Physics & Chemistry

The History and Toxicity of Chlorine

December 8, 2011, 11:31 am
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Chlorine gas from


Chlorine is an element that exists as a diatomic molecule, Cl2.  Chlorine compounds are among the most commonly produced chemicals and many are now a part of everyday life.  Although chlorine is primarily used as a disinfectant and a bleaching agent, it is also used in medication, pesticides, and sewage treatment.  As beneficial as chlorine may appear, in its pure form it can be very toxic. 


Chlorine was discovered in 1774 by the German-Swedish pharmaceutical chemist Carl Wilhelm Scheele.  He was studying the effects of heating marine (or muriatic) acid now known as hydrochloric acid (HCl) and manganese dioxide when an unfamiliar gas was produced; he called this gas dephlogisticated marine acid.  In Scheele’s time, a combustible substance was thought to contain a phlogiston that was released when burned, and the substance that was released was thus known to be dephlogisticated.  Scheele collected this dephlogisticated marine acid, later dubbed chlorine gas, in a bottle.  After some time Scheele noticed that the cork stopping the bottle was yellowing on the side exposed to the gas.  He investigated this property by placing blue litmus paper and colored vegetable flowers in the presence of the gas, and found they turned white – or bleached.

Some decades later, the French chemist Claude Louis Berthollet further explored the bleaching property of chlorine described by Scheele.  He discovered that fixed alkalis or stable bases dissolved in water aided the bleaching action of chlorine without producing the strong odor of chlorine gas. Berthollet found that cloth soaked in a hypochlorite ion (ClO-) solution bleached white in a few hours just as well as the old method of exposing cloth to the outside air for several days.  This marked the beginning of the hypochlorite bleaching industry, and soon a French chemical plant near Paris produced a commercially available hypochlorite solution known as “Eau de Javel” or the water of Javel.   

Although proposed as a disinfectant in the 1700s, it wasn’t until 1825 that chlorine became widely used for the purpose of disinfecting.  In 1825, the Royal Institute of France awarded the pharmacist Antoine Germain Labarraque for his recommendation of a chlorine solution for the enhancement of public health of France.  Labarraque recommended that a chlorinated soda solution be used for disinfecting purposes and as a deodorant.  After three years of various published clinic success using this chlorine solution, it became widely used and known as Labarraque’s solution. 

Almost two decades later, the Hungarian physician Ignaz Philip Semmelweiss introduced hand sanitation with chlorinated lime. Semmelweis worked in a hospital complex that contained both a morgue and a delivery room and noticed that the mortality rate from puerperal fever was higher on the floor where clinical students came to the delivery room straight from the morgue.  After inspecting the hands of the students, he noticed that there was an odor even after washing their hands with soap.  As a solution for cleaner hands, he proposed that the students use chlorinated lime soap.  This greatly reduced the mortality rate in the delivery room. 

In 1894, German chemist Moritz Traube preformed the first experiments on the effectiveness of chlorine as a disinfectant in drinking water.  Traube found that a small amount of calcium chloride (4.26 parts per million) added to 100 milliliters of bacteria-rich water, killed all the organisms within two hours rendering the water safe to drink.  Traube proposed this method as a cost effective way to disinfect a large quantity of water in a short time.  In 1890s, Dr. Bassenge of the German Navy built upon the experimentation of Traube.  Bassenge found that at certain concentrations, chloride could kill all bacteria within five minutes, but left the water unfit for drinking.  Bassenge’s experimentations led him to recommend 0.0978 grams of active chlorine per liter, which kills bacteria in ten minutes.  He also stated that to make the water safe to drink, excess chlorine can be removed by adding calcium bisulphate to the water.  Chlorine also lengthened the time which water could be kept without bacterial contamination. 

In 1902 chlorine was added to the first permanent water supply in Middelkerke, Belgium.  Six years later, the Jersey City, NJ Water Works was the first in the United States to continuously chlorinate water on a large scale. In 1914 the U.S. Public Health Service set standards for the bacteriological quality of drinking water marking the beginning of federal regulation of drinking water.  By 1941, 85 percent of water treatment plants in the United States used chlorination to disinfect drinking water.  In 1974, the Safe Drinking Water Act passed, guaranteeing high quality drinking water in the U.S.  

Uses today

Today, chlorine is still used to bleach and disinfect.  The Clorox Company, founded in 1913, is a major bleach production corporation with sales for third quarter of 2011 of 1.482 billion dollars.  The Clorox Company offers a variety of household disinfectants and bleaches including Clorox Regular Bleach, Clorox Outdoor Bleach Cleaner, Clorox OxiMagic Multi-Purpose Stain Remover, Clorox Disinfection Wipes, Clorox Clean-up Cleaner with Bleach, Clorox Toilet Bowl Cleaner with Bleach, Clorox Anywhere Hard Surface Daily Sanitizing Spray, Clorox ReadyMop Advanced Floor Cleaner, and Clorox Disinfecting Bathroom Cleaner.  Another such company, Chambre Syndicale Nationale de l’eau de Javel (House National Trade Union of the bleach), produces bleach products is in France. 

