The word 'lachrymator' (also found as 'lacrimator') is from the Latin verb 'lacrimare' which comes from 'lacrima' meaning tear. This first entered the English language in 1918 (sorry Webster 1913) which coincided with the end of World War I - the first war in which chemicals were used - including that of tear gas.

The simple definition of a lachrymator is that of an irritant that causes watering of the eyes. The OSHA definition of irritant is:

a chemical, which is not corrosive, but which causes a reversible inflammatory effect on living tissue by chemical action at the site of contact. A chemical is a skin irritant if, when tested on the intact skin of albino rabbits by the methods of 16 CFR 1500.41 for four hours exposure or by other appropriate techniques, it results in an empirical score of five or more. A chemical is an eye irritant if so determined under the procedure listed in 16 CFR 1500.42 or other appropriate techniques.

So reading OSHA isn't the most interesting. A less specific definition of irritant is a substance that causes tissue to produce a local inflammatory reaction.

Three of the most well known lachrymators are that of tear gas, pepper spray and onions.

When an onion is cut, peeled or crushed the tissue releases a class of enzymes known as 'allinases'. These combine with the volatile oils that give onions their flavor (known as 'amino acid sulfoxides' - say that three times fast) to form sulferic acids which then reform to create the chemical syn-propanethial-S-oxide which triggers tears.

The cornea of the eye serves the purpose of providing a barrier for physical and chemical irritants. Within the cornea are autonomic motor fibers that activate the tear glands (lachrymal glands - there's that Latin again). When the nerve endings detect syn-proanethial-S-oxide on the cornea it is interpreted as a burning sensation. A reflex activates the autonomic fibers and cause the tear glands to attempt to wash the irritant away.

As many know wearing safety glasses does not prevent one from effects of the onion, nor that of pepper spray and tear gas. The only way to prevent the effects is to prevent the chemicals (often a vapor) from coming in contact with the eye. This can be accomplished by using a fume hood or is natural equivalent of a wind (cut onions in the breeze - you upwind) or working with them under water (cutting onions under water such as a faucet). Cutting onions while wearing a gas mask works and looks funny too.

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