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Chlorine (a reactive history)
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Chlorine (a reactive history)

Swimming pools and chlorine have been synonymous for more than a century. Even today as alternative sanitizers and technologies designed to reduce, or some say eliminate, the use of chlorine, gain market share, products based on this most familiar element remain the workhorse of recreational water treatment.

By Lauren Stack

If there’s any question that chlorine remains a critical component in water treatment, consider the impact shortages of trichlor tablets have had on the price and convenience of maintaining swimming pools. When a single factory goes down, or burns to the ground, the effect spreads far and wide. Hoarding product becomes commonplace to the point that black-market sales become a reality.

Whether we like it or not, chlorine remains crucial to the watershaping industry. The urge to move away from chlorine may be strong for some people; but, for the foreseeable future, sanitizing pools, hot tubs, and interactive watershapes such as splash pads and waterpark features, will be accomplished largely with chlorine.

Of course, chlorine’s use expands well beyond pools. Public water treatment still largely relies on chlorine-containing compounds to create consistently safe and potable water for our consumption. I would argue that because of chlorine and modern filtration technology, our water supply is so reliable that most people never give it much thought. Pay the bill, and the clean water comes out the tap and the soiled water goes down the drain.

It is perhaps the greatest convenience of the modern world, at least in industrialized nations. The lack of water-treatment infrastructure in third-world societies remains a devastating and ongoing human tragedy. Waterborne diseases, most of which can be reduced or eliminated with chlorination, are needlessly life-threatening to literally hundreds of millions of people worldwide.

All of that said, it’s a useful exercise to look at chlorine’s amazing history, for it teaches us the crucial nature of this remarkable chemical. While I am all for things like ozone and UV treatment technologies, understanding the past, present and future role of chlorine offers a realistically balanced perspective.


There is no question the world before chlorine was very different. Diseases such as typhus, typhoid and dysentery were common place, and the world of products that are in one way or another based on chlorine did not exist.

The Chlorine Chemistry Council asserts that today, chlorine has an enormous economic impact, contributing 2 million jobs and around $50 billion to the annual U.S. economy. Interestingly, only a fraction exists in the most familiar applications, i.e., household bleach and swimming pool chemicals.

Consider PVC (polyvinyl chloride), which cannot be made without chlorine. In fact, without chlorine we wouldn’t have plastic wrap, nylon, microprocessors, ping pong balls, plastic toys or condoms. And it plays a crucial role as a bleach in the wood and paper industry, as well as the processing of metals.

And, let’s not forget that table salt is sodium chloride. In a very real sense, the world’s oceans are chlorinated.

Chlorine was discovered in 1774 by Swedish pharmacist Carl Wilhelm Scheele during an experiment in which he was dripping hydrochloric acid into manganese dioxide. Recognized as a chemical element a few years later, because chlorine is so reactive with other chemicals, it does not exist in its elemental form in nature.

Chlorine derives its name from the Greek “chloros”, which means green yellow, the color of elemental chlorine gas.  One of the elements known as halogens, the vast majority of chlorine is “manufactured” through the electrolysis of “brine” or seawater.

According to Cynthia Burrows, professor of chemistry at the University of Utah in Salt Lake City and senior editor of the Journal of Organic Chemistry, “A little bit of chlorine is a great way to kill bacteria, but higher concentrations turn Dr. Jekyll to Mr. Hyde. More than twice as dense as air, chlorine can settle to the ground as it did in Ypres, France, in April 1915, accounting for thousands of fatalities. Responsible use of chlorine will ensure its continued applications toward improvement of human health and lifestyle without waging war on the environment.”

The first use of chlorine to disinfect potable water is credited to British scientist Sims Woodhead, who used “bleach solution” as a sterilizing agent during an 1897 typhoid outbreak in Maidstone, Kent. This temporary measure entailed introducing the solution at the distribution mains. The first regular use of chlorine for potable water treatment in the United States began at the Jersey City Boonton Reservoir in 1908.

In 1914, the US Department of the Treasury promulgated the first bacteriological standard for potable waters in the United States, forever ensuring the delivery of safe water at American taps.


Backing up more than two millennia, swimming pools existed long before chlorine treatment. While typically attributed to the Romans, the first pools go back even further to ancient Greece and Persia, where bathing became important parts of public life, health and religious rituals. Throughout the centuries, pools built for human immersion relied on drain-and-replace routines to maintain safe water, a practice that continued well into the 20th Century.

With the fall of the Roman Empire, the construction of swimming facilities declined in the west, although they remained popular in several eastern civilizations such as India, Turkey, Japan, and Ceylon.

The nineteenth-century British enjoyed public baths in India and Japan and brought practice, and pools, back home. Elaborate swimming baths (as swimming pools were called in Great Britain) quickly spread throughout England, and the European continent, especially at fashionable spas.

In the 1860s, Boston, MA, established a saltwater-bathing program, requiring the construction of man-made vessels, effectively bringing pools to the U.S. By 1901, the city operated fourteen floating baths, ten public beaches, and two swimming pools. (The number of Bostonians bathing nearly tripled between the years of 1897 and 1898.)

