Chlorine (Cl2) is a green-yellow gas with a pungent, choking smell like bleach. It is toxic and corrosive, attacking the eyes and airways at very low concentrations, and it is around two and a half times heavier than air, so it sinks and collects at low level.
Most chlorine is used to disinfect water and to manufacture plastics and chemicals. That means it is handled in bulk at water treatment works, leisure centres and industrial plants, usually as a pressurised liquid in cylinders or drums.
Chlorine was first produced in 1774 by the Swedish chemist Carl Wilhelm Scheele, who reacted hydrochloric acid with manganese dioxide. Humphry Davy identified it as an element and gave it its name in 1810.
Other facts about chlorine:
- Chemical formula Cl2. CAS number 7782-50-5.
- Melting point: -101 degrees Celsius. Boiling point: -34 degrees Celsius.
- It does not burn, but it supports combustion in the same way oxygen does, so it can make a fire worse.
- It reacts with many common chemicals, including acidic and ammonia-based cleaners.
Fun Fact - The name ‘chlorine’ is derived from the Greek word ‘chloros’, meaning ‘yellowish green’.
What Does Chlorine Gas Smell Like?
Chlorine gas smells sharply of household bleach, and most people notice it at around 0.3 ppm, just below the short-term exposure limit. That puts chlorine between ammonia, which warns you well before it becomes dangerous, and hydrogen sulphide, which stops warning you altogether. Smell is a reason to leave, not a way to judge whether the air is within the limit.
What Is the Exposure Limit for Chlorine?
Chlorine has a UK short-term exposure limit of 0.5 ppm over 15 minutes in EH40, and no 8 hour limit at all. That reflects how it causes harm: the damage comes from short, sharp exposures rather than from a dose building up across a shift, so detector alarms are set against the short-term figure.
Is Chlorine Gas Dangerous?
Yes. Chlorine reacts with the moisture in the eyes, nose and lungs to form acids, so it burns the tissue it touches. Low exposures cause stinging eyes, a sore throat and coughing. Higher exposures cause severe breathing difficulty and fluid on the lungs, which can develop hours after someone has been moved to fresh air.
What Is Chlorine Used For?
- Drinking water and wastewater treatment
- Swimming pools and leisure centre plant rooms
- PVC and plastics manufacturing
- Pulp and paper bleaching
- Chemical manufacturing and laboratories
- Food processing and surface disinfection
How Do You Detect Chlorine?
Chlorine is measured with an electrochemical sensor reading in parts per million. A portable single gas detector protects workers during dosing, cylinder changes and confined space entry, while fixed detectors monitor pool plant rooms, dosing rooms and storage areas continuously, mounted near floor level.
Browse our chlorine gas detectors for portable and fixed Cl2 monitoring.
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Chlorine Gas FAQs
Can household cleaners produce chlorine gas?
Yes. Mixing bleach with an acidic cleaner such as a limescale or toilet descaler releases chlorine gas. Mixing bleach with an ammonia-based cleaner releases chloramines, which irritate the airways in a similar way. Both are common causes of accidental exposure in cleaning and catering work.
What should you do if you suspect a chlorine gas leak?
Move upwind and to higher ground, and raise the alarm. Never enter a plant room to trace a leak without breathing apparatus, and treat anyone who has been exposed as needing medical assessment even if they appear to recover.
Can a multi gas detector measure chlorine?
Only if it is specified with a chlorine sensor fitted. A standard four gas monitor covers flammable gas, oxygen, carbon monoxide and hydrogen sulphide, none of which is chlorine, so Cl2 has to be added as an extra channel. Our Drager X-am 5000 kit is configured that way, giving the usual four gases plus chlorine in one instrument.