Drinking water is treated by adding chlorine gas, sodium hypochloride solution or dry calcium hypochlorite.  The treatment of drinking water is critical to the public’s health.  The importance of safe drinking water is demonstrated by the populations that lack some kind of water treatment system.  It is estimated that 1.2 billion people in the world do not have access to safe drinking water, and out of that population the World Health Organization estimates 3.4 million people die each year of water borne diseases

In addition to the use of chlorine in household products and drinking water, chlorine is also used to disinfect public swimming pools and hot tubs.  Chlorine is also a major component of medication.  According to OracleThinkquest 85 percent of medication is made using chlorine chemistry.  Chlorine is used to stabilize compounds, and to alter the effects of medications.   An example of a medication that uses chlorine is potassium chloride which is used to treat hypokalemia (low levels of potassium in the blood).  Other uses of chlorine include pesticides, rubber, and fungicides. Additionally chloride (Cl-), the negatively charged ionic form of chlorine, is essential for normal biological function.  In the body, chloride does not reach toxic levels, and serves as a major electrolyte in the extracellular fluid.  The chloride ion plays a role in specific cellular pathways, as an enzyme activator, and a component of gastric hydrochloric acid.

Toxic Effects: Focus on Chlorine Gas


Although chlorine can be credited with public health with its disinfecting properties, chlorine, it is also highly toxic in its gaseous state.  On April 22, 1915, chlorine gas was used as the first chemical weapon at the start of the Second Battle of Ypres in World War I. The German troops used 180,000 kilograms of chlorine gas that day to drive back the opposing French and Algerian troops; the Germans launched a second chemical attack a few days later on the Canadians.  After World War I, chlorine gas was replaced with more potent chemical weapons, but, more recently in 2007, insurgents in Iraq rigged explosives to trucks filled with chlorine gas tanks, releasing the toxic gas in highly populated areas.   

Today, most exposures to toxic levels of chlorine gas are accidental.  More than 13 million to 15 million tons of chlorine gas are produced annually in the United States, requiring it’s transport across large distances.  The worst chlorine gas accident in the United States happened in 2005 when a freight train transporting three chlorine tank cars collided with a parked train in Graniteville, South Carolina.  This accident caused nine deaths, eight of which were attributed to chlorine inhalation from a leaking tank car, and exposed at least 1,400 people to noxious chlorine gas.  Approximately 500 people were treated at local hospitals for respiratory symptoms.

Accidents at industrial companies and with water purification systems can also occur – resulting in exposure to chlorine gas. The most frequent exposures, though, to toxic levels of chlorine gas are from misuse of household products.  Some people believe that mixing vinegar and chlorine bleach make a stronger disinfectant.  This is true since vinegar lowers the pH of bleach, and some studies show that this combination can disinfect anthrax spores.  But chlorine bleach and vinegar combined, produce chlorine gas.  Another household chemical that should not be mixed with chlorinated bleach is ammonia. This combination causes the release of chloramines, another toxic form of chlorine when inhaled. 


The toxicity of chlorine gas depends on the amount of chlorine gas that a person is exposed to, the length of exposure, and the pathway.  Chlorine is most toxic when inhaled, and so primarily affects the respiratory system.  Symptoms of the inhaled exposure to chlorine gas includes chest pain, difficulty breathing, headache, dizziness, hyperactivity, emotional disturbances, lung damage, and even death.   

Chlorine is soluble in water, so when it comes in contact with watery mucus membranes such as the lining of the lungs, it forms hypochlorous acid (HClO) and hydrochloric acid.  Some studies show that it is also likely that chlorine reacts with reactive oxygen species and other molecules in airways, forming highly reactive oxidants.  Damage to the epithelium of exposed areas may be prolonged after the initial exposure to chlorine when an immune response initiates the activation and migration of inflammatory cells such as neutrophils.  When the neutrophils reach the areas in the body of chlorine gas exposure, they release proteolytic enzymes and oxidants to combat the antigen which further propagates the toxic effects of chlorine gas. 

Chlorine is a mild irritation to the mucus membranes during a low exposure  of 1-3 parts per million (ppm), and at 30 ppm immediate chest pain, shortness of breath, and a cough develops.  When exposure reaches 40-60 ppm, toxic pneumonitis and/or acute pulmonary edema can occur, and prolonged exposures to 400 ppm can be fatal. Concentrations of 1,000 ppm of chlorine gas can kill within a few minutes.        


Although chlorine is used for beneficial purposes, it is a toxicant in its gaseous state.  The Occupational Safety and Health Administration (OSHA) recommends chemical cartridge/gas mask respirators that filter chlorine gas for workers who may be exposed to the gas.  For safe household cleaner use, the CDC advises to read all labels before use, keep chemicals in their original container, never combine household cleaners and open windows or put on a fan when using household cleaners.  In addition to information on the label on the container, information about a particular cleaner can be found online through the U.S Department of Health and Human Services Household Products Database.  


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