Why Do Onions Make Us Cry?

Chopping onions often brings tears to our eyes, but not because we are sad. This common kitchen experience happens due to a fascinating chemical reaction. When you cut an onion, it releases a special gas that turns into a mild acid when it touches your eyes. Your body produces tears to wash away that irritation.

📖 Level 1 - Beginner:

Onions can make you cry. This happens when you cut them. Onions have a special chemical inside. Cutting breaks the onion open. The chemical mixes with air. It becomes a gas. The gas floats to your eyes. Your eyes feel a small burn. Your body makes tears. Tears wash the gas away. This protects your eyes. Some onions are stronger than others. You can chill an onion before cutting. Cold slows the chemical reaction. You can also use a sharp knife. A sharp knife breaks fewer cells. Then less gas comes out.

📖 Level 2 – Intermediate:

Have you ever wondered why slicing an onion brings tears to your eyes? The answer lies in the onion’s natural defense system. Onions grow underground, where animals might try to eat them. To protect itself, the onion stores a mild chemical called syn-propanethial-S-oxide. While the onion is whole, this chemical stays separate inside cells. When you cut into the onion, you break the cell walls. The chemical escapes and mixes with enzymes, creating a gas. This gas rises toward your face and meets the water in your eyes. It turns into a weak form of sulfuric acid, which causes a stinging feeling. Your eyes produce tears to flush out the irritant. To reduce crying, try chilling the onion in the fridge for 30 minutes before cutting. The cold slows down the chemical reaction. Using a very sharp knife also helps because it causes less damage to the onion’s cells. Some onion varieties, like sweet onions, produce fewer tear-causing chemicals.

📖 Level 3 – Advanced:

The humble onion’s ability to induce lacrimation is not a kitchen accident but an evolutionary adaptation. Onions belong to the genus Allium, which includes garlic and leeks. When an onion’s tissue is damaged—by a knife, an animal’s bite, or even soil pressure—it releases an enzyme called alliinase. This enzyme reacts with sulfur-rich compounds naturally present in the onion, converting them into a volatile, sulfur-based gas known as propanethial S-oxide. This gas diffuses upward and dissolves in the aqueous film covering our corneas, forming dilute sulfuric acid. The trigeminal nerve detects this mild chemical burn and signals the lacrimal glands to secrete tears as a reflexive flushing mechanism. Interestingly, humans are among the few species that experience this intensely, as our eyes are particularly sensitive. Fortunately, several strategies can minimize the effect: refrigerating onions for 30–60 minutes reduces enzymatic activity; cutting near a steam source or under running water helps trap the gas; or simply using a sharper knife, which ruptures fewer cells per cut. Plant breeders have also developed “low-pungency” onion varieties, such as Vidalias and Walla Wallas, which contain lower levels of the precursor sulfur compounds.