A hydrotherapy pool concentrates three Legionella risk factors that rarely sit together: water held close to body temperature, a slow turnover that lets it stand for long stretches, and bathers drawn from the more susceptible groups guidance singles out [1][5]. Get the controls right and none of that bites. Get them wrong and the same warm, gently aerosolised water that soothes a frail or disabled user is also an ideal place for the bacteria to grow.

That tension runs through everything below. So does an awkward question of which rulebook applies — because a hydrotherapy pool usually sits inside a building governed by healthcare water rules, while the pool itself behaves like a spa.

The warm water paradox

A standard swimming pool is, for Legionella, fairly forgiving: kept cooler, turned over quickly, and heavily diluted by a constant stream of fresh, disinfected water. Controlling Legionella in public pools and water parks covers that high-turnover model. A hydrotherapy pool inverts almost every part of it.

The water is warm by design — held near body temperature so people with limited movement, painful joints or low muscle tone can stay in it comfortably for a long session. HSE guidance puts the active Legionella growth range roughly between 20 and 45°C [8], and a hydrotherapy pool deliberately parks its water inside that band. That is the warm water paradox: the single feature that makes the pool therapeutic is the same feature that makes it hospitable to the bacteria.

Turnover makes it worse. A physio pool turnover is slow — small bather numbers, long quiet gaps between sessions, evenings and weekends with nobody in the water at all. Low use sounds safer; for Legionella it is the opposite, because warm water that sits still is warm water that grows things. Add the gentle aerosol thrown off by movement, splashing, hoists and any jets, and you have a route for that water to reach the lungs as a fine spray [3].

Then the bathers. The people in a hydrotherapy pool — in a special school pool, a care-home pool or an NHS physiotherapy department — are frequently among the groups guidance recognises as more susceptible to infection [5]. Without making any clinical claim about an individual, the working assumption has to be that your users tolerate a lapse in control far less well than a fit adult in a leisure pool would.

Which rulebook: HSG282, HTM 04-01, or both

This is where hydrotherapy pools trip people up, so it is worth being precise. Two separate bodies of guidance meet at the pool, and they are not interchangeable.

The pool water itself — the warm, recirculating, filtered, disinfected, aerosol-generating volume — behaves like a spa-pool system, and the spa-pool guidance HSG282 is the right model for how that plant is run: continuous circulation and filtration, a maintained disinfectant residual, dilution with fresh water, and microbiological sampling [2]. Safe operation of spa pools and hot tubs under HSG282 sets out that operating discipline in full. Treat it as the template for the pool circuit, not optional reading.

The building around the pool is a separate matter. Where the pool sits inside an NHS or other healthcare premises, the cold and hot water services feeding it — the supply that fills the pool, the showers and basins in the changing area, the plant-room pipework — fall under the healthcare water-safety regime in HTM 04-01 [4]. That regime expects a water safety group, a water safety plan, and control of the building’s hot and cold systems to a healthcare standard. Legionella prevention in hospitals and healthcare facilities covers that wider picture.

In a hospital hydrotherapy pool, both apply at once: HTM 04-01 for the building’s water and the susceptible-user context, HSG282 principles for the pool plant. In a special school or a care home the building may not be a “healthcare premises” in the formal HTM sense, but the user vulnerability and the warm recirculating pool both still pull you toward those higher standards. Hydrotherapy pool water safety, then, is usually a two-document job, not one. The pragmatic call is to let the more demanding guidance set the bar wherever they overlap, and to write down in your risk assessment exactly which document drives each control.

The same pool, three settings

The principle is constant; how it lands depends on where the pool lives.

A special school pool is often run by site or estates staff rather than a dedicated pool team, used in short bursts during term and then left standing through long holidays. The holiday is the danger: a warm pool, or warm associated pipework, sitting for six weeks with the routine paused. Termly use plus extended shutdowns is exactly the pattern that needs a written plan for what happens to the water — and to the building’s hot and cold supplies — while the school is empty.

A care-home pool carries the bather-vulnerability problem at its sharpest, and frequently shares plant, staff and a hot-water system with the rest of the home. Vulnerable bather disinfection is the live tension here: you need a disinfectant regime strong enough to hold control under organic load, including incontinence and skin contact, yet managed for users with sensitive skin and airways. That balance is a job for a competent pool-plant specialist and your risk assessment, not a dial set by feel.

An NHS physiotherapy pool has the most structure around it — a water safety group, HTM 04-01, formal sampling — and usually the lowest tolerance for failure given who uses it. The risk here is rarely ignorance; it is a quiet operational gap, such as a residual not checked across a weekend, or a balance tank left off the cleaning schedule.

