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Chlorine: Making Water Safe, Inside and Outside the Body

Pools are disinfected with chlorine or sodium hypochlorite.  ©Tanya Deyukhina/Wikimedia (CC BY 3.0)
Pools are disinfected with chlorine or sodium hypochlorite. ©Tanya Deyukhina/Wikimedia (CC BY 3.0)

In developed countries, easy access to clean water that is safe for drinking and bathing is the norm. Prior to the 1900s, however, life was very different, and waterborne diseases such as cholera, dysentery, and typhoid were commonplace. Today, the water that is delivered from lakes, wells, and a variety of other sources has been made safe from bacteria, viruses, other disease-causing microorganisms by disinfection.

In most communities, disinfection of water is achieved by addition of chlorine or chloramine to water; these processes, called water chlorination or water chloramination, respectively, are considered to be among the most significant advances ever achieved in the history of public health. Having clean water that is safe from pathogens and abundant for drinking, washing, and bathing is truly a remarkable achievement.

However, this common chemical may also cause a range of adverse effects in humans in certain circumstances—especially regarding skin and hair and particularly among people who suffer from eczema and psoriasis.

Despite such cautions, there are ways in which the ill effects of chlorine on the body can be countered and any potential health risks reduced.

Water Chlorination and Its Uses

Chlorination is a process used to disinfect water and deliver water that is safe for human consumption (according to the US Environmental Protection Agency (EPA)), and for washing, bathing, and swimming. It works by killing harmful germs and other microorganisms that are naturally found in raw water sources, such as rivers, lakes, and groundwater. Some of these microorganisms (pathogens) may cause diseases in humans and can be transmitted to humans through domestic water distribution systems and through water used in swimming pools.

Chlorine was discovered in 1774 by Swedish-German chemist Karl W. Scheele. Its first use in a water treatment process occurred in 1897 in Maidstone, Kent, in the United Kingdom, when a bleach solution was used to disinfect a water main following a typhoid outbreak.

Regular use of chlorine for water disinfection began in 1902, with the first continuous application of the substance taking place in Middelkerke, Belgium. In 1908, in the US, George A. Johnson added “chloride of lime” to contaminated river water. This marked the beginning of the proliferation of chlorination as a water treatment process in numerous countries.

Chlorine in the form of hypochlorous acid is usually used to eliminate microbes in drinking water and in public swimming pools, and it does so quite effectively. Bacteria exposed to hypochlorous acid lose viability within 0.1 seconds. It is reported that the dosage of hypochlorous acid that neutralizes 50% of a bacterial population is 0.0104–0.156 parts per million (ppm), and 2.6 ppm is sufficient to prevent 100% of bacterial growth within five minutes. Despite these potent anti-bacterial effects, chlorine levels up to 4 ppm is considered safe in drinking water with harmful health effects unlikely to occur at this concentration.

Chlorine levels up to 4 ppm is considered safe in drinking water with harmful health effects unlikely to occur at this concentration.

In addition to killing pathogens, chlorination is also used to destroy organisms and substances that give unpleasant tastes and odors to water and can foul equipment. It also oxidizes undesirable substances such as ferrous iron (Fe2+) and manganese (Mn2+).

The process can be used at various points in the treatment process, such as:

  • Prechlorination of raw water prior to any water treatment

  • Added in the treatment process

  • Added after treatment but before distribution

  • Added during distribution

  • Miscellaneous use during maintenance activities

Some protozoan cysts, such as Cryptosporadium and Giardia, are resistant to chlorination. In these situations, alternative treatment processes—such as ozone and ultraviolet (UV) radiation—have proven to be effective. Where these protozoans are not present, chlorination is used because, in comparison to other water treatment processes, it is an inexpensive but highly effective method of eradicating many other possible contaminants.

Potential Adverse Effects of Chlorinated Water

If people spend hours in a chlorinated pool and don’t shower before and after swimming, they can get a chlorine rash, which is a form of irritant dermatitis. Competitive swimmers, lifeguards, and others who swim regularly in pools can experience this red, itchy rash, typically within hours of exposure.

