Legionella in Hot Tubs — Risk, Prevention and What You Need to Know

Legionella is a real risk in hot tubs if water is poorly maintained — but a properly managed hot tub carries minimal risk. This guide explains what Legionella is, why hot tubs are a risk environment, and exactly what controls keep the risk negligible.

What is Legionella?

Legionella pneumophila is a bacterium found naturally in water systems. When inhaled in fine aerosol droplets — like those produced by hot tub jets — it can cause Legionnaires' disease, a serious and potentially fatal form of pneumonia. It can also cause Pontiac fever, a milder flu-like illness.

Legionella is destroyed by correct chlorination and cannot cause disease when swallowed — only when inhaled.

Why are hot tubs a risk environment?

Legionella grows most readily at 25–45°C — exactly the temperature range of a hot tub. The aerosol produced by jets creates ideal conditions for inhalation. This is why hot tubs are one of the most common sources of Legionella outbreaks when poorly maintained.

The key controls

Legionella is straightforward to control when water chemistry is properly maintained:

• Free chlorine 3–5 ppm: Chlorine at this level kills Legionella rapidly. Below 1 ppm, the risk rises sharply.
• pH 7.2–7.6: Chlorine is significantly less effective above pH 7.8. Correct pH is as important as chlorine level.
• Regular drain and refill: Biofilm inside pipes can harbour Legionella protected from chlorine. Regular pipe flushing and drain/refill eliminates this reservoir.
• No stagnant water: Sections of pipe where water sits stagnant are the highest-risk areas. Keep filtration running.

Domestic hot tubs

For a domestic hot tub used regularly by household members, the risk from Legionella is low when water chemistry is correctly maintained. You are not legally required to carry out Legionella testing on a domestic hot tub, though it is good practice if the tub has been unused for an extended period or water quality has been poor.

If a tub has been left empty or unused for months, superchlorinate before use, run jets for 30 minutes, and refill.

Commercial hot tubs

If guests or paying members use your hot tub, Legionella control is a legal requirement under HSG282. Quarterly water sampling for Legionella culture by an accredited laboratory is required. See our HSG282 compliance guide for full details.

After a period of non-use

The highest domestic risk scenario is a hot tub that has been left with stale, unchlorinated water for weeks or months. Before anyone uses such a tub:

1. Drain and refill completely
2. Run a pipe flush product through the system
3. Superchlorinate to at least 10 ppm free chlorine
4. Run jets for 30 minutes
5. Allow chlorine to drop to 3–5 ppm before use

If you're ever unsure whether a tub is safe to use, call us — we can test on-site and advise.