ZeroWater Knowledge Center

Chlorine and THMs in Tap Water

For over a hundred years, chlorine has been added to drinking water to keep us safe from bacteria and diseases like dysentery. Indeed, water chlorination was hailed as “the major public health achievement of the 20th century”. But studies show that it isn’t all good news. Chlorine has been linked to certain cancers and birth defects. Read on to learn more about chlorine and ways you can remove it from your tap water.

  • Chlorine is used to disinfect water, but some germs have become chlorine-resistant.
  • The by-products of chlorine, THMs, are implicated in cancer and birth defects.
  • Some bladder cancer cases are directly attributable to THMs.
  • There are ways to disinfect water without chlorine, but many countries still use it and there is currently no end in sight. The solution? Filtering your tap water. 

What is Chlorine

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, thus helping to 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.

But it turns out that chlorine may not be completely effective at 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, there is a small chance it can still contain active bacteria and viruses… As well as other harmful substances.

How does chlorine end up in tap water?

Chlorine ends up in your tap water because it is used in many countries as a water disinfectant. It also interacts with the water to form other substances.

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

Even if the country you live in does not use chlorine to disinfect water, it may still be present in small amounts because chlorine is a component of PVC pipes used in many public water distribution networks

What are the potential health effects of chlorine and THMs in tap water?

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, decreased sperm quality, and a higher risk of premature birth and low birth weight.

One example that seems to prove this link is Ireland, where 90% of the water is chlorinated, and which 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.

This is not 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.

Chlorine has also been linked to an increased risk of bladder cancer. A study published by the journal Environmental Health Perspectives found that over 6000 bladder cancer cases in Europe can be attributed to THMs and disinfection by-products in drinking water.

The International Agency for Research on Cancer (IARC) does not classify chlorinated water as a carcinogen. It does, however, classify two THMs – chloroform and bromochloromethane – as Group 2B carcinogens (meaning that they are “possibly carcinogenic to humans”.

Bromoform and dibromochloromethane, the other two regulated THMs, are not classed as carcinogens. But that does not mean they are not potentially damaging. One one study, researchers found that bromoform reduces levels of hemoglobin (a red blood cell protein that transports oxygen throughout the body). In another study, researchers found that THMs can trigger chronic inflammation, which can open the door to immune problems and other chronic diseases.

The World Health Organisation’s (WHO) latest Drinking Water Guidelines give the following health-based guidelines:

  • Chlorine: 5 milligrams per litre (mg/L)
  • Chlorite: 0,7 mg/L
  • Chlorate: 0,7 mg/L
  • THMs:
    • Chloroform: 0,3 mg/L
    • Bromoform: 0,1 mg/L
    • Dibromochloromethane: 0,1 mg/L
    • Bromodichloromethane: 0,06 mg/L

The updated EU Drinking Water Directive gives the following maximum limits:

  • Chlorite: no more than 0,25 mg/L
  • Chlorate: no more than 0,25 mg/L
  • If the disinfection method used generates both chlorite and chlorate (such as when chlorine dioxide is used), a maximum value of 0,70 mg/L is applicable.
  • THMs (total amount for chloroform, bromoform, dibromochloromethane, and bromodichloromethane): no more than 0,1 mg/L

Chlorine in tap water in France

In 1844, only 10% of the public fountains in Paris provided drinking water. After WWI, chlorine became more generalised and widespread in France. These days, it is still the method used to guarantee safe drinking water. However, chlorine can create unhealthy by-products.

According to the latest analysis, the average level of THMs in French drinking water is 0,0117 mg/L, which is below the limits set by the WHO and the European Union. However, 1% of the population is exposed to water containing over 0,05 milligrams of THMs per litre.

Chlorine in tap water in Italy

In Italy, chlorine was first introduced to treat drinking water at the beginning of the 1900’s, and this represented an important step forward for drinking water distribution and quality.