The reason for this is that chlorine can make the skin increasingly porous as well as removing its protective oil (sebum), which then allows the chlorine to enter the underlying cells. These, in turn, react to the chemical, causing inflammation, redness, swelling, and itching.

People with existing skin conditions, such as eczema or psoriasis, can be affected more easily. Even in mildly chlorinated water, the combination of ammonia (in sweat and urine) with chlorine can form (mono)chloramine. However, chlorine rash can usually be treated effectively by application of over-the-counter hydrocortisone cream.

People with existing skin conditions, such as eczema or psoriasis, can be affected more easily by chlorine rash.

Swimming regularly in chlorinated pools may also cause dry skin, in which the skin becomes rough, itchy, flaky, or scaly.

As for hair, chlorine can cause damage by eating away at the protective cuticles and exposing the cortex layer. It can also break down amino acids, depleting the hair’s natural strength and drying it out, and remove melanin from hair strands, potentially lightening hair and even turning hair green because of chlorine’s bleaching effect (a phenomenon known as “pool hair”). Split ends, hair frizziness, and an itchy scalp can all be common outcomes of heavy pool use.

Wearing a swimming cap is an effective way of protecting    hair from chlorine.  ©psbitney/Pixabay. Public Domain
Wearing a swimming cap is an effective way of protecting hair from chlorine. ©psbitney/Pixabay. Public Domain

A Few More Things to Watch For

Finally, chlorination can result in the formation of certain “disinfection by-products.” One such group is called trihalomethanes (THM), described by the EPA as chloroform, bromodichloromethane, dibromochloromethane, and bromoform.

These by-products are found to be associated with increased risk of asthma and allergic diseases in children, and a study of THM levels in drinking water in the EU found an increased risk of bladder cancer after exposure.

THM risks are small—“Drinking water every day with concentrations of [Total Trihalomethanes] TTHMs at or below the standard for your entire lifetime is unlikely to cause illness,” says the Florida Department of Health. Public officials can ensure water safety with optimized water treatments, and consumers can remove THM with activated carbon water filters. Still, chronic exposure to high doses of common types of THM can cause liver and kidney cancer, heart disease, unconsciousness, and death. Due to their potential carcinogenicity, regulations often require monitoring of these THM compounds in public water distribution systems.

Other by-products of chlorination include haloacetic acids (HAAs), which are produced when the chlorine interacts with naturally occurring organic matter. Canadian guidelines recommend running an annual average concentration of 80 micrograms per liter (μg/L) for HAAs in drinking water.

In June 2004, a report published by the Oregon Department of Human Services found that although the level of toxicity caused by exposure over a short term to HAAs is low, such exposure over a longer term at levels above the maximum contaminant level can “cause injury to the brain, nerves, liver, kidneys, eyes and reproductive systems.” Animal testing has also indicated that HAAs may be a possible human carcinogen.

Haloacetic acids (HAAs), [by-products of chlorination,] can “cause injury to the brain, nerves, liver, kidney, eyes and reproductive systems.”

An alternative to chlorine is monochloramine (NH2Cl), which has a longer retention time. However, this process can lead to nitrification, which in turn can stimulate the growth of heterotrophic bacteria, such as that found in composting or biomass decay. Although this presents no significant risk to humans in itself, it can indicate favorable conditions for growth of other, more dangerous bacteria, such as Legionella or E.coli, as well as causing slime growth or corrosion in pipes.

Reducing Exposure to Chlorine

In summary, it’s always a good idea to shower before entering a chlorinated pool to reduce any possible sweat or urine traces on the body and thus reduce the risk of chloramine production. Showering after a swim will help reduce the drying effect of chlorine on the skin.

While chlorinated water is widely considered safe to drink, water filters can help to reduce the amount of chlorine present in tap water. Beyond that, if ever there is a chlorine taste or smell in drinking water, pour it into a clean jug, cover it with a cloth, and allow it to stand for a while. This will help to reduce the chlorine level in the water, but such water should be consumed within 24 hours.


*Robin Whitlock is an England-based freelance journalist specializing in environmental issues, climate change, and renewable energy, with a variety of other professional interests, including green transportation.


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