Why Do Onions Make You Cry? Why Your Knife is Making It Worse

Onions make you cry because cutting them sets off a chemical reaction that produces an airborne eye irritant. When it reaches your eyes, sensory nerves trigger tears to help wash it away. ^5,7

Your knife is also making a big difference. A 2025 study found that blade sharpness and cutting speed affect the spray released during slicing. ³

What happens when you cut an onion?

An intact onion keeps an enzyme called alliinase separate from certain sulfur-containing compounds inside its cells. Slicing, dicing, or crushing breaks those compartments open and lets the ingredients mix. Alliinase helps produce an unstable intermediate, and a second enzyme, lachrymatory factor synthase, turns it into the tear-inducing compound. ^1,2

Its name is syn-propanethial-S-oxide. You may also see it called the onion’s lachrymatory factor, which simply means its tear-producing substance. It is volatile, so it can evaporate and travel through the air to your eyes. ¹

This chemistry is part of the onion’s defense against organisms that damage it. Your knife starts the same reaction that damage from a feeding animal can set in motion. Handling an undamaged onion does not mix those ingredients in the same way. ¹

Why do your eyes respond with tears?

The irritant activates sensory nerves at the eye’s surface. Those signals set off a protective response, prompting the lacrimal glands to release tears that dilute and wash away the substance. ^5,7

These are reflex tears. They are different from the basal tears that continuously help lubricate and protect your eyes, and from tears associated with emotions. Smoke and other airborne irritants can produce a similar reflex. ⁵

The watering serves a purpose, but it can also blur your vision. If that happens while you are chopping, put the knife down and step away until you can see clearly. ⁶

What did scientists discover about knives and onion spray?

Scientists have been studying onion tears for decades. They identified lachrymatory factor synthase in 2002, and a 2017 study examined its structure to explain how it helps create the irritant. ^1,2

The newer work looked at the physics of cutting. In a study published in PNAS in October 2025, researchers used high-speed imaging to observe droplets leaving sliced onions. Blunter blades compressed the tissue before it broke, allowing pressure to build. Faster cuts and blunter blades produced more energetic droplet spray. ³

Cornell’s report describes an initial mist followed by further liquid breakup. The practical takeaway is to use a sharpened blade and gentle, controlled cuts. ⁴

The study measured droplet mechanics. Its findings support reducing spray, but do not establish how many fewer tears a person will shed. ³

How can you cut onions with fewer tears?

A useful method is to limit the spray and reduce the amount of irritant that reaches your eyes. Start with a sharp knife, improve airflow, and use well-fitting goggles if onions still make chopping uncomfortable. ^4,6,8

The airflow and onion-variety suggestions come from the American Academy of Ophthalmology. An ophthalmologist at the Barraquer Ophthalmology Centre also recommends sealed protective eyewear and extraction. ^6,8

Does refrigerating an onion really help?

Chilling has a reasonable chemical basis: low temperatures slow enzyme activity. Onion researcher Irwin Goldman explains this in guidance from the University of Wisconsin-Madison. ¹³

However, the 2025 cutting study observed more droplet volume from onions refrigerated at 1°C for 12 hours, with no significant difference in droplet velocity. The authors called for further controlled experiments. ³

This doesn’t establish whether chilling causes more or fewer tears. Treat cooling as an option alongside airflow and goggles. ^3,6

Why do onions make some people cry more than others?

There is variation in both the onions and the people cutting them. The Academy suggests trying fresh or sweet onions if other varieties cause too much tearing. Your distance from the board and the kitchen’s airflow also affect exposure. ⁶

A preliminary study published in 2026 examined onion-tearing reports from 1,001 participants. People who reported more tearing also tended to report greater sensitivity to nasal stinging and burning. However, onion tearing did not have a statistically significant relationship with performance on objective smell tests. ⁷

The study relied on people describing their onion-tearing experience. It does not prove why a particular person cries more, and it does not make chopping onions a test of your sense of smell. ⁷

Do contact lenses stop onion tears?

Some contact lens wearers say onions bother them less when their lenses are in. A lens covers the cornea, which could partly explain the experience. But an optometrist interviewed by All About Vision cautioned that contacts can also affect the tear film and contribute to dryness, which may make irritation worse. ¹²

Reports of fewer tears do not establish reliable protection. Contacts do not seal off the eye. There is no reason to switch lens types or extend your wearing time for onion prep; close-fitting goggles are the more direct way to create a barrier. ^8,9,12

If your lenses become uncomfortable, remove them with clean, dry hands. Follow your eye care professional’s lens-care instructions and use your backup glasses. The CDC advises removing lenses and contacting your eye care provider if you experience discomfort. ⁹

What should you do if your eyes start burning?

Pause the chopping and move away from the onion. Avoid rubbing your eyes, particularly before washing your hands. Rubbing can add irritation, and touching the eyes can transfer substances from your fingers. ¹¹

For mild irritation, lubricating artificial tears or a cool compress may provide relief. If you wear contacts, remove them first and follow the directions on the eye-drop label. Never rinse or store your contact lenses in tap water. ^9,11

Get prompt eye care for significant pain, sensitivity to light, or vision changes. If you wear contacts and develop unusual redness or irritation that continues after removing them, contact your eye doctor instead of assuming the onion explains it. ^10,11

Are onion tears good for your eyes?

The tears help clear an irritant. That protective response does not establish that exposing your eyes to onion fumes improves their health or treats dryness. You do not need to make your eyes sting to help them produce useful tears. ⁵

For the next meal, give your eyes less to react to: use a sharp knife, cut gently, and keep the air moving away from your face. If that is not enough, put on goggles before making the first cut. ^4,6,8

Sources

1. Silvaroli JA et al. (2017). Enzyme That Makes You Cry: Crystal Structure of Lachrymatory Factor Synthase from Allium cepa. ACS Chemical Biology, 12(9), 2296-2304.
2. Imai S et al. (2002). An onion enzyme that makes the eyes water. Nature, 419, 685.
3. Wu Z et al. (2025). Droplet outbursts from onion cutting. PNAS, 122(42), e2512779122. Published online October 7, 2025.
4. Cornell Chronicle (October 7, 2025). Mist opportunity reveals how onions make cooks cry.
5. American Academy of Ophthalmology (April 18, 2023). Facts About Tears.
6. American Academy of Ophthalmology (November 6, 2023). No More Tears: Here’s How to Chop Onions Without Crying.
7. Mahmut MK et al. (2026). Severity of Onion Tearing and Chemosensory Sensitivity: A Preliminary Investigation. Laryngoscope Investigative Otolaryngology, 11(3), e70452. First published May 25, 2026.
8. Barraquer Ophthalmology Centre (March 23, 2022). Why does chopping onions make our eyes tear?
9. CDC (May 27, 2025). Preventing Eye Infections When Wearing Contacts.
10. CDC. What Causes Contact Lens-related Eye Infections.
11. Cleveland Clinic (January 13, 2023). Burning Eyes.
12. All About Vision (updated November 6, 2025). All about onions, your eyes and how to turn off the tears.
13. University of Wisconsin-Madison (2009). Curiosities: Why do onions make us cry when we cut them?