Poison in the pool: a history of chlorine

How a deadly chemical became the world’s preferred means of keeping swimming pools clean and tidy. Elaine K Howley investigates.

It’s probably safe to say unequivocally: nobody wants to swim in a dirty pool. It’s probably also safe to assert that nobody wants to swim in toxic chemicals. But in an odd twist of chemistry and history, modern pools typically get and stay clean through the application of a chemical that’s capable of felling even the strongest person.

That special cleaning agent is called chlorine. It’s a halogen gas— element number 17 on the periodic table—and swimmers the world over are intimately familiar with its hallmark, antiseptic scent.

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Over the past century, chlorine has become so closely associated with swimming pools that it may come as a surprise for some to learn that chlorine was initially a byproduct of industry used widely in textile manufacture and is still sometimes used as a tool of warfare.

Chlorine’s first release

Chlorine does not exist naturally on Earth – it likes to combine with other compounds, in particular sodium, which creates salt, the stable, abundant substance that makes chips more tasty and swimmers more buoyant in the sea.

But chlorine can be liberated by running an electrical current through seawater. The Royal Society of Chemistry reports that chlorine was first isolated in 1774 by the Swiss-German chemist Carl Wilhelm Scheele, but he didn’t recognize he’d stumbled upon a new element during his experiment.

Instead, it was English chemist Sir Humphry Davy (pictured above) who in 1810 recognised that Scheele had made elemental chlorine when a greenish-yellow gas emerged after he repeated the experiment—reacting hydrochloric acid with manganese dioxide. (In ancient Greek the word χλωρός (khlōrós) means “pale green.”)

Chlorine’s sanitising and bleaching properties were soon recognised, and by the middle of the 19th century, the first commercial bleach called Eau de Javel (Javel water, after the French town where it was produced) gained use in bleaching textiles.

A variation on that bleach, which is a dilute solution of sodium hypochlorite, soon found its way into hospitals where it was used as a disinfectant, killing off bacteria and viruses to help prevent transmission of infection.

An instrument of war

But chlorine’s ability to sterilise a hospital ward to support longevity would eventually be harnessed in the pursuit of chemical warfare—what kills bacteria could likely kill people, too, if an adequate quantity were applied. That was the idea German scientists used when they developed chlorine gas to use on the battlefields of World War I. The first time they deployed it at Ypres in April 1915, the green gas claimed the lives of more than 1,100 soldiers.

The University of Kansas Medical Center reports that chlorine gas is “about two and a half times denser than air… and with an odor which was described as a ‘mix of pineapple and pepper.’ It can react with water in the lungs to form hydrochloric acid, which is destructive of tissue and can quickly lead to death, or, at least, permanent lung tissue damage and disability.”

In lower doses, chlorine may not be deadly, but it can still be highly irritating, causing coughing, vomiting, and severe eye irritation.

It sounds truly horrific, right? But most swimmers come into intimate contact with a version of this substance on a daily basis, or at least any time you visit your local pool because, today, chlorine is the most widely used chemical to keep pools clean.

So how did a deadly poison become the go-to means of keeping swimming pools clear and swimmable?

Green gas becomes blue water

It’s well known that swimming baths and pools have been around for eons. The oldest known such facility, the Great Bath of Mohenjo-Daro, dates to the third millennium BCE in the Harappan town of Mohenjo-Daro along the banks of the Indus River in what is now Pakistan. That 39-foot by 23-foot by 8-foot deep pool had a drain at the bottom to facilitate maintenance of the brick and tar-sealed structure. Back then, pools needed to be cleaned differently than they are now, and it was a process that required a lot of water, time, and labor.

In a paper about the history of chlorine he published in 2007, Dr Kevin Olsen, an instrumentation specialist in the chemistry and biochemistry department at Montclair State University in New Jersey, writes “prior to the introduction of sterilisation chlorine, bromine, ozone, or ultraviolet light, most swimming pools were filtered to keep them somewhat clean, and the water was changed frequently. Many residential pools were built on sloping ground to facilitate drainage, and some were equipped with storage tanks to hold the replacement water.”

Most pools also included so-called “scum gutters” to help draw contaminants off the surface of the water. But as the popularity of swimming grew and more pools sprang up during the Victorian age, a new urgency for less time consuming and water-intensive means of cleaning pools developed. “Unlike a lake or a river, a pool has no source of fresh water or a means of flushing out the impurities,” Olsen notes. “Some method of sanitising the water is needed to make swimming safe and pleasurable. For almost a century, chlorinated compounds have provided it.”