What nobody tells you about hydrotherapy pools

Most generic pool advice misses the parts that actually catch hydrotherapy operators out.

The balance tank is the first. Almost every recirculating pool has one — a buffer vessel holding the surge volume — and it is warm, dark, easy to leave off the cleaning schedule, and a textbook stagnation reservoir feeding the whole loop. If you inherit a pool and read only one thing about it, find out when the balance tank was last cleaned.

Low use is the second, and it is counter-intuitive. Operators relax over a quiet holiday because “nobody’s using it”. That is precisely when warm, static water and unflushed associated outlets are doing their worst. A quiet pool needs more attention to standing water, not less — the showers, the fill point and any rarely-used outlet in the pool’s own changing block included.

Third, the hoist, the steps and the “extras” generate aerosol too. People picture risk coming only from big spa jets, but the spray that matters is whatever puts fine droplets into the breathing zone right next to a susceptible user — splashing, a hoist lowering someone in, a poolside shower. The warm, fragile-user pool shares this with the birthing pool, where the same warm-water-plus-vulnerable-user logic applies; Birthing pools and water births: managing Legionella in maternity settings works through that parallel.

Fourth, a clean sample is not a clean bill of health. Sampling to BS 7592 verifies that your regime was working at the moment the sample was taken; it does not license you to ease off the daily disinfection, circulation and dilution that produced the result [7][9].

A genuine caveat

This is general guidance, not an operating scheme for your pool. The disinfectant type and residual, the circulation and dilution rates, the pool temperature and the sampling frequencies that apply to your hydrotherapy pool come from a competent pool-plant specialist and a suitable, site-specific risk assessment, working from HSG282, HTM 04-01 where it applies, and your manufacturer’s instructions. Nothing here is legal, medical or design advice, and a special-school pool, a care-home pool and an NHS physiotherapy pool will not arrive at the same numbers.

A next step you can take today

Find out two things about your pool before the week is out: which guidance your risk assessment says governs it — HSG282, HTM 04-01, or both — and when the balance tank was last cleaned. If either answer is “I’m not sure”, you have found your first action, and it belongs in front of your water safety group or duty holder now, not after the next quiet holiday.

After that, the controls above are recurring tasks with a date and a name against each one. A residual not held over a weekend, a sample overdue or a fill outlet left unflushed through the holidays is far easier to catch in a live, visible log than in a folder nobody opens between sessions — which, for a pool used by your most vulnerable people, is the difference that counts.

FAQ

Is a hydrotherapy pool covered by HSG282 or HTM 04-01?

In practice, often both. The pool itself is a warm, recirculating, aerosol-generating system, so the spa-pool guidance HSG282 is the right model for running its plant — circulation, filtration, disinfection, dilution and sampling [2]. Where the pool sits inside a healthcare premises, the building’s hot and cold water services and the susceptible-user context fall under HTM 04-01 [4]. Your risk assessment should state which document drives each control; where they overlap, the more demanding standard is the safe default.

Why is a warm hydrotherapy pool higher risk than a normal swimming pool?

Because it reverses the things that make a swimming pool relatively safe. A swimming pool is cooler, rapidly turned over and heavily diluted; a hydrotherapy pool is held near body temperature, used in short bursts with long quiet periods, and serves more susceptible bathers [1][5]. Warm, plus slow-moving, plus vulnerable users is the combination guidance treats most seriously.

Does low usage make a hydrotherapy pool safer?

No — it usually makes it harder to control. Warm water that sits still between sessions, over evenings, weekends and school holidays, gives Legionella the time it needs to grow, and rarely-used associated outlets such as poolside showers and the fill point stagnate alongside it [6]. A quiet pool needs a deliberate plan for standing water, not a relaxed one.

Sources

[1] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [2] HSE, “Control of legionella and other infectious agents in spa-pool systems (HSG282)”. https://www.hse.gov.uk/pubns/books/hsg282.htm [3] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [4] NHS England, “Health Technical Memorandum 04-01: Safe water in healthcare premises”. https://www.england.nhs.uk/publication/safe-water-in-healthcare-premises-htm-04-01/ [5] NHS, “Legionnaires’ disease”. https://www.nhs.uk/conditions/legionnaires-disease/ [6] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [7] BSI, “BS 7592:2022 - Sampling for Legionella bacteria in water systems. Code of practice”. https://knowledge.bsigroup.com/products/bs-7592-sampling-for-i-legionella-i-bacteria-in-water-systems-code-of-practice-1 [8] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [9] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm