When you cut an onion, enzymes and sulfur compounds react to produce a volatile irritant gas named syn‑propanethial S‑oxide that reaches your eyes and triggers tears.
How it happens
Cutting an onion ruptures cells and allows enzymes to convert amino acid sulfoxides into sulfenic acids, which are then rearranged by a lachrymatory‑factor synthase enzyme into syn‑propanethial S‑oxide; this volatile molecule diffuses through the air toward the eyes and causes irritation.
What it does to your eyes
When the gas contacts the moisture on the eye’s surface it forms low concentrations of sulfuric/related acidic species that sting the cornea, prompting the lacrimal glands to produce tears to dilute and wash the irritant away.
Ways to reduce tears
Common practical tips that can help include chilling the onion before cutting to slow the chemical reactions, using a very sharp knife to minimise cell damage, cutting under running water or near ventilation to carry the gas away, and peeling or trimming the root end last where concentrations of precursors tend to be higher.
Varieties and pungency
Sweet or milder onion varieties produce lower levels of the sulfur precursors that lead to the lachrymatory gas, so they tend to cause less tearing than pungent, storage or cooking onions, which contain higher concentrations of the reactive compounds.
Takeaway
Onion‑induced tears are a chemical defence: a specific volatile, syn‑propanethial S‑oxide produced when the vegetable’s tissues are damaged irritates the eyes, and simple measures such as chilling, sharp knives and good ventilation can substantially reduce the effect.