When chlorine gas is dissolved in water, it creates a pH-dependent sterilising agent called hypochlorous acid. This liquid form of chlorine was first used on a temporary basis as a water disinfection agent during a typhoid outbreak in Maidstone, Kent in 1897. In 1908, adding chlorine to the potable water supply first became a regular occurrence at the Jersey City Boonton Reservoir in New Jersey. Just six years later, the US Department of the Treasury instituted the first bacteriological standard of potable water in the United States, limiting interstate water supply systems to just 2 coliforms per 100 milliliters of water, Olsen reports.

But chlorine wasn’t the only sanitising agent being used at the time, Oslen notes. For example, sanitation specialists in the Netherlands experimented with using ozone to treat public water supplies as early as 1893. However, “high start-up and equipment costs for systems like ozone disinfectant, when contrasted with an abundant supply of chlorine from caustic soda manufacture, meant that few alternative technologies could compete with the price and convenience of chlorine,” Olsen writes. What’s more, in addition to a sterilising agent, pools also need a filtration system.

Oslen reports that Paisley, Scotland, was the first modern city with a water supply purified by filtration. Such systems remove sediments, organic odor-causing materials and can clarify the water. The first such American system was built in 1872 in Poughkeepsie, New York.

“By the early 1900s waterworks engineers had mastered the use of chlorine and filtration,” Olsen writes, and before long, these systems made the leap to swimming pools, coinciding with a rise in swimming and bathing as both a means of supporting both fitness and hygiene. As the number of pools across the UK and the United States proliferated, many sought a more efficient way of keeping clean. It didn’t take long for pool managers to take a page out of the public works playbook.

Olsen writes that as far as he’s been able to determine, the first pool in the US to use chlorine as a sanitiser was Brown University’s Colgate Hoyt Pool (pictured above left) in Rhode Island. When the pool opened on March 2, 1903, it was filled with city water, but “the university quickly discovered that while ‘pure enough for drinking purposes,’ a large mass of the water exhibited a brown colour. Once a mechanical filter had been installed, the 70,000-gallon pool could be filled in about 18 hours with filtered water.”

For the next eight years, the pool was maintained as most others had been down the ages, but during a 1911 experiment undertaken at Brown, chlorine as shown to dramatically reduce the levels of bacteria in the pool. To do this, the researchers placed a powdered chlorine compound in a cheesecloth bag and dragged it over the pool “until the material was distributed…. Surface bacteria counts fell from 500 to 30 in only 15 minutes to 10 in 30 minutes and ‘complete sterility’ after an hour,” Olsen reports.

This near-miraculous result along with other ongoing research into the use of chlorine to purify baths, pools, and drinking water helped opened the floodgates for chlorine’s use in most pools around the world. The attraction was easy to understand – chlorine was cheap, abundant, easy to use, and effective. Its ability to quickly purify swimming pools only aided in the growth of swimming, and its use expanded quickly. By 1963, chlorine was used to disinfect some 95% of pools in the United States, Olsen reports. It’s still the go-to sanitising agent used today.

Ancient pool maintenance

Prior to the development of chlorine and other disinfectant chemicals that are widely used to keep pools clean today, it took a lot of work to keep a pool clean. This is still the case for the Roman Baths at Bath, which have stood for more than 2,000 years.

Lined with 45 sheets of lead and filled with naturally occurring hot spa water from a nearby spring, the Great Bath is a UNESCO World Heritage Site and a popular draw for history buffs visiting southwest England. The property is managed by the Heritage Services section of Bath & North East Somerset Council, and the organisation’s operations team drains, cleans, and refills the pool completely about four times per year. The water’s surface is also skimmed daily to remove floating algae and any other debris that could collect there.

When the team is ready to commence a more thorough cleaning, they cut a valve that controls the flow of waterfrom the thermal spring into the bath, diverting the water into a Roman drain. This water flows along the drain across a half-kilometre distance and discharges into the nearby River Avon. If you’re on site, you can watch as the water surges along beneath a glass floor inside the Roman Baths Museum.

At the same time, a sluice gate is opened, which allows the water contained in the Bath to flow out via that same drain. It takes about five hours for the Bath to completely drain, Heritages Services notes. “It is a controlled process because if all the water went out at once it would, in combination with the flow from the spring, flood the drain entirely.”

As the tub drains, workers climb down into the Bath and gently brush away scum and algae left behind on the sides and floor of the bath. Additional water is used to rinse away loosened debris. Once that job is finished, workers reopen the sluice gate and the bath refills at the rate of 13 litres per second.

While that clean-ish water certainly is beautiful to look at, sadly, you can’t take a dip in the Great Bath. But if you listen carefully while gazing across its green surface, you might just be able to hear an echo of ancient voices enjoying the pleasures of a warm swim across the millennia.

Read about the rise in popularity of natural swimming pools.