Why do onions make you cry? Onions make you cry because cutting them triggers a chemical reaction that releases an irritating vapor into the air. When that vapor reaches your eyes, it reacts with the moisture on their surface and stimulates nerves that tell your tear glands to produce tears.
When asking why do onions make you cry, it helps to know the onion is not storing “tear gas” as a finished chemical. The irritating compound forms only after cutting, crushing or otherwise damaging the onion’s cells lets substances that were kept apart mix together.
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Why Do Onions Make You Cry?
The short explanation for why onions make you cry starts with sulfur-containing compounds inside the onion. Cutting breaks open cells and allows an enzyme called alliinase to interact with compounds that had been separated inside the intact plant.
That reaction ultimately produces a volatile chemical called syn-propanethial-S-oxide, often called the onion’s lachrymatory factor. “Lachrymatory” simply means tear-producing.
What Happens When You Cut an Onion?
An intact onion keeps key ingredients of this reaction in different parts of its cells. A knife damages those cells. The contents mix, enzymes begin working, and a series of reactions produces the volatile tear-inducing compound.
Because the compound is volatile, some of it becomes airborne. It can then travel from the cutting board to your eyes. This is why chopping, slicing or crushing an onion can cause more irritation than simply handling an uncut onion.
Why Do Your Eyes Produce Tears?
Your eyes are sensitive to irritating chemicals. When onion vapor reaches the eye’s surface, sensory nerves detect the irritation. The lacrimal glands respond by producing extra tears, which help dilute and wash away the irritant.
So the tears are a protective response. The onion is not directly “making” tears; your own tear system is responding to an airborne irritant.
How Can You Cut Onions With Fewer Tears?
Methods that reduce the amount of irritating vapor reaching your eyes are the most sensible approaches.
- Use a sharp knife. A clean cut crushes fewer cells than a dull blade.
- Improve ventilation. A kitchen exhaust fan or nearby airflow can carry some vapor away from your face.
- Chill the onion first. Cooling can slow chemical reactions and reduce how quickly volatile compounds reach the air.
- Keep your face back from the cutting board. More distance can reduce direct exposure.
Internet tricks range from chewing gum to holding bread in your mouth, but the chemistry suggests that limiting cell damage, slowing the reaction or moving the vapor away from your eyes addresses the actual cause.
Do All Onions Make You Cry?
Not necessarily to the same degree. Onion varieties differ in their chemistry, and growing and storage conditions can also affect their composition. Your cutting method, ventilation and individual sensitivity matter too.
That is why one onion may seem much harsher than another even when you prepare them in the same kitchen.
Why Does Garlic Usually Not Make You Cry the Same Way?
Garlic and onions are related plants, and both contain sulfur chemistry, but they do not produce the same mix of volatile compounds when their cells are damaged. The onion’s specialized lachrymatory-factor pathway is a major reason cutting onions is particularly associated with tearing.
Frequently Asked Questions
What chemical in onions makes you cry?
The main tear-inducing compound is syn-propanethial-S-oxide, a volatile sulfur-containing chemical produced after onion cells are damaged.
Why do onions burn your eyes?
The airborne lachrymatory factor irritates sensory nerves at the surface of your eyes. Your tear glands respond by producing extra tears.
Does refrigerating an onion help?
Cooling an onion before cutting can help reduce the release of volatile irritants, although it may not eliminate tearing completely.
The Short Answer
Why do onions make you cry? Cutting an onion breaks its cells and starts chemical reactions that create a volatile tear-producing compound. When the vapor reaches your eyes, irritation triggers your lacrimal glands to make tears that help wash it away.
For another everyday science question, see how many pints of blood are in the human body, or browse more answers in Food & Cooking.