Whenever cooling towers become contaminated with legionella bacteria they can quickly proliferate since the temperature is often ideal and there are plenty of nutrients.
By design, cooling towers discharge a plume of water vapour. These plumes of vapour are carried by the wind and can infect many people at the same time, even at considerable distance from the installation.
The HSE state that cooling tower operators should specifically test for legionella bacteria at least every 3 months.
Specialist water treatment companies provide treatment to minimise the risk of legionella proliferating, their efforts are supplemented by AquaCert who provide independent testing.
Most companies operating cooling towers insist that the legionella testing is a separate contract to the water treatment.