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Cl2 Gas and its Sensor

Intro to Chlorine Gas

Chlorine gas is a chemical compound with the formula Cl2. It's easily distinguishable by its greenish-yellow color and a bleach-like odor. Chlorine gas is a very toxic chemical element that can cause eye irritation, coughing, vomiting, and other symptoms. Cl2 was even weaponized during World War I and is estimated to have caused at least 1100 deaths in its first usage. Chlorine gas is known for reacting with other organic chemicals in the atmosphere, often leading to dangerous reactions. Chlorine gas should always be properly stored and managed as its found in many worksites and commonly used in the sanitation of industrial waste and sewage.

Gas Characteristics

  • Penetrating and irritating odor (smells similar to bleach) 
  • Color: greenish-yellow 
  • Second only to CO for accidents causing injury  
  • Can occasionally explode if heated  
  • Can combust under certain conditions  
  • Can intensify existing fires  
  • Violent reactions can happen with many other organic chemicals  
  • Reacts explosively with Hydrogen (when Hydrogen is in range of 4%-93%) 
  • VERY toxic gas
  • Heavier than air  
  • Other names: bertholite, dichlorine, molecular chlorine
  • CAS 7782-50-5
icon-ghs-environmental-hazard warning of chlorine compounds
GHS WHMIS Toxic Icon examples include hydrogen chloride and hydrogen chloride gas
icon-ghs-harmful examples include concentrated hydrochloric acid and poisonous chlorine gas

Industrial Cl2 hazards and sources

  • Pulp and paper - commonly used as a bleach to manufacture paper.   
  • Manufacturing - a component of plastics, pesticides, rubber, and solvents.  The production of textiles such as fabric & fiber.
  • Water and wastewater - used as part of the sanitation aspect of industrial waste and sewage, can gradually release chlorine.
  • Mining - chlorination used in the extraction of nonferrous metals.
  • Pharmaceuticals: 85% of manufacturing use a Cl2-based compound for oxidizing or substitution reactions.
  • Oil and Gas - present in petroleum production and refining. 
  • Food processing - microbicide and algicide in processing systems.
  • Rail transportation: Chlorine is often transported in freight train cars.
  • Cleaning bleach can mix with other chemicals to release Cl2 gas.  
  • Chlorine can be manufactured by the electrolysis of a sodium chloride solution.

Most incidents of chlorine exposure are through accidental industrial or household exposures.

(National Library of Medicine)

High Risk Scenarios

  • As chlorine gas is heavier than air, it can collect in dangerous amounts near ground level. 
  • Chlorine gas is most dangerous when it comes into contact with moist tissues such as the eyes, lungs, or throat. When this occurs, an acid forms which can severely damage these tissues. 
  • Because Cl2 is an oxidizer, it can cause and/or intensify fires. 
  • Gaseous chlorine can react with organic compounds found in sources of water to create chloroform, which is a carcinogen. 
  • Workers that face consistent long-term exposure to chlorine gas increase their risk of suffering respiratory effects, such as airflow obstruction or irritation to the throat and eyes.
  • Chronic exposure can also lead to the senses adapting to the presence of Cl2; as a result, it can become increasingly difficult to detect when there is Cl2 around.
  • Chlorine gas can combine with moist mucous membranes to create hydrochloric acid, which can pose a significant risk to humans.
  • The 2005 Graniteville chlorine disaster is considered one of the worst chemical accidents in the US where 120,000 lbs of chlorine gas was released, resulting in nine fatalities and 550 hospitalizations.       

Cl2 Sensor Info

Type: Electrochemical
Range: 0-20 ppm (0.1 ppm resolution)

Default Alarm Levels

Low Alarm: 0.5 ppm
High Alarm: 1 ppm
STEL — 15 minute — Short Term Exposure Limit: 5 ppm 
TWA  — 8 hour time weighted average: 0.5 ppm 

Blackline devices that can detect Cl2

Questions about the detection of Cl2?


Special Applications and Considerations

  • As with many other gases, Cl2 is particularly dangerous when dealing with confined space entry. If someone is working in a confined space with poor ventilation, chlorine gas can accumulate to fatal concentrations, especially lower down. 
  • While not flammable itself, chlorine gas can explode if it is combined with many other common gasses in the atmosphere, such as hydrogen, ammonia, and fuel gas. 
  • Cl2 has significant bleaching properties; it is dangerous to humans as well as the environment. 
  • Chlorine gas can easily corrode and cause significant damage to metals and other materials if it is not properly accounted for.
  • Too much exposure to Cl2 can begin to deaden a person's sense of smell. 
  • Cl2 destroys ozone molecules, making it a strong contributor to ozone layer depletion. 
  • Chlorine gas damages the environment in several other ways as well: it propels global warming, and is often found in acid rain. 
  • Compounds of interest - chlorine atoms hydrolyze to hydrochloric acid (HCI) when added to water. Hydrochloric acid and hydrogen chloride gas are composed of one hydrogen atom and one chlorine atom. 

Health Risks and Handling of Cl2

1 - 4 ppm
Likely will experience mild mucous membrane irritation
5 - 29 ppm
Could experience moderate respiratory tract irritation
30 - 39 ppm
Many people will suffer immediate coughing, vomiting, difficulty breathing, and chest pain
40 - 429 ppm
Lungs are likely to face accumulation of fluid and/or inflammation
430 - 999 ppm
Fatal past 30 minutes
1000 ppm +
Certain death within minutes
Carbon Monoxide First Aid Icon
  • Ensure anyone conducting first aid is adequately protected (safety goggles & chemical protective clothing) 
  • If INHALED, move victim to fresh air and make sure they are in a resting position that is comfortable for breathing. Keep the victim still. Trained personnel can administer additional oxygen if necessary. Call poison control/a doctor, and take the victim to a hospital 
  • If SKIN CONTACT is made, rinse the impacted area with gently flowing lukewarm water for 5 minutes. See a doctor if the pain/irritation does not subside
  • If the EYES are contacted, follow the same steps as for skin contact
Carbon Monoxide (CO) gas dangers
  • Evacuate the area ASAP, keep out any personnel who are not wearing protective equipment 
  • Isolate the area that is concentrated with chlorine gas
  • If you have the necessary protection to do so, attempt to remove any incompatible/hazardous materials that could escalate the situation 
  • If the Cl2 leak is relatively small, attempt to stop or reduce the presence of the chlorine gas. Try to ventilate the area 
  • If the leak is larger, follow the same steps as for a small leak. However, you can also consider knocking down the gas with fine water spray or fog. If the leak cannot be contained, contact local emergency services 


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