Potentially toxic “forever chemicals” have been detected in the drinking water sources at 17 of 18 England’s water companies, with 11,853 samples testing positive, something experts say they are “extremely alarmed” by.

Per- and polyfluoroalkyl substances (PFAS) – a group of 10,000 or so human-made chemicals widely used in industrial processes, firefighting foams and consumer products – were found in samples of raw and treated water tested by water companies last year, according to the Drinking Water Inspectorate (DWI), the Guardian and Watershed Investigations has found.

Some PFAS, including PFOS and PFOA, which are now mostly banned, have been linked to cancers, thyroid disease, immune system and fertility problems as well as developmental defects in unborn children.

The DWI says the “dangers of PFAS have become a growing concern due to their persistence in the environment, ability to accumulate in the human body, and potential health effects”.

The DWI categorises PFOS and PFOA contamination risk in three tiers, with tier 3, equal to or more than 100 nanograms a litre, being high risk and the point at which action must be taken to dilute the PFAS or remove the water source from public supplies.

PFOS was found in raw untreated water at 18 times the tier 3 100ng/l limit for drinking water. PFOA was detected at 1.5 times the limit.

Despite growing concerns about the health impacts of other PFAS, there are no limits set for the rest of the 10,000 or so substances. Of the 47 PFAS which water companies have been told to look for, 35 were detected and an additional PFAS compound was also found.

Affinity Water appears to have the biggest PFAS problem, with 73 raw water samples above the maximum DWI limits at five sites, followed by Anglian Water with 22 raw samples above the limit from two groundwater sources, according to a DWI report. Southern Water found two samples at or above the top limit in its treated water.

The DWI says these high concentrations never made it to people’s taps because the contaminated water is blended with another source to bring the levels down. However, given there has been a lag between production of PFAS – some have been manufactured for decades – and the water sector being required to test for them, it is likely that some people will have consumed high levels of PFAS in tap water.

“The report shows that there are people who are drinking medium-risk water,” said Stephanie Metzger, a policy adviser at the Royal Society of Chemistry (RSC).

Of the 12 water companies that provided data for PFAS in treated water, eight had a total of 398 samples with PFAS above 10ng/l, putting them in the DWI’s second tier category, also termed medium risk. Blending is not required at this level.

“We don’t think anyone should be drinking medium-risk water … the toxicology data shows the risk of health effects becoming more over time as PFAS builds up in our body,” said Metzger.

The RSC is pushing for a tenfold reduction of the limit for individual PFAS types – from 100ng/l to 10ng/l – as well as an overall limit of 100ng/l for the total amount of PFAS.

It would bring England and Wales closer to the EU and Scotland, which have a stricter limit of 100 ng/l for the sum of 20 specific PFAS in treated water. Some countries have stricter limits, with Denmark’s set at just 2ng/l for four individual PFAS, and the US Environmental Protection Agency has proposed dropping limits on some to just 4ng/l.

Dr David Megson, a forensic environmental scientist from Manchester Metropolitan University, said: “Our guideline values for PFAS in drinking water are not as stringent as other countries, yet it is still a challenge for water companies to provide water with PFAS levels below these limits.

“Ultimately it is water companies and consumers who are picking up the bill to try to manage these contaminated supplies, not the polluters. Urgent action and investment is required.”

The RSC wants to see more regular monitoring and the creation of a chemicals agency to deal with PFAS and other contaminants.

“There are so many PFAS out there and we are only testing for 47 and there are so many information gaps. There could be more PFAS out there we are exposed to. There needs to be more broad testing,” said Metzger.

Other information gaps exist because some companies do not know whether or not they use PFAS in their products and processes.

“We really think companies need to do a PFAS audit so they can know what PFAS is in their supply chain and factories,” Metzger added. “How can water companies do a risk assessment [of their area] if companies don’t even know they’re using it?”

Dr Clare Cavers from the environmental charity Fidra, described the findings as “extremely alarming, in particular as the acceptable limit set by the DWI for the banned toxic forever chemical PFOS is much higher than in other parts of the world”.

She added:“With a recent study finding that PFOS can pass to children in the womb, [it] is gravely concerning.”

Anglian Water said it had “robust monitoring and reporting systems in place” and that it tested for PFAS compounds “at all of our water treatment works on a risk-based frequency as per regulatory requirement and between”.

It added that it was “proposing more than £68m of investment to upgrade our treatment processes to reduce PFAS at 15 sites across our region” between 2025 and 2030.

Southern Water said it had been “monitoring levels of key forever chemicals in water sources, even before new guidance requiring us to do so was issued. No results have been found in our sources of drinking water exceeding the maximum permitted of 100ng/l of PFAS.” It said it was investing £3bn between 2020 and 2025 on its network.

Cavers said: “The levels of PFOS detected in these samples from England water companies are especially worrying because PFOS restrictions have been in place for over a decade, and meanwhile other forever chemicals with similar or greater toxicity continue to be used widely, and to accumulate around us.

“The persistence, bio-accumulation and toxic properties of PFAS, with some lasting thousands of years in the environment, mean the pollution we cause today will last for generations to come.”

In a statement received after publication, an Affinity Water spokesperson said: “We want to reassure customers that we have processes in place to monitor sources of water in the environment closely for PFAS concentrations and remove or blend sources, where necessary, to ensure there are no typical concentrations of PFAS in the drinking water that is supplied to customers.”

A Water UK spokesperson said companies “adhere to high standards set by regulators, with virtually all samples meeting their strict tests”.

The Department for Environment Food and Rural Affairs, the Environment Agency and the DWI declined to comment.

• This article was amended on 28 November 2023. A reference in an earlier version to bringing “the UK closer to the EU and Scotland” should have said “England and Wales” rather than the UK.