Which chemical is a constituent of tear gas?

Tear gas is the common name for chemical agents called lachrymators (from the Latin lacrima, meaning "tear"). They irritate the eyes, nose, throat and skin, causing tears, pain, coughing and temporary blindness. They are used by police to disperse crowds and control riots, because their effects are usually temporary.

Chloropicrin, also called nitrochloroform (chemical formula CCl3NO2), is a strong lachrymator. It was used as a chemical weapon in the First World War. Today it is used mainly as a soil fumigant and pesticide to kill fungi, insects and nematodes in soil before crops are planted. It is often mentioned in exams as a constituent of tear gas.

Other well-known tear gas agents are:

• CS gas (2-chlorobenzalmalononitrile), the most widely used tear gas today.
• CN gas (chloroacetophenone), used in early types of mace sprays.
• CR gas (dibenzoxazepine).

Pepper spray, which uses oleoresin capsicum (OC), a substance from chilli peppers. Its active compound is capsaicin. It is classified as an inflammatory agent rather than a true tear gas.

Tear gas is usually delivered by grenades, shells or spray canisters. Exposure in a closed space or in large amounts can be harmful. The Chemical Weapons Convention (1993) bans the use of riot control agents as a method of warfare, but allows them for domestic law enforcement.

Why not the other options?

• ✗Phosgene (COCl2) is a highly poisonous choking gas used in the First World War.
• ✗Mustard gas is a blister agent that causes severe burns to the skin and lungs.
• ✗Nitrous oxide (N2O) is "laughing gas", used as a mild anaesthetic.