What it is
Legionella are bacteria that live naturally in fresh water at low levels and multiply in warm, stagnant water between about 20°C and 45°C. Spa pools, hot tubs, showers and cooling systems are the classic places for them to build up. Results are reported per litre.
Where it comes from
Legionella enters a building with the incoming water and grows where conditions suit it: warm temperatures, standing water, scale, sludge and biofilm that provide nutrients. Spa pools are particularly vulnerable because the water is warm, aerated and shared.
Why it matters
Legionella is a risk when it is inhaled in aerosol, the fine spray from a spa pool, shower or spray tap, rather than when swallowed. It can cause Legionnaires' disease, a serious form of pneumonia, and the milder Pontiac fever. Older people, smokers and anyone with lung problems or a weakened immune system are most at risk. Operators of spa pools and hot tubs used by the public have legal duties to control it, set out in the HSE's HSG282 guidance.
What can be done about it
Control is about temperature, cleanliness and disinfection: keeping cold water cold and hot water hot, maintaining the right disinfectant level in spa pools, regular cleaning to remove biofilm, flushing little-used outlets and keeping records. A positive result in a spa pool normally means closing the pool, shock dosing, cleaning and retesting. Pozzani can advise on the programme and the retest schedule.
Information last reviewed 5 October 2026 by Pozzani Pure Water.