Myth: Chlorine in Tap Water Is Dangerous

On 22 April 1915, near Ypres, the German army opened the valves on several thousand cylinders of chlorine gas and let the wind carry a green cloud across the French lines, and the men who breathed it drowned in their own lungs. A century later chlorine is still a weapon, still a hazard on every railway that carries it, still the gas that empties a neighbourhood when a tanker leaks, and it is in the water that most of the people reading this will drink today, put there on purpose by the utility that supplies them. The myth follows directly: a chemical that dangerous cannot be safe in a glass. It is one of the oldest worries about tap water and one of the most durable, and it deserves the honest answer, which starts with the dose.

This article is about what the chlorine in a tap is and how much of it there is, what it did to the cities that first added it, what the real argument about chlorine is and who should have it, which people and animals do need chlorine free water, and what a glass left standing has lost.

The dose

Chlorine gas, in the cylinder, is a yellow green gas that attacks the lungs at a few parts per million in air. Chlorine in water is a different substance: dissolved, it reacts at once to form hypochlorous acid, the same thing that household bleach releases, diluted far beyond anything a bottle of bleach contains. A water works doses it at the end of treatment, in a tank that gives it half an hour of contact, at about one or two milligrams per litre, and by the time the water has travelled through the pipes to a tap most of that has been used up on whatever it met on the way, so that the water at a kitchen tap carries a few tenths of a milligram a litre, which is a few parts in ten million. The World Health Organization's guideline for chlorine in drinking water, the concentration judged safe to drink every day for a lifetime, is five milligrams a litre; the US limit is four; a swimming pool is kept at one to three. A litre of tap water with half a milligram of chlorine in it holds about a thousandth of the chlorine in a teaspoon of bleach.

At those doses chlorine has been drunk by billions of people for over a century and studied in more of them than any substance in water, and the finding is consistent: chlorine itself, at drinking water concentrations, has no measured effect on the health of the person drinking it. It is tasted and smelled by some people at around half a milligram, which is the complaint most utilities actually receive, and the taste is the strongest when the water is cold and freshly drawn.

What it did

The disinfection article on this site explains how chlorine kills; this one is about what happened when it was added. The first city to chlorinate its whole supply continuously was Jersey City, in 1908, where a lawyer's dispute over a reservoir contract ended with a doctor named John Leal dosing the water with bleach without asking anyone, and winning the case when the courts found the water was now clean. Within a decade most large American cities had followed, and the cities of Europe in the years after the war that had used chlorine as a weapon. The effect on the death rates is one of the best measured things in public health. Typhoid, which killed tens of thousands of Americans a year at the turn of the century, fell by nine tenths in the chlorinated cities within twenty years, and cholera disappeared from the cities that had it; the economists who have gone back over the records put filtration and chlorination together as responsible for nearly half of the fall in deaths in the large American cities between 1900 and 1936, and for three quarters of the fall in infant deaths. No drug, no vaccine and no hospital of the period comes close. The cholera article on this site describes what the cities without it still face.

The real argument

There is an argument to have about chlorine, and it is about what it makes rather than what it is. Chlorine reacts with the traces of leaves, soil and algae that any surface water carries, and the reaction forms a family of compounds, trihalomethanes and others, that were discovered in tap water in 1974 and have been studied and regulated since; the chlorine byproducts article on this site is about them, about the limits of a hundred micrograms a litre, and about the studies that found a modest increase in bladder cancer over a lifetime of drinking water at the higher end of the range. That argument is real, it is the reason works have moved to removing the organic matter before they chlorinate and to alternatives like chloramine, ozone and ultraviolet, and it is the argument the industry has, in the professional journals and the regulators' offices, every year. It is also an argument about micrograms of byproduct against the cholera and typhoid that the chlorine prevents, and every regulator who has weighed it has come to the same place: keep the chlorine, cut the byproducts, never let the second goal threaten the first. The one time a country was said to have done otherwise, in Peru in 1991, where officials were accused of reducing chlorination over byproduct concerns before a cholera epidemic that killed thousands, the story turned out to be more complicated, and the lesson that the water industry drew from it was the same either way.

Who needs it out

Some people and some animals do need the chlorine removed, and knowing who they are is the honest end of the myth. Fish and the creatures in an aquarium are killed by chlorine at tap concentrations, which is why fish keepers let the water stand or dose it with a neutraliser; the fish's gills are the lungs that the gas attacked at Ypres, and half a milligram is enough for them. A dialysis patient, as the hospitals article on this site describes, needs the chlorine and especially the chloramine out of the water that crosses the membrane into the blood, where it would destroy red cells, and every dialysis unit strips it with carbon. Home brewers and bakers who care about their yeast remove it. And the people with a strong sense of taste who do not like it have the simplest fix on this site: a filter jug of the kind the article on them describes removes chlorine completely, as does an hour in an open jug on the counter, because chlorine leaves standing water on its own, which is the point of the next section.

The glass on the counter

Chlorine does not stay. It is a reactive substance, which is why it works, and in an open glass it reacts with the air and whatever is in the water and is gone within an hour or two; in a covered jug in the fridge it lasts a day. That is also why it has to be there in the first place. The chlorine at the tap is a residual, the small amount left after the dose at the works has done its killing, and its job is to go on killing in the pipes between the works and the tap, where a crack lets in groundwater, a dead end lets a biofilm grow, or a repair lets the street in. The Legionella article on this site describes the building whose pipes let the residual die; the Amman article describes a city whose roof tanks hold water for a week after the residual is gone. A water with no chlorine at the tap is a water that has had nothing to protect it since it left the works, and the utilities that supply it, in parts of the Netherlands and Switzerland, do so from groundwater of a quality and through pipes of a tightness that almost nobody else has. For everyone else the residual is the guarantee, and the taste is the evidence it is there.

Verdict

False, at the dose in a glass. Chlorine in a cylinder is a poison and in a tap is a few tenths of a milligram of the compound in bleach, diluted ten thousand fold, drunk by billions for a century with no measured harm and with the largest measured benefit of any public health measure of its age. The argument worth having is about the byproducts, which the regulators have, and the people who should remove it are the fish keepers, the dialysis units, the brewers and anyone who dislikes the taste, who can do it with a jug and an hour. The chlorine is in the glass so that nothing else is.

Five milligrams the limit, a few tenths in the glass, and a century of cities that stopped burying their children of typhoid.