The legionella bacteria will only grow slowly at these temperatures but since they have minimal water losses it takes years for there to be a complete turnover of water.
When sprinklers are activated they emit a huge amount of spray and aerosol, however, the average size of each particle of water is so big that most of us would have an automatic reflex cough if we inhaled some. A small proportion of the aerosol would have particle sizes too small to trigger a reflex cough response so there remains a risk.
The greatest risk is to those engineers who test the sprinklers and discharge fire hoses and to anyone else nearby.
By testing the fire tank, the type and concentration of any legionella bacteria found can help produce a risk assessment and provide a safe system of working.