Why onions make you cry

Onions cells contain small amounts of propanethiol S-oxide. When you cut one open, this compound mixes with enzymes that release a sulfur compound which is itself not noxious. However, when it encounters the water in our tear ducts, it creates sulfuric acid which is a strong irritant. Rubbing ones eyes will only make things worse if some of the compound has adhered to the skin while cutting the onion.

Some people have noticed that running cold water on your wrists prior to cutting an onion can reduce the irritation. The explanation for this is related to putting cold keys on your back to reduce a nosebleed. The cold sensation makes blood vessels in the nasal passages constrict. The route to the tear glands is through the nose and constriction of the blood vessels may slow this process down.

Propanethial-S-oxide is a gas that's released when the cell wall of an onion is broken. This gas reacts with the water in your eyes and produces sulphuric acid. Getting acid in your eyes sucks, it also burns.

There are some ways to prevent this:

• Handle onions underwater
• Slice from the sides, rather than ends, avoid the root where the most gas is
• Wear goggles

Beakman, on Beakman's World, taught me that onions make your eyes water because of fumes that you've inhaled. I would assume that these fumes are the beforementioned Propanethial-S-oxide. Wearing goggles would not combat the eye-wateryness, since we don't inhale with our eyes. However, holding your breath or hooking yourself up to oxygen would.

I believe it's a combination of both. Inhaling the fumes would cause the production of sulphuric acid in your sinuses, causing eye and nose leakage. However, the amount of water normally present in the sinuses is greater that that on the surface of the eye, so any acid produced would be more dilute, and take less flushing out to return the pH level to normal* . The acid produced in your eyes is obviously not concentrated H₂SO₄ (i.e. it doesn't transform your eye sockets into steaming holes of raw flesh),so that which is produced from inhalation is likely to cause an even weaker reaction. Hence, achan's goggle idea, while it may not completely stop eyewatering, would at least cause a decrease in the severity of the response.

Actually, I noticed several years ago that wearing contact lenses massively decreased my onion-tearyness, and made the rest up from there. Sounds logical though.

* I have no clue as to what normal nose pH is, and very little inclination to shove test paper or a pH electrode up my nose. Anyone wishing to take part in a worldwide survey of nasal pH should feel free to /msg me with their results.

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Update

ToasterLeavings will be attempting a nasal pH reading using a fish tank test kit, just as soon as 15mL of nasal fluid become available. More information will be provided as it comes to hand.
If I get a few more volunteers, I'll send out pH strips and node the results.

Onions make you cry because they remind you that your own identity, your "self," is as ill-defined as the core of an onion. In a quest to slay inferior selves in order to perfect our own lives and come out as our own ideal person, are we simply peeling back layers of onion, searching for a core? The core, you see, is naught but more small layers - or, perhaps, a point from which concentric layers begin to radiate. Decide for yourself where its boundaries lie.

Henrik Ibsen's Peer Gynt makes extraordinary effort to stay true to himself. When the fellow finds himself starving in the forest, he digs up a wild onion and begins to peel back its layers. Each one, he remarks, is a different stage of himself throughout the play. Where is the core, the thing he's been seeking through the entire story?

I could say that it's not there. I would rather say that it's there, and up to us to define.

No onion ever made me cry as much as the budding vidalia I found with two cores.