Why do onions make you cry?

A chemical trap that springs when you cut

Inside an intact onion, the ingredients for tears are stored apart. A sulfur compound sits in the cell's cytoplasm, while the enzyme alliinase is usually described as tucked away in a separate compartment, the vacuole¹⁴. Slicing ruptures the cells, and the two meet^{7, 14}. Alliinase splits the sulfur compound, called isoalliin, into a short-lived, highly reactive molecule: 1-propenylsulfenic acid^{2, 3, 14}.

A second enzyme, lachrymatory factor synthase (LFS), rearranges that acid into syn-propanethial S-oxide, the 'lachrymatory factor' or tear factor^{1, 2}. For a long time, chemists assumed this step happened on its own^{1, 2}. In 2002, Japanese researchers showed that the onion uses a dedicated enzyme for it^{1, 2}. Sulfenic acid that LFS does not convert instead forms thiosulfinates, compounds linked to the sharp smell of freshly cut onion^{3, 6, 7}. In other words, the tear factor is not stored: it is made on demand, the moment the knife breaks the cells^{1, 2, 14}.

From cutting board to eyeball

The tear factor is volatile, so it escapes into the air^{2, 9}. Instruments above freshly cut onions detected it right away, and it faded within roughly 10 to 30 minutes⁶.

High-speed filming suggests the air is not the only route. Cornell engineers found that a knife pressing on an onion builds up pressure in the tissue until the thin skin of a layer bursts, spraying fine droplets at up to about 40 meters per second^{10, 11}. Blunter blades produced up to about 40 times more droplets, and faster cuts about four times more¹⁰. The researchers suggest that these hovering droplets may carry the irritant toward the eyes, an idea first proposed in 1947¹⁰.

Once the irritant reaches the eye, it is thought to stimulate free nerve endings in the cornea that belong to the trigeminal nerve^{7, 8}. The brain reads the signal as burning and reflexively tells the tear glands to flush the eye^{7, 8}.

Why would an onion do this?

Scientists see the whole sulfur system as a defense, part of the plant's chemical warfare against microbes and animals: harmless while stored, irritating the moment something bites in^{2, 14}. Garlic mostly stores a slightly different sulfur compound and lacks LFS, so its sulfenic acids turn into thiosulfinates such as allicin instead of the tear factor^{1, 3, 12}.

In 2008, researchers used genetic engineering to silence the LFS gene in onions. Enzyme activity dropped by up to about 1,500-fold, and the cut onions released far less tear factor and more thiosulfinate-derived sulfur compounds⁴. A Japanese team later produced a tearless onion without genetic engineering by inducing random mutations with an ion beam. In those plants, the first enzyme, alliinase, was barely made at all, while LFS worked normally⁵.