The U.S. produces over 13 million tons of chlorine each year, much of it shipped by rail. The worst chlorine accident in the U.S. happened in Graniteville, S.C., when 18 freight train cars derailed and released 120,000 pounds of chlorine gas, killing 9 people. 1,400 people were exposed, resulting in 550 hospitals visits, many with serious lung injuries. 5,000 people were evacuated from the surrounding area. The incident occurred in 2005.

When you think of chlorine, you think of the chemical that is added to swimming pools, the stuff that makes your eyes itchy and bloodshot and gives your hair an odd green hue, right? And aside from that, it’s fairly harmless, right?

But it should be remembered that chlorine is a dangerous chemical. It is a highly reactive gas and it is incompatible with many other substances. In fact, chlorine was used during World War I as an agent of chemical warfare, and it was used quite effectively.

For many of today’s consumer products, chlorine is an indispensable part of the manufacturing process. It’s used in the production of pretty much everything. Every time you drink a glass of water, read your newspaper, clean your teeth, or drive your car, you are using chlorine in some form. It is also used in the manufacture of pharmaceuticals, pesticides, paper, and resins. It is generally regarded as the most effective disinfectant and bleaching agent available today. Chlorine is used in 99% of our treated drinking water to prevent the spread of disease. It is also used to disinfect equipment and utensils in hospitals and food processing plants. It’s even used to disinfect sewage and to control odor.

Chlorine is greenish-yellow in color, with a characteristic suffocating, pungent, and irritating odor. It condenses to an amber liquid at very cold temperatures or at high pressures. Exposure to chlorine can occur in the workplace or in the surrounding environment following accidental releases. It can enter the body when breathed in with chlorine contaminated air.

Chlorine is a naturally occurring element that is used in a variety of ways because of its ability to bond with other elements. However, chlorine’s bonding ability is also what makes it a very dangerous chemical. By itself, chlorine is a stable element and is classified as non-flammable. But, in combination with other elements, chlorine becomes extremely reactive. Also, because chlorine gas is 2 ½ times as heavy as air, it will usually remain close to the ground and tend to collect in low areas or pockets and in confined spaces, where it will displace clean breathing air. Additionally, when chlorine collects in low areas or pockets it creates a very hazardous atmosphere if combustibles are present.

Although chlorine is classified as non-flammable, it will support the burning of most combustible materials, just as oxygen does. Contact between chlorine and flammable or combustible substances should be avoided, because of the potential for fires and explosions. In addition to being highly reactive, chlorine will combine readily with moisture, steam, and water. It is a strong oxidizer and is corrosive to most metals. In solution with water, it forms a corrosive material that will attack many forms of plastics, rubber, and other coatings. So, you can imagine how it can be so damaging to the human body.

Chlorine is a strong irritant to the eyes, the upper respiratory tract, and the lungs. Acute exposure is described as both blinding and completely suffocating, stealing air through displacement and chemically scorching the lungs—all of which causes panic.

Chlorine exposure has its most serious effects on the lungs when it is inhaled. This can cause the airways to close up or fluid to develop in the lungs. Chlorine can also be ingested, or eaten, with chlorine contaminated food or water and it can be absorbed into the blood stream if it comes in contact with the eyes and skin.

Exposure can cause irritation of the eyes and respiratory tract, runny nose, sneezing, coughing, choking, and chest pain. Severe breathing difficulties may occur later. With severe exposures, fluids may leak into the lungs, causing pulmonary edema. In high concentrations, chlorine can irritate, inflame, and blister the skin, in addition to causing burning and prickling sensations. Liquid chlorine can cause frostbite burns of the skin and eyes on contact. Severe exposures may be fatal.

The effects of chlorine exposure on human health are a function of the exposure concentration—that is, how much chlorine is present in the air to which the employee is exposed—and the length and frequency of exposure. The effects also depend on the health of a person. According to Scientific American, “Studies show 40-60 ppm produces lung injury; 430 ppm usually causes death in 30 minutes, and 1,000 ppm is fatal within a few minutes. Under federal standards, workers are never supposed to be exposed to concentrations exceeding 1 ppm.”