# Why Do Onions Make You Cry? The Tricks That Actually Work

> Cutting an onion breaks its cells, releasing an enzyme that reacts with sulphur compounds to produce **propanethial S-oxide gas**. The gas reaches the eye and turns into a weak sulphuric acid there, and the tears are simply a defensive reflex to wash that acid off the surface of the eye.

**4 min read**

Everyone has a trick they swear by: bread in the mouth, a candle on the counter, the onion in the fridge, a wet knife. Most are built on a partial understanding of what actually happens — so some work and some are just habit.

## What actually happens

An intact onion has almost no smell. The compounds that make you cry are not sitting there ready — they form **at the moment of cutting**.

Inside the onion's cells are sulphur compounds, and in separate compartments sits an enzyme called **alliinase**. When the knife breaks the walls, the two meet for the first time.

The reaction produces a volatile gas called **propanethial S-oxide**. The gas rises, reaches the moist surface of the eye, and converts there into a small amount of **sulphuric acid**.

The eye reads that burning as a threat and produces tears to wash it away. The tears are not emotion — they are rinsing.

## Why some onions make you cry more

The amount varies widely, and not by chance:

**Yellow and white onions** are higher in sulphur compounds than **red and sweet** ones. That is why red onion is eaten raw in salads while yellow is not.

And **long-stored onion** makes you cry more than fresh, because the compounds concentrate as water is lost.

And **the soil** matters: onions grown in sulphur-rich soil are stronger.

## The tricks that genuinely work

**1. Chilling.** This is the strongest proven fix. Put the onion in the fridge for **30 minutes** or the freezer for **15 minutes** before cutting.

The reason is that the enzymatic reaction slows dramatically in the cold, and the gas produced evaporates more slowly. It does not stop the tears entirely but it reduces them noticeably.

**2. A very sharp knife.** A blunt knife crushes the cells and breaks a greater number of them, so more gas is released. A sharp one cuts cleanly with the least tearing.

**3. Ventilation.** An extractor fan, an open window, or a small fan moves the gas away from your face. Simple, and it makes a real difference.

**4. Cutting under running water** or submerging the onion. The gas dissolves in the water instead of rising. A little awkward, but it works.

**5. Glasses.** Any glasses that cover the sides form a physical barrier. Not stylish, but the most reliable method there is.

## The tricks that do not work

**Bread in the mouth:** no scientific basis. The gas reaches the eye, not the mouth.

**A lit candle:** the idea is that the flame burns the gas. The heat is insufficient and the distance too great.

**Holding your breath:** the same problem — you breathe through your nose, not your eyes.

**A spoon in the mouth:** a folk habit with no understood mechanism.

**Onion in the sun:** makes it worse, because heat speeds the reaction.

## The cutting order matters

One part of the onion makes you cry more: **the root end** (the end with the hairs). The sulphur compounds concentrate there.

**So:** cut the top (the pointed end) first, peel, and leave the root **until last** — it also holds the layers together, which makes cutting easier.

## Why onion turns bitter when it burns

Onions contain sugars, and those sugars caramelise with heat to give the sweetness most cooking is built on.

But caramelisation has a limit: a little past deep brown, the sugar carbonises and turns into bitter compounds. That bitterness spreads through the whole dish and there is no way to remove it.

The only remedy is to throw the onion away and start again. Which is why proper caramelisation happens over **medium, not high heat** — high heat burns the edges while the middle is still raw.

## Why the smell stays on your hands

The sulphur compounds react with the proteins in your skin and bind to them, and ordinary soap does not break that bond.

**What works:** rub your hands on **something stainless steel** under running water — the kitchen sink or a spoon. The metal reacts with the sulphur and converts it into a water-soluble compound.

Lemon or vinegar help too — the acid breaks the compounds down.

## Raw onion in salad

If raw onion is too sharp in a salad, there is a simple fix: **soak it in iced water for 10 minutes** and rinse.

The soak draws out a large part of the harsh sulphur compounds, and keeps the onion crisp at the same time.

You can also soak it in water with vinegar or lemon — that adds a light acidity that balances its sharpness.

## Correct storage

Onions want a **dry, dark, ventilated place** — not the fridge and not a sealed plastic bag.

Moisture is their first enemy: it makes them rot and sprout. The fridge is humid, so onions stored there soften and rot far faster than outside.

And an important point: **do not store onions with potatoes**. Onions release gases and moisture that make potatoes sprout and rot quickly, and together they last weeks instead of months.

## A cut onion

The remaining half keeps **two to three days** in the fridge in an airtight box or well wrapped.

And there is a folk belief that a cut onion absorbs toxins or germs — that is **not true** and there is no evidence for it. The real problem is that it absorbs **smells** from the fridge and transfers them to whatever it goes into.

## FAQ

### ليه البصل بيدمّع أصلاً؟
التقطيع بيكسّر خلايا البصل فبيتقابل إنزيم مع مركّبات كبريتية وينتج غاز طيّار. الغاز بيوصل لسطح العين الرطب وبيتحوّل لحمض كبريتيك خفيف، والدموع مجرد رد فعل دفاعي عشان تغسل الحمض ده من على سطح العين.

### إيه أقوى حيلة بتشتغل؟
التبريد — نص ساعة في التلاجة أو ربع ساعة في الفريزر قبل التقطيع. البرودة بتبطّئ التفاعل الإنزيمي وبتقلّل تطاير الغاز. والنظارة أضمن حل على الإطلاق لو مش مهتمّة بالشكل.

### الخبز في البق بيشتغل؟
لأ، مفيش أساس علمي. الغاز بيوصل للعين مش للفم، وكذلك الشمعة المولّعة وحبس النفس والملعقة في البق. اللي بيشتغل: التبريد والسكينة الحادّة والتهوية والمية.

### إزاي أشيل ريحة البصل من إيدي؟
افركي إيدك في حاجة من الاستانلس تحت مية جارية — حوض المطبخ أو ملعقة. الحديد بيتفاعل مع الكبريت وبيحوّله لمركّب بيدوب في المية. والليمون أو الخل بيساعدوا كمان.

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Source: https://hanakoleah.com/en/article/why-do-onions-make-you-cry
Edited by: Marwa Mohamed
Last updated: 2026-09-12