Why Onions Make You Cry: Sulfur Gas and Reflex Tears
Written and checked with AI using the references below. This is not human expert review.
Quick answer
Slicing an onion breaks cells, mixing enzymes that produce an airborne sulfur gas, which irritates eye nerves and triggers protective reflex tears. Your brain senses this sting and signals the lacrimal glands to flood the eye. This rapid surge of tears dilutes the chemical and washes it away.
When you slice into an onion, your eyes quickly burn and water. This reaction is not an allergy. It is a rapid defense system from the plant. An uncut onion will not make you cry because its irritants stay locked inside tiny compartments. Cutting the vegetable breaks those cellular walls. That damage mixes separate chemicals together. The reaction makes a light sulfur gas that drifts straight toward your eyes.
The Chemical Components Inside an Onion
An uncut onion bulb holds all the ingredients needed to make your eyes sting. Even so, these chemicals cannot react while the vegetable is whole. The plant stores its reactive substances in separate parts of each cell. Slicing the onion breaks those cellular walls and brings three main components together.
Isoalliin and Amino Acid Sulfoxides
The reaction begins with sulfur molecules stored in the onion. Scientists call these starting materials amino acid sulfoxides. The most common one in bulb onions is isoalliin. Undamaged onions keep isoalliin dissolved in the liquid part of each cell. The plant keeps it away from enzymes that could break it apart. This stored sulfur acts as a weapon. The plant releases it whenever pests, animals, or kitchen knives damage the bulb.
Alliinase Enzymes
The onion stores helper proteins called alliinases. In an uncut onion, alliinase never touches the stored isoalliin. Slicing the bulb breaks open the cells. The enzyme spills out into the cell fluid. Alliinase attacks isoalliin immediately. It turns this precursor into a brand-new chemical compound. Scientists call this intermediate product 1-propenesulfenic acid. Its chemical formula is written as (E)-CH3CH=CH-S-OH.
Lachrymatory Factor Synthase
Many plants in the allium family carry alliinase enzymes, but they do not all make you cry. Onions possess a second enzyme called lachrymatory factor synthase. People often shorten this name to LFS. This enzyme acts on the newly formed acid right away. Biochemists classify this helper protein as EC 5.3.99.12. It rearranges the sulfur atoms within seconds. Without this enzyme, the acid changes on its own into pleasant flavor compounds. It forms the sweet and savory smells you expect from cooked dishes. With LFS present, the reaction takes a different path. It creates an irritating chemical that causes tears.
How Component Interaction Creates Airborne Gas
The transformation from an intact vegetable to an eye-watering vapor happens through a sequence of steps. Each step takes place as soon as your knife crushes the plant tissue.
- Cellular rupture: Slicing or dicing breaks the cell walls. This damage lets the trapped alliinase enzymes touch the isoalliin.
- First enzymatic step: Alliinase splits the isoalliin molecules apart. This step generates 1-propenesulfenic acid.
- Second enzymatic step: Lachrymatory factor synthase grabs the sulfenic acid. It rearranges the molecules into syn-propanethial S-oxide. Its formula is written as (Z)-CH3CH2-CH=S+-O-.
- Gas evaporation: Syn-propanethial S-oxide is a volatile liquid. It has a light molar mass of 90.14 grams per mole. At normal room conditions of 25 degrees Celsius (77 degrees Fahrenheit) and 100 kilopascals, it evaporates rapidly. The gas floats up from the cutting board into the air.
Chemists call this volatile chemical syn-propanethial S-oxide. It turns into a vapor right away at room temperature. Over time, these molecules can join together to form a stable pair. Scientists call that pair trans-3,4-diethyl-1,2-dithietane 1,1-dioxide.
Related plants make similar compounds through matching steps. For example, the wild relative Allium siculum makes syn-butanethial S-oxide. A plant named Petiveria alliacea uses its own synthase enzyme to make syn-phenylmethanethial S-oxide. Cooking often links simple kitchen tasks with plant chemistry. You can explore similar topics in our collection of everyday science questions.
How the Human Eye Detects and Flushes the Chemical
Once the onion gas floats into the air, it reaches your face. Human eyes have sensitive protective nerves. These nerves detect irritating chemicals and work quickly to rinse them away.
Nerve Detection Along the Trigeminal Pathway
The rising gas dissolves in the thin layer of moisture covering your eye. This fluid coats both the cornea and the conjunctiva. The sulfur gas stimulates tiny nerve endings in the ophthalmic nerve. This nerve is the V1 branch of your fifth cranial nerve, known as the trigeminal nerve. The nerve endings contain microscopic sensors called transient receptor potential channels. TRPA channels react strongly to onion vapor. These same channels respond to other threats like extreme heat or sharp acids. You can learn more about acid effects in our guide on how hydrogen ions define water acidity. When onion gas activates these channels, they fire an urgent pain signal straight to your brain.
Lacrimal Gland Activation and Fluid Surge
Your brain reads the nerve signal as an emergency. It orders your tear glands to wash away the stinging chemical. The brain sends instructions through the great petrosal nerve, which branches from cranial nerve VII. The parasympathetic nervous system releases a chemical messenger called acetylcholine. This messenger reaches the lacrimal glands sitting above the outer corner of each eye. It locks onto special receptors called nicotinic and muscarinic receptors.
