# How long can Legionella survive at 60°C?

I'm coding some stuff for water heaters and one of the functions is anti-Legionella.

Anyway, I've read on wikipedia that 90% Legionella dies in first two minutes if the water temperature is 60°C, but I'm interested in how long should the temperature be 60°C for 99.99999% of Legionella to die? (I guess I can never get to 100%)

Follow-up question: On what intervals should this be done so that the water isn't contained? Should it be one week, two weeks, several days?

Final question: If the water in a water heater has 60°C for 30 minutes every 2 weeks, would the water be safe?

• If the water can be repeatedly contaminated, you would probably need to continuously keep its temperature at the disinfection level to limit the risk of contamination to 0.00001%. – Jan Jul 10 '19 at 11:42

Usually cell death would follow first order kinetics.

$$X=X_0e^{-kt}$$

where $$X$$ is the number of cells at time $$t$$, $$X_0$$ is the initial number of cells and $$k$$ is the death rate constant. You can get the constant by solving for 90% reduction in 2 minutes. Then just substitute and get the time required for 99.99999% reduction — 10 minutes.