What's in your drinking water?

A disinfectant added to virtually all UK public water supplies. Not a contaminant in the traditional sense — chlorine is essential for water safety — but residual levels can affect taste and odour at the tap.

Deliberately added during municipal water treatment. Residual levels at the tap typically range from 0.1–0.5 mg/L.

No health risk at the concentrations used in UK water treatment. Main concerns are aesthetic: chlorine taste, swimming pool smell. Can cause skin and eye irritation for sensitive individuals.

All UK public water supplies are chlorinated. Taste is most noticeable in areas close to treatment works or where higher doses are needed for long distribution networks (common in rural areas). Some water companies use chloramine instead, which is less noticeable.

A group of chemical by-products formed when chlorine reacts with natural organic matter in water. The most common is chloroform. THMs are an unavoidable consequence of chlorine disinfection and represent a balancing act between disinfection effectiveness and by-product formation.

Formed during water treatment when chlorine reacts with humic and fulvic acids from decomposing vegetation. Highest concentrations occur in surface water supplies with high organic content, particularly in autumn.

Long-term exposure associated with increased risk of bladder cancer. Possible links to adverse reproductive and developmental effects. Risk is from chronic low-level exposure over many years, not acute.

THM levels tend to be higher in supplies sourced from upland surface waters with high colour — common in Wales, Scotland, the North West, and South West England. Water companies actively manage THM formation through treatment optimisation.

An essential trace element at low concentrations, but harmful at elevated levels. Copper contamination in drinking water is almost always from domestic plumbing rather than the source water itself.

Corrosion of copper pipes and fittings, particularly in newer installations (first 1–2 years) or where water is soft and acidic. Some industrial discharge.

Nausea, vomiting and diarrhoea at high acute exposures. Long-term excess linked to liver and kidney damage. People with Wilson's disease are especially vulnerable and must monitor intake carefully.

Most common in newer properties with recently installed copper plumbing, particularly in soft water areas (Wales, Scotland, South West England). Running the tap for 30 seconds before use first thing in the morning helps flush standing water from pipes.

A naturally occurring mineral that is deliberately added to some UK water supplies for dental health. Beneficial at low concentrations but harmful at elevated levels. One of the most debated topics in UK public health.

Natural geological deposits (some borehole supplies), deliberate water fluoridation schemes, some industrial processes. Around 10% of the English population receives fluoridated water.

Dental fluorosis (mottling of teeth) at moderate excess. Skeletal fluorosis at high chronic exposure. The UK scientific consensus supports fluoridation at 1 mg/L for dental health, though debate continues.

Fluoridation schemes currently operate in parts of the West Midlands, North East England, and East Midlands. In 2022, the Health and Care Act gave the UK government (rather than local authorities) the power to extend fluoridation schemes.

A broad class of chemical substances used to control pests in agriculture and other settings. UK regulations are among the strictest globally, with an overall precautionary limit rather than individual risk assessments for most compounds.

Agricultural application and run-off, domestic garden use, historical soil contamination (e.g. organochlorines like DDT), golf courses, railway line maintenance, road verge management.

Varies by compound. Potential links to cancer, endocrine disruption, neurological effects, reproductive problems. Chronic low-level exposure is the primary concern rather than acute poisoning.

Metaldehyde (slug pellets) was historically one of the most problematic pesticides for UK water companies, particularly in arable regions. Its outdoor use was banned in 2022. Propyzamide and chlortoluron remain common treatment challenges in parts of southern and eastern England.

An intermediate compound in the nitrogen cycle, substantially more toxic than nitrate. Can form in water distribution systems through incomplete nitrification, particularly where chloramine disinfection is used.

Microbiological conversion of nitrate in distribution systems, fertiliser breakdown, some industrial processes. Can form in premise plumbing where water stagnates with biofilm present.

More potent cause of methaemoglobinaemia than nitrate, especially dangerous for formula-fed infants. Can form carcinogenic nitrosamines in the body.

Nitrite is rarely found at concerning levels in UK public supplies. Private water supplies near intensive agriculture and properties with long internal plumbing runs where water stagnates are at greatest risk.

