Understanding Chlorine

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Because they are in such wide use, the hazards of common chemicals sometimes are taken for granted. Complacency can set in and improper procedures may be used by those who work with the chemicals regularly and by emergency responders who deal with the materials during a release, resulting in injury and death. One of these chemicals is chlorine.

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Photo by Robert Burke
One-ton chlorine containers are loaded on a flatbed truck bearing a poison gas placard and four-digit identification number.

Chlorine is a common hazardous material found in most communities in the United States as a gas or in compound with other chemicals that can release the chlorine when in contact with water or other chemicals. It is generally transported and stored as a liquefied compressed gas and will be found in 100- to 150-pound cylinders, one-ton containers and railroad cars.

Chlorine (elemental symbol Cl) is a nonmetallic element, a member of the halogen family of elements with an atomic number of 17 on the Periodic Table. Other halogens include fluorine, bromine and iodine. Chlorine was discovered in 1774 by Carl Scheele, who also discovered oxygen and several other important compounds. Scheele called his discovery “dephlogisticated marine acid.” Chlorine has an atomic weight of 35.453 and is a greenish-yellow diatomic gas with a pungent irritating odor, but does not exist freely as a gas in nature. Diatomic gases are elements that do not exist as a single molecule, in this case Cl, but rather as the diatomic molecule Cl2. Other elements that are diatomic are hydrogen, nitrogen, bromine, iodine, fluorine and oxygen. (Oxygen is often referred to as O2 because it is a diatomic element.)

The primary source of chlorine is in the minerals halite (rock salt), sylvite and carnallite and from the chloride ion (sodium chloride) in sea water. It can be liquefied for more economical shipping, storage and use.

Chlorine is toxic by inhalation (1 part per million in air), non-flammable, non-explosive and a strong oxidizer (stronger than oxygen). Because chlorine is a strong oxidizer, it will support combustion even though it is non-flammable. Chlorine has a National Institute for Occupational Safety and Health (NIOSH) immediately dangerous to life and health (IDLH) rating of 10 ppm and exposure limit time-waited average (TWA) of 1 ppm. The Occupational Safety and Health Administration (OSHA) ceiling for chlorine is 1 ppm. The maximum airborne concentration is 3 ppm. This is the amount to which a person could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms that could impair the ability to take protective action.

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Photo by Robert Burke
Chlorine railroad car with the name of the material stenciled on the container, an inhalation hazard warning and a bulk placard with a four-digit identification number.

Chlorine gas irritates the mucus membranes and the liquid burns the skin or causes irritation to the skin and may cause burning pain, inflammation, and blisters. Tissue contact with cryogenic liquid chlorine can cause frostbite injury. Chlorine’s odor threshold is about 3.5 ppm, although some report that odor can be detected below the 1 ppm OSHA ceiling and TWA. Short-term exposure to low concentrations of chlorine (1 to 10 ppm) can result in a sore throat, coughing, and eye and skin irritation. After a few breaths at 1,000 ppm, chlorine can be fatal. Exposures to chlorine should not exceed 0.5 ppm (an eight-hour time-weighted average over a 40-hour week).

Chlorine is not known to cause cancer. Reproductive and developmental effects are not known or documented. Chlorine has a boiling point of 29 degrees Fahrenheit, a freezing point of –150F, a gas density of 2.5 (making it heavier than air), a specific gravity of 1.56 (heavier than water) and a vapor pressure of 5,168 mm Hg at 68F. The vapor pressure of chlorine is 53.1 psi at 32F and 112.95 psi at 77F.

Chlorine is slightly soluble in water and reacts with a variety of other chemicals, including aluminum, arsenic, gold, mercury, selenium, tellurium, tin and titanium. Carbon steel ignites near 483F in contact with chlorine. It also reacts with many organic materials creating violent or explosive results. Chlorine will react violently with acetylene, ether, turpentine, ammonia, fuel gas, hydrogen and finely divided materials. Chlorine is placarded and labeled as a Class 2.3 Poison Gas in transportation and OSHA-mandated fixed storage. Non-bulk containers will also have the corrosive label displayed. Chlorine has a United Nations four-digit identification number of 1017 and a National Fire Protection Association (NFPA) 704 designation of Toxicity 3, Flammability 0, Reactivity 0 and special information oxy (oxidizer).

