Elemental chlorine is a highly reactive, greenish yellow gas. It has a suffocating, irritating, pungent bleach like odour that is detectable at low concentrations (above 0.3 – 0.5 ppm). Continuous or high-level exposure of the Cl2 can quickly deaden a person’s sense of smell, making the odour of Cl2 an unreliable indicator of its presence. Hence, other means such as chlorine monitoring is a viable solution to provide adequate warning of hazardous exposure. This article covers information on chlorine gas, its sources in the ambient air, permissible levels, health and environmental impact, possible corrective measures, need for chlorine monitors as well as different methods of H2S monitoring.
What is Chlorine?
Chlorine is a highly reactive, greenish yellow gas. It has a suffocating, irritating, pungent bleach like odour that is detectable at low concentrations (above 0.3 – 0.5 ppm). Cl2 is heavier than air which causes it to remain in low-lying areas or areas near ground with little air movement.
Chlorine gas is slightly soluble in water, Hence, it reacts to form hypochlorous acid (HClO) and hydrochloric acid (HCl). It is not flammable but has an ability to react explosively when combined with other substances such as hydrogen, ammonia, fuel gas, acetylene, ether, etc. Cl2 gas is oxidizing in nature and has bleaching properties. It is toxic to human and plant life and can corrode metals and other materials.
Chlorine in the Atmosphere
Elemental chlorine is rarely present itself in nature due to its high reactivity. It can be formed from atmospheric reactions of chlorine-containing compounds with NO2 and through oxidation of chlorides in presence of strong oxidants in the atmosphere such as ozone.
Once released, it rapidly combines with other chemicals/compounds in the atmosphere to form secondary compounds instead of remaining in pure elemental state. Chlorine dissolves in the water and reacts to form chloride salts and chlorinated organic chemicals such as sodium chloride (NaCl, common salt), sodium hypochlorite (bleach), chloroform, etc.
Sources of Chlorine
Chlorine gas is commonly used
- as household cleaner and disinfectant
- oxidizing agents in water treatment to disinfect drinking water
- as a disinfectant in industrial applications such as food processing, pulp and paper mill and water cooling systems, etc.
- in the bleaching process in pulp and paper mills and ink removal from recycled paper
- in the manufacturing of many products such as paper, antiseptic, insecticides, paints, petroleum products, textiles, medicines, etc.
Permissible levels of O3
The permissible exposure limits of Cl2 in terms of continuous occupational exposure are given below:
| Cl2 concentration | |
| OSHA (PEL) | 0.5 ppm TWA |
| OSHA (PEL) | 1 ppm STEL |
| NIOSH (REL) | 1 ppm STEL |
| NIOSH (IDHL) | 10 ppm |
| AIHA ERPG-1 | 1 ppm |
| AIHA ERPG-2 | 3 ppm |
| AIHA ERPG-3 | 20 ppm |
Permissible Exposure Limit (PEL) given by OSHA (Occupational Safety and Health Administration) defines the maximum concentration of Cl2 to which an unprotected worker may be exposed to. PEL may reference an eight-hour time-weighted average (TWA) or a 15-minute short term exposure limit (STEL) concentration that cannot be exceeded for any period. Similarly, Recommended Exposure Limit (REL) and Immediately Dangerous to Life of Health (IDLH) are the occupational exposure limit recommended by NIOSH (National Institute for Occupational Safety and Health) and the Emergency Response Planning Guidelines (ERPGs) are exposure guidelines given by AIHA (American Industrial Hygiene Association).
Health & Environmental Impact of Chlorine
Health impact
Chlorine is majorly toxic due to its oxidizing and corrosive properties. When in contact with Cl2 gas, the hydrogen in moist tissue splits from water. Thus, producing hydrogen chloride (HCl) which damages tissue. It also forms hypochlorous acid (HOCl) that penetrates cells in the body and destroys the cell structures by reacting with the cytoplasmic proteins.
Cl2 + H2O ↔ HCl + HOCl
Low level exposure of chlorine causes eye, skin and respiratory tract irritation, sore throat, coughing, etc. Higher levels of exposure to chlorine causes burning of eyes and skin, chest tightness, narrowing of bronchi, rapid breathing, wheezing, blue coloring of the skin, accumulation of fluid in lungs, etc. Very high exposure may cause severe burns to eye and skin, lung collapse, noncardiogenic pulmonary edema, etc and may lead to death.
