Why do onions make you cry

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Last updated: April 8, 2026

Quick Answer: Onions make you cry because when you cut them, they release a volatile sulfur compound called syn-propanethial-S-oxide, which reacts with the water in your eyes to form sulfuric acid. This compound is produced when the enzyme alliinase breaks down amino acid sulfoxides in the onion's cells, a process that occurs within seconds of cutting. The sulfuric acid irritates the nerve endings in your eyes, causing them to produce tears as a protective response. This reaction is estimated to affect over 90% of people who cut onions, with tear production typically starting within 30 seconds of exposure.

Key Facts

Onions contain amino acid sulfoxides that convert to syn-propanethial-S-oxide when cut, a process discovered in 2002 by Japanese researchers
The enzyme alliinase is released within 30 seconds of cutting an onion, triggering the chemical reaction
Syn-propanethial-S-oxide reacts with eye moisture to form sulfuric acid at concentrations of 0.0001-0.001%
Tear production increases by approximately 50-100% when cutting onions compared to normal eye moisture levels
Over 90% of people experience tearing when cutting onions, with sensitivity varying by onion variety and individual

Overview

The phenomenon of onions causing tears has been observed for centuries, with historical records dating back to ancient Egyptian tomb paintings from 2500 BCE showing onion cultivation. Onions (Allium cepa) belong to the Allium family along with garlic, leeks, and shallots, and have been cultivated for over 5,000 years. The specific chemical responsible for eye irritation was identified in 2002 by Japanese researchers at the House Foods Corporation, who isolated syn-propanethial-S-oxide as the primary tear-inducing compound. This discovery built upon earlier work in the 1940s when scientists first identified that cutting onions released volatile sulfur compounds. Today, approximately 93 million metric tons of onions are produced globally each year, making them one of the world's most widely cultivated vegetables. The tear-inducing property varies among onion varieties, with sweet onions like Vidalia containing about 50% less of the precursor compounds than standard yellow onions.

How It Works

When you cut into an onion, you damage its cells and release two previously separated components: the enzyme alliinase and amino acid sulfoxides. Within approximately 30 seconds of cutting, alliinase begins converting these sulfoxides into sulfenic acids. These unstable compounds then rearrange to form syn-propanethial-S-oxide, a volatile gas that quickly evaporates and reaches your eyes. This gas dissolves in the water layer covering your eyes (the tear film) and converts to sulfuric acid through hydrolysis. The sulfuric acid, though extremely dilute at concentrations around 0.0001-0.001%, stimulates the cornea's nerve endings, specifically the ciliary nerves. Your autonomic nervous system responds by signaling the lacrimal glands to produce tears, which can increase tear production by 50-100% above normal levels. This reflex attempts to dilute and wash away the irritant, with the entire process typically causing tearing within 60-90 seconds of initial cutting.

Why It Matters

Understanding why onions make you cry has practical applications in both culinary science and agriculture. Chefs and home cooks use various techniques to minimize tearing, such as chilling onions before cutting (which slows enzyme activity by approximately 30%), cutting under running water, or using sharp knives that cause less cell damage. Food scientists have developed tearless onion varieties through selective breeding and genetic modification, with the first commercially available low-tear onion introduced in New Zealand in 2008. From a biological perspective, the tear-inducing compounds serve as a defense mechanism for the onion plant against pests and herbivores. The sulfuric acid produced in this process is chemically similar to compounds used in some industrial applications, demonstrating how natural biochemical pathways can mirror synthetic processes. This knowledge also helps people with sensitive eyes manage food preparation and has led to innovations in kitchen ventilation and cutting board designs.