Chlorine was used during World War I as a choking (pulmonary) agent. On April 22, 1915, the German army released a large cloud of chlorine at Ypres, France, resulting in the deaths of 5,000 Allied soldiers and the injury of 10,000 more. It could also be a potential weapon for terrorists because of its common use and availability. As a result of the military use of chlorine, much data is available about human exposure and the expected effects, both long and short term. Release of chlorine from containers as an act of terrorism could be very effective in killing hundreds if not thousands of people.

One of the primary uses of chlorine around the world is in the chlorination of drinking water and treatment of sewage. It is also widely used in swimming pools. Chlorine is used in the production of paper products as a bleach and in dyestuffs, textiles, petroleum products, medicines, antiseptics, insecticides, food, solvents, paints, plastics and many other consumer products.

Exposure to chlorine can cause various signs and symptoms depending on the amount and length of time exposed. There is no available antidote for chlorine exposure. Effects can be treated and most people exposed that survive acute exposure will likely recover with little if any side effects. Listed below are potential symptoms:

  • Coughing
  • Chest tightness
  • Burning sensation in the nose, throat and eyes
  • Watery eyes (contact with liquid can cause blindness)
  • Blurred vision
  • Nausea and vomiting
  • Burning pain, redness and blisters on the skin if exposed to gas and skin injury; similar to frostbite if exposed to liquid (cryogenic) chlorine.
  • Difficulty breathing or shortness of breath
  • Fluid in the lungs

Chlorine Kits Stop Valve Leaks

Leaks in chlorine container valves can be stopped with the use of commercially available specially designed prefabricated chlorine kits. Kits are available for 150-pound cylinders, one-ton containers and railroad cars. The kits are known as Chlorine Kits A, B and C. Every fire department should know where the closest chlorine kits are located and how to get them to a scene when needed.

Not all kits are located with fire departments and hazardous materials teams. They may be found at water and sewage treatment facilities and private industry. The Chlorine Institute also has a mutual aid system called CHLOREP. It is made up of chlorine industry emergency teams that are available to respond to chlorine emergencies, who also have chlorine kits available.

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Photo by Robert Burke
A 150-pound chlorine cylinder with Chlorine Kit A in place on a leaking valve.

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Photo by Robert Burke
Railroad car dome valves with Chlorine Kit C in place on a leaking valve.

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Photo by Robert Burke
A one-ton chlorine container with Chlorine Kit B in place on a leaking valve.

Exposure to low concentrations (1 to 10 ppm) is likely to result in eye and nasal irritation, sore throat, and coughing. Higher concentrations (greater than 15 ppm) are likely to result in rapid onset of respiratory distress with airway constriction and accumulation of fluid in the lungs (pulmonary edema). Additional symptoms may include rapid breathing, blue discoloration of the skin, wheezing, rales or hemoptysis. Pulmonary injury may progress over several hours and lung collapse can occur. It is estimated that the lowest lethal concentration for a 30-minute exposure is 430 ppm. While these symptoms can also be present with exposure to other inhalation hazards, investigation of the site and circumstances should clear up the chemical involved in most cases.

Chlorine usually does not just appear, it has a distinctive color and odor. Examinations of containers and reports of witnesses can be helpful in positive identification. There are usually not any long-term health effects from sudden exposures to chlorine vapor for those who survive. Complications can occur such as pneumonia during treatment. Chronic bronchitis can also develop in people who contract pneumonia.

While it is a gas, chlorine can cause irritation and burns in contact with the skin. Therefore, firefighter turnouts are not appropriate for chlorine exposures inside the “hot zone” of a hazardous materials incident. In the past, firefighters were known to wear firefighter turnouts with petroleum jelly covering exposed skin. Chlorine is a poison gas and requires self-contained breathing apparatus (SCBA) and full Level A chemical protective clothing for anyone knowingly going into an atmosphere where chlorine is present. OSHA allows Level B protection for unknown atmospheres, which could include chlorine, but as soon as it is known that chlorine is present, protection should be changed to Level A.

