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.