Depending on where you live, many substances are added to tap water. It's anything but pure H2O

Tap water in the UK is anything but pure H2O.

Depending on where you live, many substances are added to make it cleaner, clearer and supposedly better for you.

With Scottish researchers investigating whether lithium should be added to the water to boost mood, we look at what's being put in your water before it reaches your glass - and its impact on your health.

Should lithium be in the water supply?

Scientists in Scotland are looking into whether adding lithium to water supplies could help mental health. Lithium is prescribed as a mood-stabilising drug (a typical daily dose is 300 mg), mainly for bipolar disorder, and is thought to work by modifying certain chemicals in the brain.

But it occurs naturally in many water sources in Scotland, leaching out from volcanic rock at very low concentration (providing a daily dose of about 2 mg per two litres of water).

Now researchers at the University of Glasgow School of Medicine are investigating where there is a link between lithium in water and lower suicide rates - previous research in Austria and Japan suggests that people whose water supply naturally contains lithium are less likely to take their own lives.

'We want to improve the methodology by looking at smaller postcode areas,' explains Daniel Smith, a professor of psychiatry, who is heading the research.

Results are expected next year and could spark discussions about adding lithium to the water supply.But, despite the fact that doses would be extremely low, it's a controversial step. One scientist has reportedly received death threats over his involvement in the research.

Chris Exley, professor of bioinorganic chemistry at Keele University, thinks it 'unlikely' that lithium will be added to water supplies soon. But he says that such low amounts are unlikely to cause harm or make any difference to mood anyway.

Chlorine's not just for the pool

Chlorine is added to most water supplies across the UK. An excellent disinfectant, it kills bacteria such as campylobacter and E.coli.

'We enjoy some of the highest-quality drinking water in the world, but it does need to be disinfected first,' explains Sue Pennison, principal inspector of the independent Drinking Water Inspectorate, which upholds water industry standards in England and Wales.

At low quantities chlorine poses no danger to health, and most water companies aim to keep levels well below the maximum set by World Health Organisation's guidelines.

But in high amounts, chlorine can lead to eye and skin irritation, and difficulty in breathing because it irritates the lining of the airways, says Professor Exley.

Chlorine is added to most water supplies across the UK as it's an excellent disinfectant which kills bacteria

In 2011, a study from Birmingham University linked water chlorination to birth defects, although a previous study by Imperial College London found little evidence of a link.

Chlorine can also form cancer-causing compounds when combined with oxygen.

An alternative to chlorine is to destroy bacteria with ultraviolet light - water is passed through a pipe containing special tubes that shine UV light on to it. This method is becoming increasingly popular because it leaves no residue or smell.

Fluoride for stronger teeth

Around a tenth of the UK population drinks water containing added fluoride, to help reduce dental decay. Fluoride is the only compound added purely for health reasons.

Studies show that fluoride helps mineralise teeth (it promotes formation of the mineral fluorapatite, which is harder than natural enamel).

Last March, a Public Health England report showed that children in areas with water fluoridation have less tooth decay than those in areas without it.

Whether to add fluoride or not is the decision of the local health authority. Most fluoridation happens in the West Midlands and the North-East of England. There is none in Scotland or Wales.

Nigel Carter, chief executive of the British Dental Health Foundation, is calling on the Government to introduce the mineral to more areas. However, fluoridation is controversial.

Scientists are looking into whether adding lithium to water supplies could help mental health. It is prescribed as a mood-stabilising drug, mainly for bipolar disorder (posed by model)

At high levels, fluoride is a toxin - an early sign of overdosing is dental fluorosis, when white marks develop on the tooth enamel - and the maximum allowed in Europe is 1.5 parts per million.

Fluoride has been linked to bone cancer in boys, according to a Harvard School of Dental Health study. And a 2012 report in the journal Environmental Health Perspectives concluded there were 'strong indications' fluoride exposure 'is highly problematic for proper cognitive development and brain formation' of children.

The National Pure Water Association is lobbying to stop fluoride being added to drinking water anywhere, claiming there are no good studies linking the practice with dental health.

Chemical to stop lead poisoning

If you live in Glasgow, London or Manchester, where there are still rows of Victorian terraces, chances are you have phosphate in your drinking water.

This chemical - a compound of phosphorus and oxygen - is added to water likely to pass through lead pipework.

'Lead pipes are still common in homes with plumbing done before the Seventies,' explains Sue Pennison. 'Lead is poisonous and can affect the neurological development of young children.'

If lead is found in the water supply, the pipe can be lined with phosphate which acts like a hard impermeable scale, stopping it making contact with the drinking water.

'It is added following a risk assessment and doesn't pose any threat to human health in itself,' says Sue Pennison.

However, Professor Exley says it can 'promote the growth of algae if the water is exposed to light because it is a plant nutrient' - seen, for example, in green standing water. However, this would be 'highly unlikely' to affect health.

The cost of crystal clear water

To get the crystal clear drinking water we expect, chemicals are added that make sediment form clumps which can then be filtered out easily.

One such chemical is aluminium sulphate, explains Professor Exley. Most should be extracted at the treatment works. Some may remain in minute quantities when it reaches your tap, but at too low a level to cause problems, he adds.

There are concerns that aluminium in the water supply may be linked to Alzheimer's - high levels of the metal in the brain may contribute to the early onset of the condition, although this is unproven.

A study in the Lancet did find that people living in parts of England and Wales with higher concentrations of aluminium in the water had a higher risk of developing the disease than in areas where levels were low.

Around a tenth of the UK population drinks water containing added fluoride, to help reduce dental decay

But a later Norwegian study suggested that although aluminium was a potent neurotoxin, drinking water contains only a fraction of what we get from other sources, including antacid medication and tea.

Professor Exley, who has previously reported that there is possible link between Alzheimer's and aluminium, says: 'There should not be a downside to adding aluminium sulphate to the water supply as long as the process is well controlled and never rises above 50 parts per billion.'

However, in 1988, a tanker driver in Camelford, Cornwall, emptied 20 tonnes of aluminium sulphate straight into the water supply by mistake, exposing local people to 3,000 times the permitted level.

Despite many people complaining of problems, including dementia, the final report concluded it was unlikely the exposure could have caused long-term health effects.

PS: Is soft water better for you?

Hard water - found in areas with chalk or limestone soil, such as in London and the Isle of Wight - contains higher levels of dissolved minerals such as magnesium, which may help regulate blood sugar, and bone-healthy calcium.

A 2004 Finnish study even suggested hard water may protect against heart attacks.

The downside is that hard water makes soap less effective and leads to scale build-up on kettles. Water softeners can ease this, though may remove good minerals, such as calcium, in the process.