When you cut an onion, you damage its cells and release a chain of sulfur‑based chemicals that turn into a tear‑gas–like vapor, which irritates your eyes and makes you cry.

Why Do Onions Make You Cry?

Quick Scoop

Onions aren’t out to get you personally—they’re running a tiny chemical- defense system. When you slice into them, they launch a mild “gas attack” that your eyes desperately try to wash away with tears.

What’s Actually Happening?

1. The Onion’s Built‑In Defense

  • Onions grow underground and absorb lots of sulfur from the soil, which they use to build special sulfur‑rich amino acids.
  • Inside each onion cell, enzymes and these sulfur compounds are stored separately, like ingredients in tiny sealed packets.
  • When you cut, crush, or chop the onion, you break those packets, letting the enzymes and sulfur compounds mix.

Think of it like opening hundreds of tiny “chemical capsules” with every knife slice.

2. The “Tear Gas” Chemical

Once those ingredients meet, a short but dramatic chemical chain kicks off:

  1. An enzyme called lachrymatory-factor synthase is released when you cut the onion.
  1. It converts sulfur‑containing molecules (amino acid sulfoxides) into sulfenic acids.
  1. These unstable sulfenic acids quickly rearrange into a gas called syn‑Propanethial‑S‑oxide (often written syn‑propanethial-S-oxide).
  1. This gas evaporates easily, rises off the cutting board, and heads straight toward your face.

This gas is the main reason behind “why do onions make you cry.”

Why Your Eyes Start Watering

Your eyes are coated with a thin layer of moisture and packed with nerve endings that detect irritants.

  • The onion gas dissolves into the moisture on your eyes and irritates the surface and the nerves there.
  • Your brain interprets this as a potential chemical threat and activates your lachrymal glands (tear glands).
  • Your eyes then flood with tears to dilute and flush out the irritating gas.

So you’re not “sad”—your eyes are running an emergency rinse cycle.

Do All Onions Make You Cry the Same?

Not exactly. Some onions are chemistry bullies, others are gentler.

  • More likely to make you cry:
    • Yellow onions (usually the strongest)
* Red onions

* White onions
  • Less likely:
    • Sweet onions (like Vidalia, Walla Walla)
* Green onions/scallions (usually milder overall)

These differences come from how much sulfur and related compounds each type contains, which affects how much syn‑Propanethial‑S‑oxide gas is produced.

Quick Tips to Cry Less Over Onions

Here are practical tricks people use to dodge onion tears, along with why they help.

  1. Chill the onion first
    • Putting the onion in the fridge (or briefly in the freezer) slows the chemical reactions and reduces how quickly the gas forms and evaporates.
  1. Use a sharp knife
    • A sharp blade crushes fewer cells, so fewer enzymes and sulfur compounds mix and less irritant gas is produced.
  1. Move air away from your face
    • Cutting near a fan, open window, or under a kitchen vent hood helps blow the gas away before it reaches your eyes.
  1. Cut under or near water
    • Some people cut onions near running water or on a wet board so the gas dissolves into water instead of reaching their eyes.
  1. Goggles or glasses
    • Tight‑fitting kitchen or swim goggles can physically block the gas from hitting your eyes.

Different Angles on the Question

From the Onion’s Point of View

Some scientists describe this as a kind of botanical self‑defense : the onion’s chemistry likely evolved to discourage animals and pests from eating it. Irritating eyes and noses is a pretty effective way to convince creatures to back off.

From a Human Perspective

  • Annoying in the moment, yes—but the same sulfur chemistry is also linked to onions’ strong flavor and some potential health benefits.
  • So the thing that makes you cry is also a big part of what makes onions so tasty in soups, curries, and stir‑fries.

Mini FAQ

Q: Are onion tears harmful to my eyes?
For most people, no—the irritation is temporary, and the tears actually help protect your eyes by washing away the irritant.

Q: Why do some people cry more than others?
Everyone’s eye sensitivity is a bit different, and factors like dry eyes, contact lenses, or room ventilation can change how strongly you react.

Q: Are there “no‑cry” onions?
Breeders have developed varieties that produce less of the tear‑inducing chemical, though they may also have a milder flavor.

TL;DR: Onions make you cry because cutting them triggers a chemical chain that creates a volatile gas (syn‑Propanethial‑S‑oxide) that irritates your eyes, and your tear glands respond by flooding your eyes to wash the irritant away.

Information gathered from public forums or data available on the internet and portrayed here.