Swimming Pool Water Standards UK: The PWTAG Numbers

The swimming pool water standards UK spas, hotels and leisure centres work to are not written into a single law. They come from the Pool Water Treatment Advisory Group (PWTAG), whose Code of Practice the Health and Safety Executive and local councils treat as “the standard to be achieved in effectively managed swimming pools”. If a pool makes people ill, that is the benchmark an inspector measures it against. This page pulls out the numbers that matter to you as a swimmer: chlorine, pH, how often the water is tested, how fast it is filtered, how many people the pool should hold, and what that famous “chlorine smell” is really telling you.

For hot tubs and spa pools, which have their own HSE guidance and much higher chlorine levels, see our page on spa pool safety rules and HSG282.

The numbers at a glance

Figures from the PWTAG Code of Practice. PWTAG also stresses that any chlorine figure only counts if the monthly lab results are satisfactory: the numbers are a means, clean water is the goal.

Chlorine: why less is the aim

Many people assume a pool with more chlorine is safer. PWTAG’s view is the opposite. Free chlorine should be “as low as is consistent with satisfactory microbiological quality”, and the Code puts that at 1 mg/l or below when the pH is held at 7.0 to 7.2. A pool that cannot hold its pH that low has to run more chlorine, up to 2.0 mg/l at pH 7.2 to 7.4, because chlorine works less well as pH rises.

The reason is simple chemistry. PWTAG’s technical note on free chlorine explains that at pH 7.0, about 75% of the free chlorine in the water is in its strong, active form; at pH 7.5 it is 50%; at pH 8.0 just 25%. So a pool at pH 7.0 with 1 mg/l of chlorine is doing better work than a pool at pH 8.0 with 2 mg/l. The Code says levels above 3 mg/l should not be needed in any pool using hypochlorite, and if a dosing fault pushes free chlorine to 5 mg/l, dosing should stop and, if the level keeps climbing, swimming should stop until it is under control.

Pools that add a secondary treatment, ultraviolet (UV) or ozone, can run lower chlorine still. PWTAG recommends UV in particular because it breaks down chloramines and kills Cryptosporidium, the stomach bug that ordinary pool chlorine struggles to deal with.

That “chlorine smell” is not chlorine

The sharp swimming-pool smell that catches your throat and stings your eyes is mostly combined chlorine, also called chloramines. They form when chlorine reacts with what swimmers bring in: sweat, urine, skin, cosmetics. The most irritant of them, nitrogen trichloride, is the one you smell in the air above a busy indoor pool.

PWTAG’s rule is that combined chlorine should be as low as possible, always less than half the free chlorine, and never more than 1 mg/l whatever the free chlorine level. A strong smell suggests one or more of:

• too many bathers for the treatment plant;
• not enough fresh water being added (PWTAG asks for 30 litres per bather);
• poor circulation or a turnover that is too slow;
• people getting in without showering;
• cleaning products with ammonia or detergents getting into the water.

A well-run pool smells of very little. If a hotel or leisure pool hits you with chlorine from the changing room door, the water is not cleaner; it is working hard.

Testing: how often and by whom

Pools without automatic control should test the water before opening, every two hours while open and after closing. Pools with automatic dosing still need manual tests at least three times a day to check the sensors, and the controllers should be checked against a manual test daily. Automatic systems do not measure combined chlorine, so that still has to be tested by hand.

Monthly laboratory tests sit on top of that. PWTAG says swimming and spa pools should be tested at least monthly by a UKAS-accredited laboratory for:

• Aerobic colony count: ideally no more than 10 colony-forming units per millilitre; over 100 is unsatisfactory.
• E. coli: absent in a 100 ml sample. Its presence points to faecal contamination.
• Coliforms: a wider indicator of contamination.
• Pseudomonas aeruginosa: absent in 100 ml. It causes skin rashes and ear infections, and can live in filters, channels and pool covers.

Hydrotherapy pools, warmer and often used by people with health conditions, should be tested weekly. A pool that fails a lab test should retest; three unsatisfactory results in a row call for immediate action, which may mean closing.

