Why do onions make you cry when you cut them?

An intact onion does not contain the chemical that makes you cry. Damaging its cells mixes enzymes with sulfur compounds, and the reaction produces a volatile irritant called propanethial-S-oxide, also known as lachrymatory factor. It drifts into the air and reaches your eyes, which produce tears to wash it away. The same reaction is part of the onion's chemical defense against pests. [1] [2] [3]

The Royal Society of Chemistry compares it to a glow stick, which only lights up when two chemicals mix. In an onion, one compartment of the cell holds an enzyme and another holds sulfur compounds that the plant stores from the soil. A cut or an insect bite breaks the compartments open. The enzyme alliinase then converts the sulfur compounds into sulfenic acids, including 1-propenyl sulfenic acid. [1] [3]

Scientists once thought alliinase alone produced the irritant in a single step. In 2002 Japanese researchers showed that a second enzyme, lachrymatory-factor synthase, converts the sulfenic acid into propanethial-S-oxide. The Library of Congress and the Royal Society of Chemistry both describe this two-step process. A 1999 Scientific American answer, written before that finding, describes the sulfenic acids as rearranging on their own into the irritant. The same acids also condense into thiosulfinates, which give onions their smell and taste. Scientific American notes that the odor is not what makes the eyes water. According to that source, the irritant peaks about 30 seconds after the damage and finishes forming over about five minutes. [4] [1] [5]

The cornea, the front surface of the eye, is densely supplied with sensory nerve fibers. Free nerve endings there detect the irritant, and the nervous system reads their activity as burning. That activity reflexively signals the tear glands to flush the irritant out. The Royal Society of Chemistry and the University of Wisconsin–Madison both say the compound reacts with water on the eye to form an acid that irritates the nerves. The University of Wisconsin–Madison compares this to a sulfuric-acid-like effect, and the RSC says only that an acid is formed. [5] [1] [3]

Garlic does not make the same irritant. Only three lachrymatory factors are known in nature, and Poison Control lists onions, the shrub Petiveria alliacea and Allium siculum as their sources. Red and white onions cause the most tears, while spring onions have much lower levels of the enzymes involved. [2] [1]

Chilling onions for about 30 minutes before cutting is widely suggested. The University of Wisconsin–Madison's Irwin Goldman explains that cold slows enzymes down. Leaving the root end intact is also recommended, because it holds the most sulfur compounds. Sweet varieties grown in low-sulfur soil are less pungent, though Poison Control notes they spoil faster. Other tips include sealed goggles and cutting under water. Poison Control says no method is foolproof. One source, The Conversation, instead advises trimming off the root end to let the sulfur compounds escape, which conflicts with the advice to leave it intact. [3] [2] [6]