Chopping an onion is a familiar kitchen chore often accompanied by stinging eyes and profuse tearing. This involuntary reaction has led to many household remedies, including the belief that wearing contact lenses can prevent the tears. While the idea of a physical barrier protecting the eye is appealing, the underlying science must be examined to determine if this popular notion holds up.
The Chemical Reaction That Causes Tears
The burning sensation experienced when cutting an onion is a defense mechanism evolved by the plant to deter pests. This reaction begins the moment a knife ruptures the onion’s cellular structure. When the cells are damaged, sulfur-containing compounds and enzymes that are normally kept separate are allowed to mix.
The enzyme Lachrymatory Factor Synthase (LFS) is a primary component in this chain reaction. LFS acts on the sulfenic acids formed when the onion’s amino acid sulfoxides are broken down. The enzyme then converts the unstable sulfenic acids into a volatile chemical irritant called syn-Propanethial S-oxide.
The syn-Propanethial S-oxide quickly vaporizes and wafts toward the face. When the gas contacts the moisture on the eye’s surface, it dissolves and forms a dilute sulfuric acid solution. This acidic solution stimulates the nociceptors, or pain receptors, in the cornea, triggering a protective reflex. The brain signals the lacrimal glands to produce tears to dilute and flush out the chemical irritant.
How Contact Lenses Interfere (Or Fail To)
The popular theory suggests contact lenses function as a shield, physically blocking the irritating gas from reaching the sensitive corneal surface. In practice, however, this barrier’s effectiveness is minimal and unreliable. Modern soft contact lenses, the most widely used type, are designed to be highly permeable to oxygen and water vapor.
This permeability means the syn-Propanethial S-oxide gas can still pass through the lens material or readily dissolve into the tear film bathing the lens surface. While the lens covers the central cornea, the gas can still reach the unprotected conjunctiva and the limbus, the surrounding tissues of the eye. The irritant dissolves in the moisture around the edges of the lens, still triggering the tear reflex.
Rigid Gas Permeable (RGP) lenses are also not a dependable solution, as their name states they allow gases to pass through them. Any minor protection experienced by wearers is due to the lens covering a small portion of the sensitive cornea. The lenses do not prevent the chemical from dissolving into the eye’s moisture layer, which is the source of the irritation.
Science-Backed Methods for Tear Prevention
Instead of relying on contact lenses, several scientifically sound methods can minimize or eliminate the chemical reaction and gas exposure. One approach is to reduce the activity of the enzyme responsible for irritant gas production. Chilling the onion in the refrigerator for about 30 minutes before cutting slows the enzymatic reaction rate, allowing for a longer, tear-free chopping time.
A primary method focuses on the physics of the cut itself: using a very sharp knife with a slow, steady cutting motion. Researchers found that dull blades crush more cells and create a higher-pressure release, propelling irritant-laden droplets into the air. A sharp knife minimizes cell damage and prevents the explosive ejection of the tear-inducing mist.
Another effective strategy involves diverting the volatile gas away from the eyes entirely. Cutting the onion directly under a powerful kitchen range hood or next to a small fan pulls the fumes away from your face. The most reliable physical barrier is a sealed pair of protective goggles, which prevents the irritant gas from making contact with the eye’s surface.