A naturally occurring metal that is also widely used as a coagulant (aluminium sulphate) in water treatment. Residual aluminium in treated water has been the subject of health debate, though evidence of harm at regulatory levels remains inconclusive.

Natural geological sources, but primarily from aluminium sulphate used as a coagulant in water treatment. Treatment process failures can cause spikes in aluminium levels.

The relationship between aluminium in drinking water and Alzheimer's disease has been extensively studied but remains unproven. At very high levels, aluminium can cause discolouration and taste issues.

The Camelford water pollution incident in 1988 — where 20 tonnes of aluminium sulphate was accidentally added to the supply — remains the UK's most significant aluminium contamination event. Modern treatment controls make such events extremely unlikely.

A metal that exists in multiple forms. Chromium(III) is an essential nutrient; chromium(VI) is highly toxic and carcinogenic. Current water standards do not distinguish between the two forms, though there is growing pressure to set separate limits.

Industrial discharge (particularly from chrome plating, leather tanning, and textile manufacturing), natural geological deposits, some stainless steel corrosion.

Chromium(VI) is a known human carcinogen when inhaled and is under active review for drinking water exposure. Long-term ingestion linked to stomach and intestinal effects.

The UK standard of 50 µg/L covers total chromium. The DWI advisory group (December 2024) is reviewing whether a specific chromium(VI) limit should be introduced. Industrial areas may have elevated levels in groundwater.

An essential trace element that is toxic at elevated levels. The margin between nutritional benefit and toxicity is narrow, making selenium one of the more complex drinking water parameters.

Natural geological deposits (particularly in some chalk and limestone aquifers), agricultural drainage from selenium-rich soils, some industrial processes including glass manufacturing.

Selenosis from chronic excess: hair and nail loss, skin lesions, neurological effects, nausea. Deficiency is also harmful. The therapeutic window is narrow.

Selenium levels in UK drinking water are generally very low. Some groundwater sources in chalk aquifer areas of southern England may have slightly elevated natural levels, but exceedances of the standard are rare.

A metal contaminant that primarily enters drinking water from nickel-plated taps and fittings rather than from the source water. One of the poorest-performing parameters in UK compliance testing.

Leaching from nickel-plated taps and fittings (particularly new installations), stainless steel components, some industrial discharge, natural geological deposits.

The primary concern is allergic contact dermatitis — nickel allergy affects approximately 10–15% of the population (predominantly women). High concentrations can cause gastrointestinal effects.

Nickel consistently has one of the lowest compliance rates in UK drinking water testing (99.74% in 2024), primarily due to leaching from domestic taps and fittings. Running the tap before drinking water in the morning reduces exposure.

Compounds formed by combining chlorine with ammonia. Used as an alternative disinfectant to chlorine in some UK water supplies because chloramines are more persistent in long distribution networks, though they are less effective as primary disinfectants.

Deliberately formed during water treatment by adding ammonia to chlorinated water. Some water companies use chloramination for areas with long pipe networks where maintaining chlorine residual is difficult.

Less odour and taste than free chlorine. However, chloramines are toxic to fish and aquatic life, and dialysis patients must use chloramine-free water. Some people report skin irritation.

Several UK water companies use chloramination in parts of their distribution networks, including Yorkshire Water, Severn Trent, and some zones of Thames Water. The practice is less common in Scotland.

A group of disinfection by-products formed alongside THMs when chlorine reacts with organic matter. Emerging as a regulated parameter — the DWI advisory group recommended introducing a new UK standard in December 2024.

Formed during chlorination of water containing natural organic matter. Concentrations tend to be higher in surface water supplies and can increase in distribution systems.

Animal studies link HAAs to cancer and reproductive effects. Human epidemiological evidence is limited but growing. Considered a greater emerging concern than THMs by some researchers.

The DWI advisory group recommended an 80 µg/L standard for the sum of nine haloacetic acids — going beyond the EU's five-compound, 60 µg/L limit. As of 2026, this has not yet been enacted in UK legislation.

