The familiar sensation of burning, stinging eyes while chopping onions is a common kitchen irritation. This discomfort is a natural physiological reaction to a volatile chemical compound released from the vegetable’s cells. Knowing how to quickly relieve the irritation and prevent it can make preparing meals much more pleasant.
The Science Behind the Sting
The burning sensation begins when an onion’s cellular structure is damaged by a knife. Inside the onion cells, two components—a sulfur-based amino acid derivative and the enzyme alliinase—are kept separate until cutting starts. Once the two mix, a quick chemical reaction occurs, generating unstable sulfenic acids.
Another enzyme, lachrymatory-factor synthase (LFS), immediately converts these sulfenic acids into a gas called syn-propanethial S-oxide. This airborne compound is volatile and quickly wafts up toward the face. When syn-propanethial S-oxide gas contacts the moisture on the surface of the eye, it reacts to form a dilute acid, which triggers the nerve endings, causing burning and tearing.
The eyes respond to this acidic irritant by activating the lachrymal glands to produce tears, a natural reflex designed to flush the substance away. This process is similar to the effect of tear gas, though the onion’s chemical is milder. The irritation is a protective measure, and the tears are the body’s way of attempting to neutralize and wash out the offending compound.
Immediate Steps for Eye Relief
The first action when your eyes begin to burn is to stop cutting and move away from the immediate area to escape the lingering gas. Since the irritation is caused by an airborne chemical, stepping back from the cutting board allows the fumes to dissipate quickly enough to halt the reaction.
If the burning sensation persists, flushing the eyes with cool water or a sterile saline solution is the most effective step. Gently rinse the eyes for a few minutes, allowing the water to wash over the irritated surface and dilute the remaining irritant. If you have them, artificial tears can also help by lubricating the eye and washing away the residual chemical compound.
It is important to resist the impulse to rub your eyes, as this can worsen the irritation and potentially scratch the sensitive surface with residual chemical or debris. Before touching your eyes, thoroughly wash your hands with soap and water to remove any trace of the volatile compound.
Applying a cool compress to the closed eyelids offers immediate comfort by cooling the inflamed tissues. A clean cloth soaked in cold water works well, as the cooling effect helps constrict blood vessels and reduce the burning sensation. The discomfort is generally temporary and should subside once the irritant is cleared from the eye’s surface.
Effective Prevention Strategies
To avoid the burning, focus on minimizing the release of the gas or preventing it from reaching your eyes. Placing the onion in the refrigerator or freezer for about 15 to 30 minutes before cutting can significantly slow the enzyme activity that produces the gas. The colder temperature reduces the volatility of the compounds, keeping them contained within the onion.
Using a sharp knife is helpful, as it cleanly slices through the cell walls rather than crushing them, minimizing enzyme release. Cutting the onion near strong ventilation, such as an exhaust fan or an open window, helps pull the gas away from your face.
Cutting the onion under running cold water can absorb the released chemical compounds before they become airborne. If these methods are not practical, wearing close-fitting protective eyewear, like swimming or specialized onion goggles, creates a physical barrier that prevents the gas from contacting the eye surface. Finally, try to keep the root end intact for as long as possible while cutting, as it contains a higher concentration of the enzymes.