What is actually in your tap water — and which of it matters

Ask what is in tap water and you will usually get one of two useless answers. Either it is full of chemicals and you should be alarmed, or it meets all legal standards and you should stop asking. Both skip the only question that helps: which of the things in this particular water were put there deliberately, and which arrived on their own?

That distinction does almost all the work.

Put there on purpose

A treatment plant adds things to water, and the additions are the reason the water will not make you ill. Chlorine, or chloramine, kills the bacteria and viruses that used to kill people in large numbers — dysentery, cholera, typhoid. The taste and smell people dislike is the evidence that the disinfection is still working when the water reaches them.

It is not entirely free: chlorine reacts with organic matter in water to form disinfection by-products, trihalomethanes among them, and those are regulated because at high long-term levels they carry a small cancer risk. But the arithmetic is not close. Waterborne disease outbreaks kill quickly and in numbers; by-products present a small statistical risk over decades. Every public health authority in the world makes the same trade, and they are right to.

Plants also correct pH and dose corrosion inhibitors — usually orthophosphate. This one is worth understanding, because it is invisible and it is critical. Correctly conditioned water leaves a protective scale inside old pipes and does not dissolve the metal. Change the water chemistry without accounting for that and the pipes start giving their lead to the water. That is, in one sentence, what happened in Flint, Michigan: not a poisoning that someone added, but a protection that someone removed.

You may also find fluoride, where a country adds it, and traces of the coagulants and treatment aids used to settle particles out. In properly run systems these sit far below any level of concern.

So the honest summary of the deliberate additions: they are why the tap is safe, and the small print on them is small.

Not put there on purpose

Everything below arrives without anyone choosing it — from geology, from farming, from industry, or from the pipes between the treatment plant and the tap. This is the list that deserves attention, and each one has its own article here.

Notice what the middle column does to the usual advice. Lead does not come from the source water; it comes from the last few metres of pipe, which means a citywide test result tells a particular building almost nothing. Arsenic is geology, so it clusters by location and is a question for boreholes rather than for municipal networks. Nitrate is agricultural, so it clusters by farming. None of these is evenly spread, and none of them is answered by a national average.

The one thing almost everybody gets wrong

Boiling water does not remove nitrate, lead or arsenic. It concentrates them. The water evaporates and the contaminant stays behind, so the concentration in what is left goes up.

Boiling is excellent against bacteria, viruses and parasites, and if that is the problem — after a mains break, or under a boil-water notice — it is exactly the right response. For anything dissolved and chemical, it makes matters slightly worse while feeling responsible. It is worth knowing which problem you have before applying the remedy.

What actually removes what

The point of that table is not to sell anything. It is that a filter is not a general-purpose safety device. It is a tool for a named contaminant, and the commonest mistake in this whole subject is fitting one that does not address the thing actually present — because everyone then stops worrying, which is worse than worrying.

What to do, in order

First, find the published analysis for your own supply, from your own water company rather than from a national figure. In the EU this is information you are entitled to.

Second, work out whether your building changes the answer. Plumbing installed before the mid-1980s, a private borehole, a storage tank nobody has opened in years — each of these means the supply analysis is not the whole story for your tap.

Third, match the treatment to what you found, and only then. And plan for the servicing at the same time as the purchase: a filter past its life does not announce itself, it just quietly stops working while everyone assumes it has not.

Why we go on about this

The Foundation funds water systems for children’s homes and shelters. In those buildings nobody living there chose the address, nobody can change the plumbing, and nobody is going to run a laboratory test on their own initiative. The gap between water that is legally compliant and water that is genuinely clean is not an abstraction there — it is a specific tap, in a specific building, that specific children drink from every day.

So we test first, we fund equipment matched to what the test found, and we pay for the servicing afterwards. That last part is the one charities usually skip, and it is the one that decides whether any of it was worth doing.

If you would like to help us do that in more buildings, the ways to give are on our donate page — and the individual articles linked above go into each of these contaminants properly.