Why Onions Make You Cry, and How to Stop It

Onions make you cry because cutting them releases a gas that irritates your eyes, and tears are your eyes’ way of washing it away.

An onion’s cells contain sulphur-based compounds, stored separately from an enzyme called alliinase. As long as the onion is whole, the two never meet. When you slice it, you break open thousands of cells, and they mix.

Alliinase turns the sulphur compounds into unstable acids. A second enzyme, identified by Japanese scientists in 2002 and called lachrymatory-factor synthase, then quickly converts one of them into syn-propanethial-S-oxide. This is the “crying factor”. It’s a volatile gas, so it drifts up from the chopping board into your eyes.

There, it stimulates the sensory nerves in the cornea, the clear front surface of the eye. Your brain reads this as irritation and tells your tear glands to flush it out. That’s why the tears come fast, and why they stop soon after you finish cutting.

Why some onions are worse than others

Onions vary in how much of these sulphur compounds they contain, depending on the variety and how they were grown. Strong, pungent onions tend to cause more tears than milder, sweeter ones. Cooking breaks down the enzymes, which is why cooked onions don’t sting.

Tears are your eyes’ way of washing the onion gas away.

Tricks that actually help

• Chill the onion first. Fifteen to thirty minutes in the fridge slows the chemical reaction and makes the gas less volatile.
• Use a sharp knife. A sharp blade cuts cleanly through cells; a blunt one crushes them and releases more of the irritant.
• Cut near moving air. A fan, an open window or a running kitchen chimney carries the gas away from your face.
• Leave the root end until last. The root has a higher concentration of the compounds, so cut it at the very end.
• Wear glasses or goggles. Swimming goggles look silly but work, because they physically keep the gas away from your eyes.

Some popular tips, such as holding bread in your mouth or chewing gum, have little evidence behind them. They may help a little by making you breathe through your mouth, but they don’t stop the gas reaching your eyes.