Packed with happy children, council-run paddling pools and splash parks have offered welcome relief for families from the heatwaves of this summer.As a father of two, aged six and eight, I know how irresistibly drawn to these pools children are. But you might want to pause for thought before they launch into the cool water.For experts warn vital checks of water quality and levels of chlorine – used to disinfect pool water – are being missed, leaving children, whose immune systems are less robust, at risk of exposure to bugs including E. coli, norovirus and Pseudomonas (which can trigger painful ear and skin infections).Another cause for alarm is that water isn’t being tested for a particularly nasty parasite, known as cryptosporidium (or ‘crypto’), which can lead to extreme watery diarrhoea, vomiting, dehydration and hospitalisation.The Pool Water Treatment Advisory Group (PWTAG), an independent body whose water quality standards are used by the Health and Safety Executive and councils to assess pools, warns that sweat, urine, faeces and skin cells washed from people’s bodies react with chlorine in the water.As the chlorine works to neutralise this contamination, it is used up. Sunlight can break chlorine down further, which is why levels can fall quickly in busy outdoor pools on hot summer days, explains Professor Kelly Reynolds, an environmental microbiologist at the University of Arizona, who has researched swimming-pool water quality.‘The more people there are taking the chlorine out, the less chlorine concentration is available in the pool,’ she explains.If chlorine levels fall too low, bacteria introduced into the pool can multiply, with some doubling in number every 20 minutes, says Dr Primrose Freestone, an associate professor in clinical microbiology at the University of Leicester. Water at a splash park may look clean, but harmful bacteria could be lurking Will Stoddart investigates contamination levels in six splash parks in and around LondonFor this reason, the PWTAG says chlorine levels at splash parks and lidos should be checked every two hours when open. Experts warn some pool operators neglect these checks, meaning chlorine levels may be too low just when pools are being exposed to the greatest contamination.More worrying still, the PWTAG warns there is ‘good evidence’ that some pool operators are failing to carry out monthly laboratory tests for potentially dangerous bacterial contamination.And as Professor Reynolds spells out, the issues may be exacerbated in paddling pools: ‘Children have poorer hygiene habits, which increases the potential to contribute a faecal load to the swimming pool.’At the same time, ‘children are twice as likely to swallow water than adults, which increases the chance of getting it in their mouths’. She has even seen children sit over water jets and drink from them ‘like it’s a water fountain’ – exactly the sort of behaviour that allows pathogens from faeces to pass from one child to another, she says.Professor Reynolds says she’s also seen parents change nappies at the poolside and then rinse their children’s bottoms in the water, believing the chlorine will kill any germs – ‘which it doesn’t, at least not instantaneously’. (Depending on the germs, this can be within minutes for E. coli – if the chlorine levels are high enough; with parasites such as crypto it can take ten days.)And while at some sites the water is collected, treated, filtered and pumped around the system regularly, others may empty or refill only several times a week or not at all.Now a Good Health investigation has uncovered alarming bacterial contamination in council-run pools and splash parks.I collected samples from six popular facilities in London and the home counties, on August 11 (when temperatures were mid-to-high 20s) and August 12 (around 30C), both while children were playing in them and afterwards (between 2pm and 8pm).At two sites, the bacterial counts were above the level set by the PWTAG; at two others, they were 1,000 times higher.Lucy Robertson, a professor of parasitology at the Norwegian University of Life Sciences, told Good Health that levels this high could mean a greater chance of disease-causing bugs being present and should trigger an investigation by the council or pool operator and corrective action.She said it also ‘raises the suspicion’ that they weren’t checking water and chlorine levels frequently enough.Chemical dosing requires ‘regular checks and maintenance’, adds Martin Wood, an environmental microbiologist and co-founder of pool-water specialists Pool Sentry – but he says that financial pressure means the pool operators don’t have the resources to do enough checks. WHAT PARENTS CAN DO The Pool Water Treatment Advisory Group advises keeping children out of pools if they have diarrhoea or vomiting for at least 48 hours after symptoms stop.Look for its PoolMark accreditation or ask the operator how often chlorine is checked and whether the water is filtered and recirculated.Try to stop them swallowing the water and drinking directly from splash jets. ‘An issue with open-access pools is there is often little or no control over the numbers of bathers which are very likely to exceed the capacity of the water treatment system on hot days during holidays,’ he says.Biodet, the lab at the University of Hertfordshire that tested our samples, also raised the alarm. Dr Richard Smith, a specialist in microbiological testing and director of Biodet, said the two ‘notably high’ readings should trigger an investigation into chlorine levels and filtration.The samples were tested for E. coli and coliform bacteria, which can signal faecal contamination; for Pseudomonas aeruginosa – and the overall bacteria level, the total viable count (TVC).The latter is used as a broad measure of water hygiene and how effectively the pool’s filtration and disinfection are working, rather than as a test for specific disease-causing bugs.According to the PWTAG, a well-run pool should normally contain no more than ten bacteria colony-forming units per millilitre (a TVC of under 10); anything above 100 is classed as ‘unsatisfactory’.Two of the pools we sampled – Gadebridge Splash Park in Hemel Hempstead and Bruce Castle Park in north London – were below five. Howard Park in Letchworth recorded a TVC of 20; twice the normal level. At