Why do onions create tear gas when cut

I want to get a sense of what's got you curious here. Are you wondering about the actual chemistry of what happens, or did something spark this — like you were just cutting an onion and got frustrated, or you're trying to figure out how to avoid it?

You've hit on exactly the right insight — and it's actually a defense mechanism, which makes it even weirder.

The onion isn't storing the tear-inducing chemical; it's creating it on demand when you damage the cells. Here's what happens: inside the onion cells, there's an enzyme (a protein that catalyzes reactions) sitting near some sulfur-containing compounds. As long as the cell is intact, they never meet. But the moment you cut or crush the cells, they mix together, and the enzyme converts those compounds into something called propanethial S-oxide — which is a volatile gas that stings your eyes.

So the onion's basically weaponizing itself in the moment you attack it. The theory is it evolved this way to discourage insects and other creatures from eating it. Your tears are the same chemical irritant that would bug a bug.

The strange part — and this gets back to your observation — is that it's a waste if the onion creates this and then just sits there. The gas needs to hit something quickly to be worth making. Which suggests the whole system is optimized for an actual threat (getting eaten), not for the weird situation of being chopped on a kitchen counter.

Does that match your intuition about it being a live reaction rather than something pre-loaded?

That search didn't give me the mechanism details I was hoping for. Let me work through your two questions from what I understand, and I'll flag where I'm less confident.

On eyes specifically: The gas is volatile — it's airborne and drifts upward. Your hands are in contact with it, sure, but eyes are exquisitely sensitive to chemical irritants because they're mucous membranes. Your skin on your hands can tolerate things your eyes can't, even at the same concentration. So it's not that the gas avoids your hands; it's that your hands just don't react to it the way your eyes do. That part I'm fairly confident about.

On the remedies: Here's where I want to be honest — I'm working from memory on the mechanism, so take this with some caution. Chilling the onion should slow down the enzyme reaction (cold slows chemistry), so less gas gets produced. That follows from the mechanism. Running water might dilute or wash away the compounds before the enzyme can act on them, or it might just help disperse the gas as it forms. Breathing through your mouth instead of your nose makes less intuitive sense to me — the gas still reaches your eyes either way. That one feels like it might be cargo-cult advice.

Does the temperature part ring true to you from your experience — have you noticed chilled onions actually being gentler?

You've got it exactly right. I'd say it the same way you just did.

The one thing I might add is just to make the causation explicit: the enzyme converts those sulfur compounds into the propanethial S-oxide. So it's not like two separate things that both happen to be volatile — the enzyme is doing the work to create the irritant specifically. But that's already in what you said, so I'm really just making the "what the enzyme does" part more visible.

You nailed the core insight, though: intact cell keeps them apart, damage allows the reaction, volatile product reaches your eyes because it's airborne and your eyes are sensitive. That's the mechanism. The rest — which remedies actually work, how much each one matters — we can come back to if you hit onions again and want to test something.