First lets start by looking at chlorine itself. Chlorine is the name of an element, and its name comes from the Greek word khloros, meaning pale green because chlorine in its natural form is a pale green gas. Chlorine is also an atom essential for living things. It is in the group of elements called halogens. Halogens are very reactive elements and chlorine is no exception. Due to the number of electrons in its outer atomic shell, chlorine is very electronegative, meaning it is quite good at pulling electrons off of other atoms. Even though by itself chlorine is very reactive, when it is in some compounds, like when it is an ion form paired with the metal sodium, it makes common table salt! Chlorine in other compounds forms can still be very corrosive, such as with hydrochloric acid (HCl) which is in our stomach juices and which was also used as early at 800 A.D. by alchemists.
Chlorine is well-known for being part of sodium hypochlorite, which is another name for bleach. Sodium hypochlorite is used to whiten fabric and paper, for disinfection (killing microbes) and sanitizing water. Chlorine in this and similar forms is used for these applications because it is a strong oxidizer, meaning it will readily donate oxygen to other molecules. This property also makes concentrated chlorine toxic to animals too; it is a strong irritant and can cause significant damage to skin, lungs and eyes. Its not easy to say whether exposure to chronic low-concentration levels of chlorine, like in swimming pools, would be harmful, but it may lead to drying of the skin and dry skin can become irritated. It may also aggravate already existing conditions, such as asthma.
Chlorine is used to remove potentially dangerous microbes from drinking water and swimming pools. While the lower levels of chlorine in pools and drinking water are not themselves generally harmful to humans, recent studies show that chlorine can react with organic molecules in the water and then create harmful compounds called disinfection by-products (DBPs). These compounds might react with DNA and lead to higher rates of cancer. This is another reason that it is very important to shower before and after swimming in a pool: before to wash off the organic compounds that can react with chlorine, and after to wash off the chlorine and DBPs.
Some drinking water information:
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