This signal triggers a sudden flood of tears. Under normal resting conditions, your lacrimal glands make basal tears slowly. They release about 0.5 to 2.2 microliters of fluid each minute. When onion gas irritates the eye, tear production jumps by up to 100-fold. Output can reach 300 microliters per minute. This rush of fluid flows across the eye surface. It dilutes the sulfur compound and carries it away from sensitive corneal tissue.
Tear Drainage and Nasal Congestion
Normal tears drain through tiny holes in the inner corners of your eyelids called lacrimal puncta. From there, the fluid moves through small canals into the lacrimal sac. Then it flows down the nasolacrimal duct straight into your nasal cavity. A sudden burst of reflex tears quickly overfills this drainage system. Excess fluid spills over your lower eyelids and runs down your cheeks. At the same time, lots of fluid drains down into your nose. That sudden overflow is why chopping onions makes your nose run.
Comparing Basal, Reflex, and Emotional Tears
Human eyes make three distinct types of tears. These are basal tears, reflex tears, and emotional tears. Each type serves a different purpose. Each one uses different nerve paths and contains different chemicals. Looking at these tear types shows how your body reacts to different kinds of stress across broader scientific topics.
Healthy eyes rely on a tear film made of three layers. The outer lipid layer comes from the Meibomian glands. It measures 0.015 to 0.160 micrometers thick and stops tears from drying up too fast. The middle aqueous layer comes from the lacrimal glands. It is about 4 micrometers thick and supplies water, salts, and protective proteins. The inner mucous layer comes from conjunctival goblet cells. It measures 2.5 to 5 micrometers thick and helps tears stick to the cornea.
| Tear Classification | Secretion Rate | Primary Source | Key Chemical Components | Main Biological Role |
|---|---|---|---|---|
| Basal Tears | 0.5 to 2.2 microliters per minute (0.75 to 1.1 grams per day) | Lacrimal and Meibomian glands | Lysozyme, lactoferrin, lipocalin, electrolytes, and nonpolar lipids | Lubricates the cornea, smooths light refraction, and fights bacterial entry |
| Reflex Tears | Up to 300 microliters per minute (up to 100-fold increase) | Lacrimal glands via trigeminal nerve stimulation | High-volume aqueous solution with diluted protein content | Flushes out foreign particles, smoke, and chemical irritants like onion vapor |
| Emotional Tears | Variable, driven by central autonomic signaling | Lacrimal glands via limbic and hypothalamic stimulation | Higher levels of prolactin, adrenocorticotropic hormone, and leucine enkephalin | Releases stress hormones and provides social communication during emotional distress |
Nerve studies show a clear difference between reflex tears and emotional crying. Reflex tears react to physical pain on the surface of the eye. Emotional crying comes from feelings in deeper parts of the brain. When surgeons cut or block the sensory trigeminal V1 nerve, reflex tears from onion fumes stop completely. The pain signal cannot travel to the brain. Yet emotional crying still works normally. Psychic tears do not need eye surface sensors. Instead, they start in brain regions called the limbic system and the hypothalamus.
Practical Ways to Lessen Onion Irritation
You cannot change the biology of an onion, but you can control the air and temperature around it. Simple physical adjustments slow down chemical reactions and stop the gas from spreading.
- Chill the onion before cutting: Chemical reactions happen faster at warm temperatures. At a standard room temperature of 25 degrees Celsius (77 degrees Fahrenheit), the onion compound turns into gas rapidly. Chilling the onion means less gas enters the air.
Frequently asked questions
Do other plants make chemicals similar to the ones in onions?
Yes, related plants make similar compounds through matching steps. For example, the wild plant Allium siculum produces syn-butanethial S-oxide. Another plant named Petiveria alliacea uses its own synthase enzyme to make syn-phenylmethanethial S-oxide.
Why do whole onions not make your eyes tear up?
Whole onions keep their chemical parts apart. The helper protein called alliinase is stored away from another compound called isoalliin. As long as the onion stays uncut, they never touch. When a knife cuts the bulb, cells break. The parts mix together and form the gas that makes you tear up.
Why does your nose run when you cut an onion?
When onion gas triggers reflex tears, excess fluid flows across the eye into the lacrimal puncta. The fluid then drains down through the nasolacrimal duct straight into the nasal cavity. This sudden rush of fluid overwhelms the normal drainage path, causing tears to pool in your nose and making it run.
How do reflex tears differ chemically from emotional tears?
Reflex tears consist mainly of a high-volume watery liquid that dilutes surface irritants. Emotional tears, triggered by brain regions like the limbic system, contain different chemicals. They carry higher levels of protein hormones, including prolactin, adrenocorticotropic hormone, and leucine enkephalin, which acts as a natural painkiller.
Sources
- syn-Propanethial-S-oxide - Wikipedia · en.wikipedia.org
- Tears - Wikipedia · en.wikipedia.org
Written by AI; checked through automated evidence and editorial review. References retrieved October 12, 2026.
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