London’s tap water contains PFAS, but at surprisingly low levels

London’s tap water contains PFAS, but at concentrations well within current safety limits. That is the key, and somewhat unexpected, finding from the largest UK survey of PFAS in household water supplies. ‘None of us knew what we were expecting going into this … but we were a bit anxious what we would discover, says Alexandra Richardson from Imperial College London, who led the work. ‘Overall, this is a good news story. London’s surface waters are some of the most contaminated in the country, and the drinking water abstracted from those source waters is within guideline limits.’

Per- and polyfluoroalkyl substances (PFAS), often called ‘forever chemicals’, are a group of thousands of highly stable fluorinated compounds used widely in industry and consumer products because of their chemical stability and water resistance. That same stability, however, means they linger in the environment and in our bodies, where long-term exposure has been linked to health problems including cancer, immune disruption and reproductive toxicity.

Richardson and her colleagues analysed 210 samples from 92 homes and 12 public drinking fountains across 28 of London’s 33 boroughs, screening for 38 PFAS compounds using analytical methods capable of detecting contaminants at low nanogram-per-litre concentrations. PFAS concentrations in London tap water ranged from 3–41ng/l, with all samples below both the UK Drinking Water Inspectorate’s individual threshold of 10 ng/L or the 100ng/L cumulative limit introduced in January 2025. Estimated weekly exposure to the four PFAS monitored by the European Food Safety Authority – perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorononanoic acid (PFNA) and perfluorohexane sulfonate (PFHxS) – were also below the recommended safety thresholds of 4.4ng/kg body weight for every sample.

Part of the explanation lies in how London’s water is managed. Suppliers blend water from multiple ground and surface water sources before it reaches the treatment works, diluting any localised contamination. The team’s own data directly captures the effect of treatment: while 5.1% of London’s raw source water exceeded the current lowest Drinking Water Inspectorate threshold of 10ng/L, that figure fell to 3.0% after treatment and to 0% at the tap.

The study saw researchers mail sampling kits to volunteers and included field blanks to track contamination at home or in transit. Five commercial filter jugs were also tested, each removing at least 85% of PFAS with the best achieving 99%.

Ruth Godfrey , an analytical chemist specialising in the detection of environmental contaminants from Swansea University in the UK, welcomes the filter finding. ‘It’s certainly interesting to see that the filter jugs with commercially available filters featured in this study can appear to clean up these chemicals; it offers something people can proactively do to suppoprt their own health.’

The study, however, did not measure ultrashort-chain PFAS, including trifluoroacetic acid (TFA), which has previously been detected in London’s water and requires different analytical methods. Patrick Byrne, a PFAS expert at Liverpool John Moores University whose research has helped identify major contamination sources across the UK, says ‘there’s well-publicised concern that we’re only monitoring the tip of the iceberg. Concentrations of ultrashort-chain compounds are usually much higher than legacy PFAS since some older PFAS degrade to these compounds, which is why their concentrations are so elevated.’

Richardson says that the findings are reassuring, but not a reason to ease off. ‘This is not, “it’s fine, we should stop”. We should continue to improve drinking water quality and investigate further. At least we now know something we didn’t know before.’