddenly in use) is exactly the pattern that releases aerosol into the breathing zone of occupants. A documented flushing and disinfection protocol before the start of term is what the control scheme should require.
The role of nutrients
Legionella grows faster where there are nutrients available. Rust and iron oxide scale on the inside of pipes, sediments in the base of hot water cylinders, amoebae in cold water storage tanks, and soft rubber components that leach plasticisers into the water all support faster growth. Systems with any of these features reach critical populations sooner than clean systems at the same temperature.
Legal basis
HSG274 Part 2 Appendix 2.1 on microbiology. ACOP L8 paragraphs 13 to 22 on factors affecting growth.
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Free legionella risk assessment template
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