A by-product formed when water containing naturally occurring bromide is treated with ozone. Bromate is classified as a potential human carcinogen and is tightly controlled in UK water treatment.

Formed during ozonation of bromide-containing water. Can also form when sodium hypochlorite (bleach) used for disinfection degrades during storage.

Classified as a potential carcinogen (Group 2B by IARC). The UK standard is set at a level considered to represent a negligible lifetime cancer risk.

Bromate is primarily a concern for water companies using ozone treatment. It is not a widespread issue for consumers but has caused compliance failures at specific treatment works.

A broad group of bacteria used as indicator organisms in water quality testing. Total coliforms include both environmental species and those of faecal origin. Their presence doesn't necessarily mean faecal contamination but indicates that the water system may be vulnerable to pathogen entry.

Soil, vegetation, animal and human intestines. Environmental coliforms can colonise biofilm in pipes and fittings. Faecal coliforms indicate sewage or animal waste contamination.

Most coliform bacteria are not directly harmful, but their detection triggers investigation because it indicates the water supply's disinfection barrier may be compromised.

Coliform bacteria had one of the lower compliance rates in UK public water supplies (99.78% in 2024), primarily due to detections at consumer taps rather than treatment works. Detections often relate to biofilm in domestic plumbing.

A polycyclic aromatic hydrocarbon (PAH) and known carcinogen. Historically entered UK drinking water from coal tar linings used to protect the inside of water mains. Extensive pipe refurbishment has greatly reduced this issue.

Coal tar pipe linings (historical), industrial emissions, vehicle exhaust, incomplete combustion of organic material. Modern sources in drinking water are rare following extensive UK mains replacement programmes.

Classified as a Group 1 carcinogen by IARC. Chronic exposure linked to increased cancer risk. The drinking water standard is set at an extremely low level to minimise lifetime risk.

Benzo(a)pyrene in UK drinking water has been virtually eliminated through decades of water mains refurbishment and renewal. It is now very rarely detected. Historical contamination was most common in areas with older distribution infrastructure.

A naturally occurring radioactive element found in some geological formations. Chemical toxicity to the kidneys is considered a greater concern than radiological effects at the concentrations found in drinking water.

Natural dissolution from uranium-bearing rocks and minerals, particularly granite formations. Mining and nuclear industry discharge (rare in UK drinking water context).

Kidney damage from chemical toxicity is the primary concern. Radiological risk is considered negligible at drinking water concentrations. The WHO guideline is based on chemical toxicity, not radioactivity.

The DWI advisory group recommended introducing a new UK standard for uranium in December 2024. Elevated natural uranium is found in some groundwater sources in granite areas (parts of Cornwall, Devon, Scotland). Currently no UK-specific standard exists.

A by-product formed from the degradation of sodium hypochlorite (bleach) used in water disinfection. Concentrations increase when hypochlorite solutions are stored for extended periods or at warm temperatures.

Degradation of sodium hypochlorite disinfectant during storage and use. Also formed from chlorine dioxide treatment. Concentrations are higher when hypochlorite stock solutions are old or stored in warm conditions.

Inhibits iodine uptake by the thyroid gland, potentially affecting thyroid function. Of particular concern for people with thyroid conditions and for bottle-fed infants.

The DWI has prosecuted water companies for high chlorate levels and issued multiple legal instruments. The DWI advisory group recommended introducing a standard of 0.7 mg/L with a rolling annual average of 0.35 mg/L, aligning with EU requirements.

A highly toxic chemical compound that inhibits cellular respiration. Rarely found in drinking water in the UK but tightly regulated due to its extreme toxicity. Even very small amounts can be lethal.

Industrial discharge (metal finishing, mining, chemical manufacturing), some natural sources, road salt runoff (ferrocyanide anti-caking agent), combustion products.

Acute cyanide poisoning inhibits cellular respiration and can be rapidly fatal. Chronic low-level exposure can cause thyroid effects, neurological damage, and cardiovascular effects.

Cyanide is very rarely detected in UK drinking water. Industrial areas with a history of metal finishing or chemical manufacturing may have contaminated groundwater, but public supplies are well monitored.