Facts About Chlorine
Chlorine is a respiratory irritant. Inhaling 1,000 ppm for a few deep breaths can be fatal.
Credit: Pi-Lens | Shutterstock

If you have ever taken a prescription medicine, driven a car or drunk tap water, you likely have been exposed to chlorine.

Chlorine, element No. 17 on the Periodic Table of Elements, has multiple applications. It is used to sterilize drinking water and to disinfect swimming pools, and it is used in the manufacturing of a number of commonly used products, such as paper, textiles, medicines, paints and plastic, particularly PVC, according to the Royal Society of Chemistry. Moreover, chlorine is used in the development and manufacturing of materials used in products that make vehicles lighter, from seat cushions and seat covers to tire cords and bumpers, according to the American Chemistry Council

The element is also used in organic chemistry processes — for example, as an oxidizing agent and a substitution for hydrogen, according to the Los Alamos National Laboratory. An oxidizing agent has strong disinfecting and bleaching qualities. When used as a hydrogen substitute, chlorine can bring many desired properties in organic compounds, such as its disinfecting properties or its ability to form useful compounds and materials like PVC and synthetic rubber.

But chlorine also has a dark side: In its natural gas form, it is harmful to human health. Chlorine is a respiratory irritant, and inhaling it may cause pulmonary edema — an excessive buildup of fluid in the lungs that can lead to breathing difficulties. The gas can also cause eye and skin irritation, or even severe burns and ulcerations, according to the New York State Department of Health. Exposure to compressed liquid chlorine can result in frostbite of the skin and eyes, the agency reports.

  • Atomic number (number of protons in the nucleus): 17
  • Atomic symbol (on the Periodic Table of Elements): Cl
  • Atomic weight (average mass of the atom): 35.453
  • Density: 3.214 grams per cubic centimeter
  • Phase at room temperature: Gas
  • Melting point: minus 150.7 degrees Fahrenheit (minus 101.5 degrees C)
  • Boiling point: minus 29.27 F (minus 34.04 C)
  • Number of isotopes (atoms of the same element with a different number of neutrons): 24. Number of stable isotopes: 2
  • Most common isotopes: Chlorine-35 (76 percent natural abundance)

In 1774, Swedish pharmacist Carl Wilhelm Scheele released a few drops of hydrochloric acid onto a piece of manganese dioxide in his lab, and a greenish-yellow gas was produced in a matter of seconds, according to the American Chemistry Council. However, chlorine was not recognized as an element until several decades later, by English chemist Sir Humphry Davy, and before that, people thought it was a compound of oxygen. Davy named it "khloros," from Greek word for greenish-yellow, and in 1810, he updated the name to "chloric gas," or "chlorine."

Chlorine belongs to the group of halogens — salt-forming elements — together with fluorine (F), bromine (Br), iodine (I) and astatine (At).

Chlorine is a very "sociable" element, meaning it likes to bond with other elements rather than occur on its own. A highly reactive substance, it is usually present in nature, forming compounds with sodium, potassium and magnesium. In fact, probably the most known form of a chlorine compound is sodium chloride, otherwise known as table salt. Potassium chloride is a drug used in the prevention or treatment of low potassium levels in the blood, whereas magnesium chloride is used to prevent or treat magnesium deficiency.

Industrial chloride is made via electrolysis of sodium chloride solutions, according to The Essential Chemical Industry - online.

  • Some frogs have a chlorine compound in their skin that is a very strong painkiller, according to Los Alamos National Laboratory. The compound, called epibatidine, has no side effects in small doses, but in large doses, it is fatal to humans.
  • Due to its toxic properties, chlorine was used as a chemical weapon during World War I, according to the Royal Society of Chemistry.
  • When isolated as a free element, chlorine takes the form of a greenish-yellow gas, which is 2.5 times heavier than air and smells like bleach.
  • Chorine is the second-most-abundant halogen and the second-lightest halogen on Earth, after fluorine.
  • Sodium chloride (salt) is the most common compound of chlorine and occurs in large quantities in the ocean.
  • There may be some chlorine in the chicken you eat. Chicken carcasses that come from U.S. factory farms are often drenched in chlorine to get rid of fecal contamination.
  • Chlorine destroys ozone, contributing to the process of ozone depletion. In fact, one chlorine atom can destroy as many as 100,000 ozone molecules before it is removed from the stratosphere, according to the U.S. Environmental Protection Agency.

Chlorine has caused quite a stir among researchers over the years because of certain harmful effects it may have on human health. Those effects, however, remain debatable.

Previous research has linked drinking chlorinated water to an increased cancer risk. For example, in a study published in 1992 in the American Journal of Public Health, researchers found that people who drank chlorinated water had a 21 percent higher risk of getting bladder cancer, and a 38 percent higher risk of getting rectal cancer, than people who drank nonchlorinated water. And, in another study, published in 2010 in the journal Environmental Health Perspectives, investigators found that people who swam in a chlorinated pool for 40 minutes had increased biomarkers (i.e., certain molecular indicators) related to cancer risk. 

However, the U.S. Department of Health and Human Services, the International Agency for Research on Cancer, and the U.S. Environmental Protection Agency have not classified chlorine as a human carcinogen, according to the Centers for Disease Control and Prevention.

So, is chlorine bad for your health? Not exactly, said Preston J. MacDougall, a professor of chemistry at Middle Tennessee State University in Murfreesboro.

"You don't want to use excessive amounts of chlorine, but we shouldn't fear chemical substances because we do not understand them," MacDougall told Live Science. 

In fact, the lack of appropriate chlorination to kill harmful bacteria, such as E. coli, can have devastating consequences on human health and life, he added. For example, in May 2000, in Walkerton, Ontario, seven people died and more than 2,300 got sick after the town's water supply became infected with E. coli and other bacteria, according to the Water Quality and Health Council. If the required chlorine levels had been maintained, the disaster could have been prevented, even after the water was contaminated, according to a report published by the Ontario Ministry of the Attorney General.

In addition, there is some promising research-related news about chlorine. MacDougall pointed to a recent study on chlorine atoms found in a novel class of antibiotic compounds that have been discovered in tiny marine organisms in North Atlantic waters near Norway. Those chlorine atoms are essential to the antibiotic activity of the compounds, which can be effective against methicillin-resistant Staphylococcus aureus, a bacterium that causes hard-to-treat infections in people and is resistant to commonly used antibiotics, he said. 

"The drug discovery community is very excited about these naturally occurring compounds because they are effective against MRSA," said MacDougall, who was not involved in the research, published in April 2014 in the journal Angewandte Chemie International Edition

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