According to author, Kevin Olsen, an instrumentation specialist with the Chemistry and Biochemistry Department at Montclair State University, Montclair NJ, “By the early 1900s waterworks engineers had mastered the use of chlorine and filtration. Educators and health professionals appreciated the value of swimming for physical fitness. All of the elements of the modern swimming pool were in place.

As near as the author can determine, the first attempt to sterilize a pool in the United States with chlorine was made at Brown University in Providence, RI, which opened in 1903.” Coinciding with the early dawn of public water treated with chlorine, the impact was immediate. A number of influential experiments at Brown University and other pools indicated a precipitous drop in bacteria levels. It wasn’t long before chlorine products formulated for pools, such as sodium hypochlorite and calcium hypochlorite, came on the market.

During the first two decades of the century, chlorine-based treatment of public water spread rapidly, driven by radical reductions in common waterborne diseases, leading to dramatic increases in life expectancy. Swimming pool treatment followed that trend.

By 1923, seven states had passed regulations for the control of swimming pool sterilization. The technology of pool chlorination was still not fully developed, however. During the 1920s pools still had to be drained periodically and the entire volume of water replaced.

During this period, several concepts defining chlorine treatment were introduced, which are still in use today. For example, according to Olsen, “The amount of oxidizing agent required to destroy the impurities present at the time of addition is the ‘chlorine demand’ and the unchanged material is the ‘residual chlorine.’ Most pool chemical suppliers recommend keeping the residual chlorine at a concentration of no more than 2 ppm. ‘Shocking’ a pool refers to adding excess chlorine so that once the chlorine demand is met, a massive excess of chlorine ‘shocks’ the water.”


During the first half of the twentieth century, there were three major forms of chlorine used for swimming pool sanitization: chlorine gas, liquid chlorine (bleach) and granular or tableted calcium hypochlorite., All except chlorine gas continue to be widely used today.

By the middle of the twentieth century a new class of chlorinating compounds was available. It was soon discovered that on a sunny day, as much as 70% of chlorine may be dissipated from an unstabilized pool, which led to the introduction of “isocyanurate” products that contain both chlorine and cyanuric acid. Like earlier compounds, these materials function as a source of hypochlorous acid.

A vast subject unto itself, cyanuric acid, also commonly referred to as stabilizer, reduces UV degradation, while also retarding the amount of free-available chlorine. A vibrant, and sometimes even bitter, debate about the optimum levels of stabilizer and associated chlorine residuals continues to this day.

It’s worth noting that from the very beginning of chlorine water treatment, alternative methods were also being developed. Ozone used for killing pathogens and oxidizing organic compounds was developed in the 1870s and has grown in use ever since. The use of metal ionizers and most recently UV sterilization have also gained ground in the market, as has chlorine’s close elemental cousin, bromine.

Many watershapers, builders and service technicians alike, have now turned to combined treatment methods, often relying on ozone to carry the majority of the treatment load, with chlorine reduced to a back-up role with low residuals under 1 ppm.


According to Life magazine, Americans were spending $250,000,000 on private pools in 1960. Chlorinating these pools with tablets had become easier when, in 1956, Olin Mathieson introduced a floating polyethylene mesh basket that could be suspended in a pool and allow chlorination tablets to dissolve, leading to chlorine floaters and stationary feeders.

Liquid chlorine was among the most popular types of chlorine at that time, but an average sized pool required as much as one gallon per day. Lithium hypochlorite had also recently come onto the market. This material released only 35% available chlorine compared to the 70% released by calcium hypochlorite. These two materials were the most popular types of solid chlorine products.

The chlorinated isocyanurates were also relatively new; and while they were more resistant to photodegradation than other forms of chlorine, their effectiveness as biocides was still being debated. Bromine, iodine, and silver ions were available but not widely used. Chlorine in one form or another was used to disinfect some 95% of pools in 1963.

The evolution of chlorine products continued in the1990s with the introduction of saltwater chlorine generation. The now familiar method of adding salt to the pool water and circulating it through an electrolytic chamber to generate hypochlorous acid was a radical departure at the time. It steadily gained acceptance for the convenience and cost savings.

Through its entire history, along with all of its advantages, chlorine does come with its drawbacks. Chief among those is the formation of disinfection byproducts (DBPs). As chlorine oxidizes compounds such as ammonia, urea and many others, it forms byproducts such as chloramines, trihalomethanes, chloroform and several others, many of which have potential negative health effects on swimmers, ranging from respiratory, eye and ear ailments.

And, the infamous chlorine smell is due to one of the most common DBPs, trichloramines.

Because water is the main ingredient in watershapes, and chlorine remains the most popular way to keep it safe, understanding chlorine’s long and colorful history might just help inform the future of water treatment.

Lauren Stack is current Vice President of Operations for Watershape University. She has a bachelor’s degree in chemistry from the University of Pittsburgh, graduating summa cum laude. Her research career – in electrolytic processing — lasted all of 18 months when she left the lab for business, obtaining an MBA and working in project management, new product development and marketing for chemical-based firms.  Later, her career turned toward non-profit and educational endeavors for the watershape industry in which she still happily remains.  

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