It is difficult to have a completely clear picture of the present situation in Italy because the data is collected in a decentralized manner. However a report carried out by the Barcelona Institute for Global Health (ISGlobal) found that the average amount of THMs in Italian water is 0,0031 mg/L. However, a maximum level of 0,129 mg/L was also reported, which is higher than the limit value set by the Drinking Water Directive.

A study revealed that 366 of 27,297 bladder cancer cases in Italy were attributable to THM exposure. The ISGlobal study author stated that these cancers could be avoided by optimizing drinking water treatment, disinfection, distribution techniques and other measures.

Chlorine in tap water in Germany

In Germany, five substances are approved as disinfectants for drinking water: sodium and calcium hypochlorite, chlorine dioxide, chlorine and ozone. The active components of these ingredients destroy the proteins of potential pathogens, rendering them harmless.

Tap water in Germany contains between 0,03 and 0,05 milligrams of chlorine per liter. In exceptional cases – for example after heavy rainfall or after repairs to the supply network – the water is also chlorinated. Compared to other countries, the limit value for THM in the German Drinking Water Ordinance is low: a maximum of 0,05 milligrams of THM per liter.

Chlorine in tap water in the Netherlands

With so many studies pointing to chlorine’s potential health effects, 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 and Austria, chlorine is avoided as much as possible

Chlorine in tap water in the United Kingdom

In the UK, chlorine is used to disinfect drinking water, and it is a legal requirement for water companies to hold a residual chlorine concentration within the water distribution network. According to the Drinking Water Inspectorate, these companies typically keep the level of residual chlorine to 0,5 mg/L or less.

The UK’s Drinking Water Regulations do not set limits for chlorine, but do set a limit for THMs:

  • Total THMs (chloroform, bromoform, dibromochloromethane, bromodichloromethane): 0,1 mg/L
  • Chlorine: not mentioned in regulation
  • Chlorite: not mentioned in regulation
  • Chlorate: not mentioned in regulation

In the Department for Environment, Food and Rural Affairs’ latest report on drinking water quality, none of the above chemicals are mentioned. It is therefore difficult to know exactly how much chlorine, or related substances, is in UK tap water.

Chlorine in tap water in Ireland

In Ireland, the EU’s Drinking Water Directive applies, which sets limits for chlorine and its by-products. However, levels of THMs are sometimes above the limits set by regulations.

Irish Water recently tested over 700 drinking water samples and found that 59 of these, across 13 counties, exceeded the allowed limit of THMs.

Chlorine in tap water in the USA

The Environmental Protection Agency sets the following parameters for chlorine disinfection by-products:

  • Chlorine: 4 mg/L
  • Chlorite: 1 mg/L
  • Chlorate: not included in regulation
  • Total THMs: 0,08 mg/L
    • Chloroform: 0,07 mg/L
    • Dibromochloromethane: 0,06 mg/L
    • Bromoform: 0
    • Bromodichloromethane: 0

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.

ZeroWater eliminates chlorine from your tap water

ZeroWater filters remove chlorine from your tap water. They also remove limescale, glyphosate, lead, PFAS (PFOS/PFOA), and more. Check out our online shop and start filtering your water today.


1785: Chlorine gas first used to bleach textiles (French chemist Claude Berthollet)

1789: Invention of sodium hypochlorite, known in France as “eau de Javel” when Berthollet passes chlorine gas through a solution of sodium carbonate.

1895: Proposals are written to add chlorine to water to make it “germ-free”.

1897: The town of Maidstone, England, is the first to have its water supply treated with chlorine – a bleach solution is used to disinfect a water main following an outbreak of typhoid.

1902: Permanent water chlorination begins in Europe (Belgium).

1905: Permanent water chlorination begins in England.

1908: Permanent water chlorination begins in the United States.

1915: Chlorine gas used as a weapon in the First World War.

1974: Dutch chemist discovers the presence of chloroform in chlorinated drinking water. This sparks the phasing out of water chlorination in the Netherlands.

1994: President Clinton signs an order stating chlorinated water should be banned as soon as a safe alternative is possible. This ban has not yet been introduced, despite alternatives being available.

2005: Chlorination officially ends in the Netherlands.