Chlorine in Tap Water: Is it Safe?

Published on 06/11/2022

Water chlorination was hailed as “the major public health achievement of the 20th century”. For over a hundred years, this chemical has been added to our drinking water to keep us safe from bacteria and diseases like dysentery. But studies show that it isn’t all good news. Instead of protecting our health, chlorine in our tap water may be putting our health in danger. In Ireland, chlorine may be to blame for the high number of spina bifida cases.

What is chlorine and what is it used for?

Chlorine is a versatile chemical. You will find it in commercial bleaches and disinfectants. It is used to manufacture many consumer products, particularly polyvinyl chloride (PVC) and other plastics. It was also used as a chemical weapon (poison gas), in WWI.

But you probably know chlorine as the disinfectant used in swimming pools… and tap water.

Water companies use chlorine, in the form of chlorine gas, calcium hydrochloride or sodium hypochlorite, to cleanse water destined for human consumption. Chlorine kills bacteria, microbes and viruses that commonly grow in water reservoirs and swimming pools, and helps prevent the spread of water-borne diseases like typhoid, dysentery and cholera. This strategy was first used in the late 1800’s (the town of Maidstone, in England, was the first to have its entire water supply treated with chlorine in 1897), and is still used today.

In 1844, only 10% of the city fountains in Paris produced clean drinking water. After the first world war, water chlorination was widely deployed in France.

But it turns out that this is not a fail-safe method for keeping water safe. In fact, evidence has shown that some toxic organisms, including e.coli, have adapted and become resistant to chlorine. This means that despite tap water being chlorinated, it may still contain active bacteria and viruses… As well as other harmful substances.

Is chlorine safe?

Chlorine reacts with organic matter (such as twigs, leaves, and dirt commonly found in surface water or groundwater sources) and forms by-products called triholomethanes (THMs). Four of these THMs are included in the EU’s drinking water regulation parameters, and consequently regulated: chloroform, bromoform, dibromochloromethane and bromodichloromethane.

The question of chlorine’s safety has divided scientists for decades. Some say that the benefits of this type of disinfection process far outweigh any health risks posed by chlorine or its by-products. Others point to studies linking chlorine exposure to birth defects and an increased risk of bladder cancer.

Chlorine and cancer risk

The International Agency for Research on Cancer (IARC) does not classify chlorinated water according to its carcinogenicity. However, it has classified two individual THMs – chloroform and bromodichloromethane – as “possibly carcinogenic to humans” (Group 2B).

Bromoform and bromodichloromethane were not classified as to their carcinogenicity. But that does not mean that they don’t have potentially negative health effects. In one study, researchers found that bromoform reduced haemoglobin levels (haemoglobin is a protein in red blood cells that carries oxygen throughout the body). Bromodichloromethane can trigger chronic inflammation, something that can open the door to other diseases. A major Norwegian study found a significant association between exposure to chlorinated water and a higher risk of cardiac, respiratory and urinary tract defects.

Chlorine and reproductive health

Exposure to chlorine and its by-products has also been linked to an increased risk of reproductive problems. Studies have found that it decreases sperm quality, and increases the risk of premature birth and low birth weight.

Ireland has the second highest level of spina bifida (a birth defect that occurs when the baby’s spine and spinal cord don’t form properly) in the world. Here, 90% of the water is chlorinated. Irish Water recently tested over 700 drinking water samples and found that 59 of these, across 13 counties, exceeded the allowed limit of THMs.

Not a new problem

None of this is new information. Back in the 1960’s, question marks began appearing around chlorine’s safety when it became clear that even low levels of this chemical and its byproducts were toxic to plants and animals. In 1974, Dutch chemists found that almost all chlorinated water supplies are contaminated with chloroform, a known carcinogen.

So why is it still being used to treat drinking water? We can only speculate. Regardless of the reasons, there are other, safer alternatives to provide clean tap water. In the Netherlands, for example, chlorine began to be phased out of the water treatment system in the mid-1970’s, and it hasn’t been used since 2005. Instead, Dutch water companies treat tap water with ultraviolet disinfection, sedimentation, or ozonation, methods that do not produce any harmful disinfection byproducts. In Switzerland, Germany and Austria, chlorine is avoided as much as possible.

Water chlorination in Europe

The latest EU Directives on Drinking Water state that there should be no more than 0,25 milligrams of chlorate per litre of water and no more than 0,25 milligrams of chlorite per litre of water. If the disinfection method used generates both chlorite and chlorate (such as when chlorine dioxide is used), a maximum value of 0,70 milligrams per litre of water is applicable. The upper limit for the four regulated THMs is set at 100 micrograms per litre. Unfortunately, in some European countries, these limits are sometimes exceeded. In Ireland, for example, some measurements have been as high as 255 micrograms of THMs per litre.

The most recent studies in France show that the average level of THMs in tap water is 11.7 nanogram per litre, well below the EU’s limits. However, 1% of the population is still exposed to water containing over 50 nanogram of THMs per litre.

How to remove chlorine from tap water

We know, as the example of the Netherlands proves, that it is possible to disinfect water without using chlorine. But until other countries follow suit, it is down to you to remove chlorine from your tap water.

There are several ways you can do this:

  • Fill a jug with tap water in the morning and let it sit in the fridge or the open air for a day. Chlorine gas weighs less than air so it will slowly evaporate from the water.
  • Boil the water for 15 minutes (this speeds up the evaporation of chlorine gas) and let it cool before drinking.
  • Invest in a water filter that effectively removes chlorine (one containing activated carbon), like ZeroWater filters, which get rid of chlorine as well as glyphosate, microplastics, lead, and more. For more details, take a look at our online shop.