Onions make you cry because cutting them releases an airborne chemical that irritates your eyes. Cutting onions can make your eyes water as they produce protective tears to wash the irritant away.

That explains the watering, but there’s an interesting complication: the tear-triggering chemical forms only after you damage the onion. Understanding that small chemical chain reaction also helps explain which kitchen tricks deserve your attention.

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Why Onions Make You Cry

Cutting brings separate ingredients together

Inside an intact onion, sulfur-containing compounds, including ACSOs, are kept separate from enzymes. Think of each sulfoxide precursor as an ingredient stored in a different kitchen cupboard.

Your knife breaks cell walls and mixes those ingredients. An enzyme called alliinase starts a reaction that produces unstable sulfur compounds. Another enzyme, lachrymatory-factor synthase, helps turn some of them into syn-propanethial-S-oxide, the tear-triggering irritant. This compound is distinct from sulfur compounds associated with onion flavor.

This volatile gas readily enters the air around your chopping board. Its everyday scientific name is lachrymatory factor. The name shares its Latin roots with lacrimare, meaning “to weep.”

Research on the onion’s tear-producing enzyme describes this chemistry as part of the plant’s defense mechanism. Damaged tissue releases irritating compounds that discourage pests from eating it. Unfortunately, preparing soup also counts as damaging tissue.

Your tears are a protective response

When the gas reaches your eyes, it dissolves in their moist surface and causes eye irritation. Sensitive nerves detect it, and your tear glands increase fluid production to dilute and remove it. Tear glands produce the fluid, while tear ducts drain it from the eyes.

Only small amounts are needed. That’s why you can start watering before you’ve finished chopping a single onion.

The Royal Society of Chemistry’s molecule explainer connects this familiar response to propanethial-S-oxide. Your eyes are responding to an airborne irritant, even when no onion juice has touched them.

Which Methods Help Most?

Start with goggles or moving air

Close-fitting onion goggles or safety goggles provide the most direct protection, creating a physical barrier between the gas and your eyes. Open-sided glasses leave more room for the irritant to enter.

PureWow found onion goggles effective in its kitchen testing, although the nose could still sting. Protecting your eyes doesn’t remove the chemical from the room.

Ventilation is another useful approach. An externally vented range hood, an open window with airflow, or a carefully positioned fan can carry the gas away. Keep the airflow moving across the preparation area and away from your face.

These methods protect you without washing away the onion’s flavor compounds.

Add a sharp knife and a chilled onion

A sharp knife cuts more cleanly than a dull knife, which crushes more tissue and can damage more cells. Cleaner cuts generally mean less mixing of enzymes and sulfur compounds, though they won’t prevent every tear.

To chill onions before chopping, leave them in the fridge for an estimated 30 to 60 minutes, or in the freezer for around 10 minutes. PureWow suggests about two hours refrigerated or 30 minutes in the freezer.

These are estimates, not universal thresholds. Onion size and appliance temperature affect how quickly the center cools. The practical goal is a cold onion that’s still easy to cut, rather than one frozen solid.

Cut it soon after chilling. As the onion warms, the reactions speed up and gas release increases. Cooling slows the reactions temporarily, but doesn’t permanently switch off the chemistry.

Choose Your Method Around the Meal

Reducing irritation can also change flavor. America’s Test Kitchen notes that methods that reduce lachrymatory-factor production can affect the chemistry responsible for onion flavor.

Rinsing cut surfaces removes some flavor compounds and can soften an onion’s pungent flavor. For a raw salad or sandwich, that milder taste may be welcome. A brief cold-water soak can also soften the pungency of sliced onions.

For a soup, stew, or slowly cooked onion dish, you may prefer to preserve those compounds. Goggles and ventilation can help reduce onion tears without washing flavor away.

Sweet onions are generally milder than pungent yellow or red onions, although color alone doesn’t predict how much you’ll cry. Growing conditions and variety matter too.

Choose firm bulbs without soft spots or mold. Laura Ali notes that older stored onions can have softer cell walls, allowing more irritating compounds to escape during cutting.

Our guide to preparing and enjoying onions offers more ideas for matching raw and cooked onions to your meal.

Which Kitchen Tricks Disappoint?

Some remedies sound appealing because they’re easy: light a candle, place a damp paper towel nearby, or microwave the onion first.

PureWow found that these three approaches didn’t reduce tears in its testing. That doesn’t establish every possible setup, but it gives little reason to choose them over goggles, airflow, and chilling.

A nearby damp towel also doesn’t create a barrier around your eyes. The gas can still travel between the onion and your face.

Common Questions About Onion Tears

Does crying mean the onion is bad?

No. A fresh, sound onion can produce plenty of lachrymatory factor when cut. Tears reflect exposure to the irritant, rather than providing a reliable freshness test.

Judge the bulb by its condition, including firmness, visible mold, and signs of decay. Tearfulness tells you little about those qualities.

Are onion tears good for you?

This response performs a useful protective job, but deliberately irritating your eyes offers no established health benefit.

If your eyes start stinging, pause and move away from the preparation area. Avoid rubbing them with onion-covered fingers. Persistent pain or changes in vision deserve medical attention.

A Small Defense With a Familiar Result

An onion’s tear-triggering response begins with broken cells and ends with your eyes trying to protect themselves. That makes limiting exposure the most useful goal.

Use a sharp knife, cut the onion while cold, and favor airflow or goggles when you want to preserve its flavor. Dinner can keep its onions without requiring quite so many tissues.