Other questions

We answer your not so common questions here:

It’s is extremely unusual for drinking water to be the cause of illness. However, if you are concerned or have any further questions please call us.

Fluoride occurs naturally in soils and rocks and can therefore be found in raw water. The concentration of fluoride depends on the type of soil and rock the water passes through.

Fluoride levels in drinking water of 1 mg/l (1 part per million) are considered to have health benefits by reducing the frequency of tooth decay.

We do not add fluoride to our water. The decision to fluoridate water supplies is taken by your local strategic health authority - not by Thames Water. Your health authority should be contacted for further details.

Thames Water supplies have a natural concentration of between 0.1 - 0.4 mg/l. In the areas to the south and west of Reading the concentrations can be higher, typically around 1 mg/l. 

UK and European regulations require water companies to test drinking water for pesticides. We ensure levels are not above the drinking water standard, which is almost zero - 0.1 micro grams per litre, which is one part per 10 billion, or one drop in an Olympic-sized swimming pool.

Our water samples

Most of our 25,000 annual tests show that no pesticides have made it into our treated water supplies.  However, tiny amounts of pesticides can be transported by rainwater from agricultural sites, gardens, local authority sites and other sources into water catchment areas. 

Pesticides can also reach drinking water sources through the air and by  filtering through the soil. 

For this reason, samples do occasionally show detectable traces of pesticides like isoproturon, diuron and mecoprop, but these are almost always well below the stringent European pesticide standard.

In fact, the concentrations detected are so small that they pose absolutely no risk to public health. If they did, the Drinking Water Inspectorate wouldn't let us supply our water.

What are we doing about it?

We have an ongoing campaign to control the use of pesticides by talking to and working with local users.  As a result, many local organisations have modified their use of pesticides to protect water resources. 

We continue to lobby for additional controls on specific pesticides that are regularly found in drinking water sources.

Water treatment steps used to remove pesticides from drinking supplies include filtration using activated carbon and disinfection using ozone. Activated carbon filters tightly bind pesticide compounds as water passing through them, while ozone attacks the chemical compounds and breaks them up. 

These two processes can be used in isolation or as a combined approach.

As detection methods continue to advance,  we've recently started to look for metaldehyde, the chemical used in slug pellets.  Harmless traces have been detected in treated water supplies by water firms across the country at levels very slightly above the pesticide standard; however these do not pose a risk to health.

 It's impossible to completely remove metaldehyde from water using normal treatment methods. 

That's why we're working with manufacturers and farmers, the main users of slug pellets, to prevent metaldehyde from reaching water sources in the first place. 

Experts say an average-size person would have to drink more than 1,000 litres - more than a tonne - of the worst-affected water (which is not in Thames Water's supply area) every day of their lives to exceed the Health Protection Agency (HPA)'s 'acceptable daily intake' for metaldehyde.

There are several different kinds of home water filters and jug filters available on the market, but each type is designed to remove various compounds from tap water. 

There is no reason to use these filters on health grounds, as tap water from the mains is high quality and closely monitored. 

Water that has passed through a domestic filter should be treated as a perishable foodstuff, consumed within 24 hours, and kept in refrigerated conditions. 

The manufacturer's instructions for the filter equipment should be followed at all times. 

Visit the British Water website for advice on the installation of filters, softeners and other devices fitted to the water supply in your home.

British Water website

Cryptosporidium is the name given to a type of parasite which can infect humans and animals. These parasites can be found in the waste of infected humans and animals and can survive in the environment for several months. 

Infection with Cryptosporidium arises by ingesting the parasite. This can happen through:

  • Contact with infected animals (particularly cattle and sheep).
  • Person-to-person spread (particularly within families/households).
  • Swimming pools in this country and especially abroad.
  • Other recreational waters (e.g. boating lakes, rivers).
  • Contaminated food and water

Infection with this organism is called cryptosporidiosis and symptoms can persist for several weeks.

In healthy people the infection is usually relatively short-lived, but in immunocompromised people (e.g. people with a suppressed immune system such as AIDS patients or transplant recipients) the infection is more serious and can be life threatening.

Please consult a doctor for any medical advice relating to these issues.

The drinking water we supply is closely regulated to ensure that it is wholesome, safe, and fit for human consumption.

There are some basic steps which may make the water more suitable for fish-keeping:

Time and temperature

Fish can be sensitive to extremely small amounts of metals from water that has been in contact with pipework in the home. Run your cold kitchen tap  for a few minutes before collecting water the fish tank to remove any water that hasn’t moved in a while and has been in contact with pipework. 

This water can be collected and used for watering plants.

Rapid changes in water temperature can cause fish to go into shock. Allow the tap water to stand for several hours to reach room temperature before adding it to your pond or tank.


Chlorine is present in drinking water as a disinfectant, yet it is highly toxic to fish. Chlorine must be allowed to completely  disappear before the water is added to the pond or tank.

It is recommended that tap water is left to stand in a clean container for at least 24 hours before use. Alternatively, water can be pre-treated using other methods to de-chlorinate water; your local aquarium supplier will be able to help with this


Levels of nitrogen in the water can also impact upon the health of fish. Nitrogen is naturally produced in aquariums as a by-product of decomposing fish food and faeces. 

Partial water changes and aquatic plants can help to stabilise nitrogen levels. Please note, pet shop-bought nitrogen test kits can be inaccurate. 

Get a free summary report of the water quality in your local area:

Check your water quality

For specialist advice on breeding and caring for fish, we suggest customers should contact a local pet shop or aquarium supplier.

If your issue is not listed on this page or if you have any further questions please call us on 0800 316 9800 24 hours a day.

0800 numbers are free to call from UK landlines, but may be chargeable from mobiles. Rates vary depending on your network and call plan

If your hearing or speech is impaired, you can contact us using the Next Generation Text Service (NGTS).