Turnover: how fast the water is cleaned

Turnover is the time it takes for a volume of water equal to the whole pool to pass through the filters and treatment plant. The shallower and busier the pool, the faster it needs to be. The Code’s table includes:

Water should circulate 24 hours a day. The fast turnover for spas explains why a busy hot tub can stay clean while a slow-filtering one turns cloudy within an hour.

How many people a pool should hold

PWTAG sets the maximum number of people in the water at any one time by depth:

A typical hotel pool of 10 m by 5 m, mostly 1.0 to 1.5 m deep, works out at about 18 people. That is a safety limit, not a target. The treatment plant may be sized for fewer, and a spa that packs its pool on a Saturday afternoon will often show it in the water by teatime.

Clarity: the test you can do yourself

You do not need a test kit to judge one thing. PWTAG says you should be able to see the bottom of the pool clearly at its deepest point from the side. If you cannot, the pool should be closed: there is a drowning risk because lifeguards cannot see a swimmer on the bottom, and cloudy water usually means the disinfection is failing.

Other signs worth noticing at a spa or hotel:

• a strong chlorine smell or stinging eyes (high combined chlorine);
• slimy or greasy pool edges and steps;
• foam on the surface;
• no visible showers by the pool, or nobody using them;
• a hot tub that is crowded all day with no closed periods for checks.

Any reputable spa can tell you how often it tests and whether it uses a qualified pool technical operator. PWTAG’s Code expects a qualified operator to be on site or available within two hours for larger and busier pools, and at least weekly contractor visits for smaller ones. Pools that meet the Code in full can apply for PWTAG’s PoolMark accreditation, so it is worth asking whether a spa has it.

What you can do as a swimmer

Pool water quality is partly in your hands. PWTAG asks everyone to use the toilet and then shower with soap before swimming, and to make sure children do the same. Showering removes much of the sweat, cosmetics and other matter that would otherwise turn into chloramines. Stay out of the pool for 48 hours after diarrhoea or vomiting stops, and for 14 days after cryptosporidiosis, because Cryptosporidium can survive normal pool chlorine for days.

For more on spa days and pools that take this seriously, see our pages on outdoor spas in the UK, cold water immersion safety and the best spa hotels in England.

Frequently asked questions

What chlorine level should a UK swimming pool have? PWTAG says free chlorine should be 1 mg/l or below, with a minimum of 0.75 mg/l, when the pH is held at 7.0 to 7.2. Pools at pH 7.2 to 7.4 may need up to 2.0 mg/l. Levels above 3 mg/l should not be necessary.

What pH should a swimming pool be in the UK? Between 7.0 and 7.4 for pools using chlorine, ideally 7.0 to 7.2, because chlorine is much more effective at lower pH.

Why does a pool smell strongly of chlorine? The smell is mostly combined chlorine, or chloramines, formed when chlorine reacts with sweat, urine and skin. A strong smell usually means the pool is overloaded, under-diluted or poorly circulated, not that it is extra clean.

How often should pool water be tested? PWTAG recommends chemical tests before opening, every two hours during opening and after closing, plus a lab test for bacteria at least monthly. Hydrotherapy pools should be lab-tested weekly.

Are PWTAG standards a legal requirement? Not as a law in themselves, but the HSE and local authorities treat the PWTAG Code as the standard for a well-managed pool, and falling short of it can be taken into account in enforcement.

What are the water standards for hot tubs and spa pools? Spa pools follow HSE guidance HSG282, with free chlorine of 3 to 5 mg/l, and PWTAG gives commercial spas a 6-minute turnover. Our HSG282 page explains the detail.

Sources

• PWTAG Code of Practice (Pool Water Treatment Advisory Group)
• PWTAG Technical Notes: TN53 Microbiological testing, TN60 Free chlorine, TN61 Combined chlorine, TN34 Cloudy water
• HSE, The control of legionella and other infectious agents in spa-pool systems (HSG282)