Generally, gases do not present a serious contamination concern, because it is unlikely they will stay on chemical protective clothing. When exposed to chlorine gas, responders will need to go through a minimal decontamination reduction corridor. Liquid exposure to chlorine or compounds of chlorine may require a more extensive decontamination effort. Victims will require decontamination quickly to reduce damage to skin and eyes. Emergency decontamination would be appropriate by first responders if done from a safe distance, avoiding vapor and runoff. Exposure of victims to gas will result in minimal contamination. Removing clothing can limit the exposure to liquid chlorine and any gas that may be trapped in the victims’ clothing.

When released from a container, chlorine is most concentrated at the point of the release. As with many gases and vapors, the concentration diminishes the farther away from the source you get. Evacuation and isolation distances found in the Department of Transportation (DOT) Emergency Response Guide Book (ERG) are based on computer modeling of chlorine releases. Isolation (hot zone) for small spills (those from a small container or a small leak from a large container) is 100 feet. From a large container (several small containers or large leak from large container) the recommended isolation distance is 200 feet.

Evacuation distances are categorized into day and night spills. This is because the environment tends to be more stable at night, meaning a cloud will stay together longer and travel farther before dissipating. The evacuation distance for small day or night spills is one-tenth of a mile. The evacuation distance is three-tenths of a mile for a large day spill and seven-tenths of a mile for a large night spill. Several factors influence the amount of time a cloud of gas will stay together, including temperature, humidity, and wind direction and velocity. Chlorine dissipates best in warm, windy weather.

Chlorine is a common industrial chemical in the top 10 produced chemicals annually. It is used and transported to and through almost any community in the United States and releases do occur. On Nov. 17, 2003, in Glendale, AZ, a leak occurred as chlorine was being loaded into a railroad car. The incident forced the evacuation of the surrounding area for hours. Fourteen people were treated for symptoms such as nausea, throat irritation and headaches. A preliminary investigation indicated that safety devices apparently failed. A full investigation has been initiated by the U.S. Chemical Safety Board.

Firefighters and hazmat personnel responded to a leak at a water distribution plant in North Carolina on June 13, 2003. Chlorine alarms in a pump house alerted personnel to the leak. No one was injured as hazmat personnel entered the facility and found a manifold connected to 150-pound cylinders of chlorine had malfunctioned. Responders were able to shut off valves on the cylinders stopping the leak. Pre-planning and following proper procedures led to a successful outcome.

In St. Louis, MO, in August 2002, a chlorine release caused the injury of 63 people, including workers and nearby residents, during the off-loading of a chlorine railroad car. Once again, an automatic shutdown system failed to operate. Approximately 48,000 pounds of chlorine was released.

Another incident occurred on Oct. 3, 1998, in Sun Bay South, FL. This was the sixth release from the same facility in three years. Chlorine is pumped from railroad cars into the facility and five of the six releases occurred during off-loading operations. The most recent incident was caused by a cap that burst. A dozen people experienced difficulty breathing and one employee experienced a burned trachea and other injuries.

A massive leak of liquefied chlorine gas occurred on May 6, 1991, in Henderson, NV, in a plant that produces chlorine gas from sodium chloride. More than 200 people sought aid at local hospitals for respiratory distress caused by inhalation of the chlorine, with 30 admitted for treatment. Several first responders and the battalion chief in charge were overcome by chlorine at the main entrance of the plant when they responded. Over 700 people were evacuated and taken to shelters with 2,000 to 7,000 others taken elsewhere.

The alarm was delayed when plant employees thought they could handle the release internally. Fire department response was the result of several reports from the public of strong odor in the area. The release was caused by failure of pipes corroded by leaking acid from a heat exchanger that ate through the pipes, resulting in the release of more than 70 tons of chlorine.


Robert Burke, a Firehouse® contributing editor, is the fire marshal for the University of Maryland. He is a Certified Fire Protection Specialist (CFSP), Fire Inspector II, Fire Inspector III, Fire Investigator and Hazardous Materials Specialist, and has served on state and county hazardous materials response teams. Burke is a veteran of 24 years in fire and emergency services, with experience in career and volunteer departments. He has attained the rank of lieutenant, assistant chief and deputy state fire marshal. Burke is an adjunct instructor at the National Fire Academy and the Community College of Baltimore, Catonsville Campus, and the author of the textbooks Hazardous Materials Chemistry for Emergency Responders and Counter Terrorism for Emergency Responders. He can be reached in the Internet at robert.burke@att.net.

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