Sulfur compounds are at the heart of why onions make people cry. These compounds are produced as part of the onion’s defense mechanism. The onion contains a variety of sulfur-containing compounds, such as thiopropanolsulfonic acids, which are stored in separate cell compartments. When the onion is cut, its cells are damaged, and the compounds are released into the air through a chemical reaction.

The enzyme alliinase is activated upon cutting and begins breaking down these sulfur compounds into sulfenic acids. These acids are unstable and quickly decompose into syn-Propanethial-S-oxide, a volatile sulfur compound that easily evaporates and becomes airborne. This compound is what causes the tearing effect.

When syn-Propanethial-S-oxide reaches the eyes, it reacts with water in the eyes, producing a mild sulfuric acid solution. This irritates the sensitive nerves in the eyes, prompting the brain to respond by releasing tears. The tears help dilute and flush out the irritant, thus protecting the eyes from further damage.

Interestingly, sulfur compounds in onions serve a defensive purpose for the plant. The sulfur compounds act as natural pesticides and help protect the onion from pests. When the onion is cut, this release of irritants may also serve as a warning signal to other potential threats, such as insects, that the onion is under attack.

Sulfur compounds, particularly syn-Propanethial-S-oxide, are responsible for the formation of onion tears. These compounds irritate the eyes, causing the body to release tears in response.