Some health effects associated with chlorine gas exposure are given below:
| Cl2 levels | Health effects |
| 1 – 3 ppm | Mild mucous membrane irritation |
| 5 – 15 ppm | Moderate irritation of the respiratory tract. |
| 30 ppm | Immediate chest pain, vomiting, dyspnea (shortness of breath), and cough |
| 40 – 60 ppm | Toxic pneumonitis (inflammation of the lungs) and pulmonary edema (accumulation of fluid in the lungs) |
| 430 ppm | Lethal over 30 mins |
| 1000 ppm | Fatal within minutes |
Continuous or high-level exposure of the chlorine can quickly deaden a person’s sense of smell, making odour of Cl2 an unreliable indicator of its presence. Hence, other means such as the use of Cl2 monitors is a viable solution to provide adequate warnings of hazardous exposure.
Environmental Impact
Chlorine gas does not have a direct impact on the environment. However, it contributes to environmental problems such as ozone layer depletion, global warming and acid rain. Cl2 in the atmosphere destroys the ozone molecules as shown below.
Additionally, chlorine gas rapidly combines with other compounds to form chemicals such as dioxins. These dioxins pollute water, contaminating fish which is then consumed by larger animals and humans.
Possible corrective measures
The primary action is Cl2 monitoring i.e. to measure how much of chlorine concentrations you are exposed to. In addition to this following corrective measures can be taken:
- Avoid going to or staying in low lying areas near the source of Cl2.
- If the presence of Cl2 is detected, immediately vacate the area and provide proper ventilation to remove the gas.
- Use appropriate respiratory protection or any other personal protective equipment (PPE) when the exposure is detected.
- Also, avoid spending time in the areas downwind of industries emitting chlorine.
Measurement methods of Chlorine monitoring
Different working principles for chlorine monitoring in the ambient environment are pulsed NDUV (Non-Dispersive UV Absorption Spectroscopy) , semiconductor, and electrochemistry.
Non-Dispersive UV Absorption Spectroscopy
The Cl2 monitor working on the principle of NDUV is based on the absorption of ultraviolet radiation at a particular wavelength. When the gas sample in the chlorine monitor is exposed to ultraviolet light, the UV radiation absorbed by the Cl2 molecules present in the gas sample is measured by the detector and converted to chlorine concentration.
Semiconductor method for Chlorine monitoring
When a metal oxide semiconductor-based Cl2 monitor is exposed to an air sample, the chlorine molecules react on the metal oxide surface of the sensor and dissociate into charged ions which alter the resistance of the film. This interaction is measured as a signal and is converted to the respective gas concentration. However, when compared to other chlorine monitors, the energy consumption of such chlorine monitors is higher.
Electrochemical method for Chlorine monitoring
Chlorine monitors working on the electrochemical principle are operated based on the diffusion of Cl2 gas into the respective sensor which results in the production of electrical signals proportional to the chlorine concentration. It allows accurate measurement of even low concentrations of Cl2, which is essential in chlorine monitoring for the ambient air.
Among all the above principles of Chlorine monitoring, applications like ambient air monitoring prefer chlorine monitors based on electrochemistry. This is because they yield more accurate Cl2 concentrations and are inexpensive in comparison with the others.
Oizom’s working principle for Chlorine monitoring
Oizom’s ODOSENSE is the real-time odour emission tracking solution. It continuously detects, measures, and monitors the odourful gaseous contaminants including hydrogen sulfide, ammonia, sulfur dioxide, methyl mercaptan, TVOC, formaldehyde, methane, and weather parameters like temperature, humidity, wind speed, and wind direction. The sensor that measures Cl2 works on the principle of electrochemical sensing. With the help of meteorological data, Odosense can trace the odourant dispersion plume incited by conditions like wind speed and wind direction. Odosense is a proactive approach to measure real-time odour emissions. This makes it an ideal choice for landfill sites, wastewater treatment facilities, fertilizers, paper-pulp industries, and soil-treatment sites, etc.
5 Reasons why Cl2 monitoring is important
- Cl2 is highly reactive, greenish yellow gas that has a suffocating, irritating, pungent bleach like odour and can accumulate in enclosed, poorly ventilated, and low lying areas.
- Cl2 reacts with moisture in the tissues of the human body and forms toxic products such as hydrogen chloride (HCl) and hypochlorous acid (HOCl) that causes damage to tissues and cell structure of the body.
- Exposure to chlorine gas may cause irritation to eyes, skin, and respiratory tract, coughing, wheezing. accumulation of fluid in lungs, etc and may even lead to death. It also contributes to ozone layer depletion.
- Continuous or high-level exposure of the Cl2 can quickly deaden a person’s sense of smell, making odour of Cl2 an unreliable indicator of its presence. Hence, other means such as the use of Cl2 monitors is a viable solution to provide adequate warnings of hazardous exposure.
- Real-time Cl2 monitoring helps in determining the source of odour as well as formulating an action plan to control emission of chlorine.
