When you cut into an onion, a familiar stinging sensation often follows, quickly leading to watery eyes. This common kitchen experience is a complex biological and chemical reaction. The tears that flow are your body’s natural defense mechanism, triggered by specific compounds released from the onion’s cells.
The Onion’s Chemical Defense
Onions possess a sophisticated chemical defense system that activates when their cells are damaged. Within an onion’s cells, sulfur-containing compounds called amino acid sulfoxides are stored separately from an enzyme known as lachrymatory-factor synthase. When an onion is cut, the knife blade ruptures these individual cell compartments, allowing the previously separated compounds and enzymes to mix.
This mixing initiates a rapid chemical transformation. The lachrymatory-factor synthase enzyme acts upon the amino acid sulfoxides, converting them into a volatile compound called syn-propanethial S-oxide. This newly formed chemical is a gas that quickly disperses into the air. The release of syn-propanethial S-oxide is the onion’s natural way of deterring pests, but it also affects humans.
How Onions Irritate Your Eyes
The volatile syn-propanethial S-oxide released from the cut onion travels through the air and makes contact with the moist surface of your eyes. When this gas dissolves in the watery layer of tears covering the eye, it undergoes a chemical reaction. This reaction forms a mild solution of sulfuric acid.
This acid irritates the delicate sensory nerves on the eye’s surface. In response to this irritation, the nervous system triggers a protective reflex. This reflex causes the lacrimal glands, which produce tears, to significantly increase their output. The resulting flood of tears serves to dilute and wash away the irritant, protecting the eye from further discomfort.
Reducing Eye Irritation from Onions
Several practical strategies can help minimize the eye irritation experienced when cutting onions. Chilling the onion in the refrigerator before cutting can reduce the activity of the lachrymatory-factor synthase enzyme, leading to less irritant being produced.
Cutting onions under a kitchen vent or near an open window helps to disperse the volatile syn-propanethial S-oxide, preventing it from reaching your eyes in high concentrations. Cutting the onion near running water is also effective, as the irritant compound is attracted to water and will dissolve there instead of traveling towards your eyes.
Using a very sharp knife can also make a difference; a sharp blade creates cleaner cuts, which ruptures fewer cells and releases less of the irritating chemical compared to a dull knife that crushes cell walls. For direct protection, wearing glasses or sealed goggles creates a physical barrier, preventing the volatile compounds from ever reaching the eye’s surface.