Lordship Recreation Ground in north London, (also tested on the hotter of the two days), it was 30. These were well below the 100 level classed as unsatisfactory.But the results from Runnymede Pleasure Grounds in Egham and California Country Park in Berkshire had TVCs over 10,000; more than 100 times the level regarded as unsatisfactory.Looks can be deceptive: at Runnymede, the water looked clean and inviting.Reassuringly, E. coli and coliforms – signs that faecal matter has recently entered the water – were not detected in any of the six samples.But ‘the thing with TVC is that you do not know what the bacteria are’, says Professor Robertson. For instance, Staphylococci (which can cause skin and wound infections), can survive even when E. coli and coliforms have been killed by chlorine.Professor Robertson says a high TVC can be a sign chlorination is failing or that bacteria are growing in biofilm, a slimy layer that clings to the inside of pipework.‘This can be important – if it is loosened by something such as by a change in water pressure, then someone could ingest a concentrated gloop of the stuff.’To remove biofilm, pipes have to be scrubbed by hand and treated with high levels of chlorine, says the PWTAG.Our findings come against a background of growing concern about Britain’s ageing swimming facilities, with a report by the All-Party Parliamentary Group for Swimming this year finding more than 1,200 public pools are over 40 years old and past their expected lifespan. In Scotland, 56 per cent are over 30 years old.Some but not all pools have automatic dosing, where machinery automatically adds chlorine to keep levels stable.Both Runnymede Pleasure Grounds and California Country Park use automatic chlorine dosing systems. Yet these didn’t prevent high bacteria levels.Many older pools still use simpler fill-and-empty systems, like in Haringey – its paddling pools at Bruce Castle and Lordship Recreation Ground are emptied, cleaned and refilled three times a week during the summer.But there is another danger that’s not being tested.Crypto is a microscopic parasite spread in faeces which causes severe watery diarrhoea, stomach cramps, vomiting and dehydration. It is ‘the pathogen that most commonly causes outbreaks linked to recreational water use because it is resistant to chlorine’, the UK Health Security Agency told Good Health.The PWTAG warns that just a millilitre of infected diarrhoea can contain around a million of these microscopic parasites – as little as one may be enough to make someone sick, compared with the thousands of bacteria typically needed for infection from E. coli, says Professor Reynolds. According to the PWTAG, a well-run pool should normally contain no more than ten bacteria colony-forming units per millilitre (a TVC of under 10)‘If you’re in close proximity to somebody who’s shedding a million and then you swallow water nearby, it only takes one potentially to make you sick,’ she says.The latest crypto figures show there were 14 outbreaks linked to swimming pools or water parks in England between 2023 and 2025, affecting at least 86 people, with some hospitalised.Yet crypto is not routinely tested for. Professor Rachel Chalmers, head of the Cryptosporidium Reference Unit at Public Health Wales, says the current UK approach is that testing should be used during outbreaks or when filtration failure is suspected – not for prevention.But Dr Seak Lin Ly, a water quality analyst and author of a recent study published in The Journal of Infectious Diseases, argues that high-risk pools – particularly children’s pools in summer – should be proactively tested, rather than waiting for an outbreak.The study showed that crypto can be circulating in swimming pools on average 27 days before it’s detected and a pool being closed for treatment.The parasites are protected by a tough outer shell so levels of chlorine needed to kill them off are many times higher than are normally used in pools. But these higher levels can irritate the eyes and skin and the fumes can harm the airways – so the pool has to remain shut until the chlorine falls back to its normal range.Chlorine alone is also not often enough: crypto has to be physically removed from the water by filtration. But not all paddling pools can do this.While noting that routinely testing for crypto is difficult and expensive (as large volumes of water have to be analysed), James Ebdon, a professor of environmental microbiology at the University of Brighton, says ‘a more cost-effective, longer-term solution would be to improve treatment of the recirculating paddling-pool and splash-pad water through additional treatment steps,such as physical filtration and ultraviolet [using heat to kills germs] or ozone [a gas that destroys microorganisms].’‘These could help remove cryptosporidium as well as a range of other bacteria, viruses, protozoa and moulds responsible for waterborne disease.’Even the best treatment, however, cannot protect a child instantly. ‘If an infected swimmer excretes the [crypto] parasites into the pool, the next swimmer can get infec-ted from the water before it goes through treatment,’ says Professor Joan Rose, a water microbiologist at Michigan State University.Professor Reynolds says people can shed the parasite or other pathogens even when they don’t realise they’re infected, while many return to swimming too soon after their diarrhoea stops.Despite the concerns raised by our investigation, experts stress that parents should not be frightened away from paddling pools and splash parks.A spokesman for Runnymede Council said: ‘Water testing is undertaken routinely during opening hours. We had become aware that during the heatwave water-quality levels had been fluctuating, and have commissioned our own independent tests. In the meantime, the pool has been closed.’A Wokingham Borough Council spokesperson (California Country Park) said the water at their pool is tested every two hours during opening hours: ‘We have reviewed our records for the two-week period around August 11 and can confirm that both pH and chlorine levels remained within the levels recommended by the PWTAG during operating hours.‘Given the result reported to us, we are carrying out a shock chlorine treatment of the system and will undertake further microbial testing to confirm there is no ongoing issue.’