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What, if any, relationship is there between onions and tear gas, both of which stimulate crying?

Watering of the eyes and crying - whether from onions or tear gas - are natural responses to irritation. It is the body’s way of getting something in an eye washed out.

The term tear gas covers a variety of chemicals that irritate the eyes and trigger tearing. They are generally used by law enforcement officials to get groups of people to stop some illicit activity, such as rioting, without causing serious harm - although there is some evidence that many of the substances used can cause toxic effects, potentially including lung, heart, and liver damage.

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The proper term for such chemicals is lachrymatory agent or lachryamator, from the Latin word “lacrima,’’ meaning tear. But most lachrymators used as tear gas are not gases at all.

For example, CS “gas’’ is an aerosol of 2-chlorobenzalmalononitrile - a solid - either as a fine powder or tiny droplets dissolved in a solvent, and there are plenty of others. Some genuine gases make eyes tear up, but gases are diffused faster than aerosols and are less well suited to crowd control. Pepper spray, based on capsaicin, the chemical that gives hot peppers their fire, can be considered a sort of “tear gas’’ in this sense.

Tearing from onions is caused by a complicated process.

When onions are cut, cells are broken open, allowing enzymes called allinases to emerge. These react with sulfur-containing compounds to produce sulfenic acids that are turned into a gas, syn-propanethial-S-oxide, that irritates nerves in the eyes and induces crying.

Cooking onions destroys the enzymes, so cooked onions can be cut without causing crying. Similarly, cutting onions under water can trap the gas and keep it out of eyes.

There are claims that holding a cut onion near the eyes and nose helps protect them from tear gas. But whether this is true is uncertain, and if it is true, the chemistry involved is not clear. I once walked through the remnants of tear gas cloud in Peru, well after a protest had cleared out, and it was not fun. I would keep away from any variety of tear gas.

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