For some people, the reaction to cutting an onion is surprisingly mild or entirely absent. This lack of lachrymation, or tearing, is not a coincidence but the result of a precise interplay between the onion’s natural chemistry, the way it is handled, and individual biological differences. Understanding why this defensive mechanism fails to trigger tears requires a look into the specific chemical cascade that creates the volatile irritant.
The Chemical Process That Causes Tears
The burning sensation that leads to tears is triggered by syn-Propanethial S-oxide, a potent irritant. This compound is not present in an intact onion but is rapidly manufactured when the onion’s cells are ruptured, such as by a knife. The process begins when the physical damage of cutting releases a specialized enzyme called alliinase, which is normally stored separately within the onion’s cells.
Alliinase immediately interacts with sulfur-containing molecules, known as S-alk(en)yl cysteine sulfoxides, converting them into unstable sulfenic acids. A second enzyme, lachrymatory factor synthase (LFS), then converts the sulfenic acids into the volatile syn-Propanethial S-oxide gas. This gas is highly reactive and diffuses quickly into the surrounding air.
When the volatile compound reaches the eye, it dissolves in the tear film, forming a trace amount of mild sulfuric acid. This acidic solution stimulates pain-sensing nerve endings (polymodal nociceptors) in the cornea. The brain receives this pain signal and reflexively instructs the lacrimal glands to produce tears to dilute and flush away the noxious irritant.
Environmental and Preparation Methods That Stop Tears
The most immediate cause for a lack of tearing can be attributed to simple techniques used during preparation, which directly interfere with the chemical reaction. Chilling an onion in a refrigerator or freezer for about 30 minutes before cutting significantly slows the activity of the LFS enzyme. This reduction in enzyme speed means the irritant gas is produced at a much slower rate.
Using a very sharp knife also reduces the amount of irritant released. A sharper blade causes less crushing damage to the cell walls, minimizing the mixing of enzymes and precursor molecules. Conversely, a dull knife crushes more cells, releasing a greater volume of tear-inducing compounds at once.
Proper ventilation is another external factor that can prevent the gas from reaching the eyes. Cutting the onion directly beneath an extractor fan or positioning a small fan blows the volatile syn-Propanethial S-oxide away. Cutting the onion under running water or submerging it in a bowl of water allows the irritant to dissolve into the liquid, preventing its vaporization and travel to the corneal surface.
Natural Variations in Onion Composition
The intrinsic chemistry of the onion is a major determinant of its tear-inducing potential. Different cultivars contain naturally varying amounts of the sulfur-containing precursor compounds necessary to form the lachrymatory factor. Sweet onions, such as Vidalia or Walla Walla varieties, are often grown in soil naturally low in sulfur.
This low-sulfur environment results in the onion absorbing fewer sulfur compounds, leading to a significantly lower concentration of precursor sulfoxides. For example, some sweet onions contain less than one-third of the sulfur compounds found in pungent yellow onions. Even if enzymes are activated, the overall concentration of the irritant gas produced is much lower, resulting in little or no tearing.
Some newer, tearless onion varieties, like Sunions, have been bred to reduce LFS production over time. These onions are stored until the lachrymatory factor precursors naturally degrade, ensuring a milder bulb upon reaching the market. The high water content found in some sweet onions also acts as a diluent, effectively weakening the concentration of the irritant gas when it is released.
Physiological Reasons for Lack of Response
If an individual consistently handles pungent onions without a tearful reaction, the explanation may lie in their unique physiology. One factor is a variation in the sensitivity of the corneal nerves, which detect the irritant gas. These sensory nerves can naturally become less responsive with age, leading to hypoesthesia, or reduced sensation.
Certain ocular conditions or past eye procedures can also alter the nerves’ response threshold. Individuals who have undergone refractive surgeries like LASIK may have reduced corneal sensitivity due to the severing and regeneration of nerve fibers. This reduced sensitivity means the eyes may not register the irritant gas intensely enough to trigger the full reflex tear response.
A pre-existing condition like tear-deficient dry eye syndrome may also contribute. While reflex tearing is intended to flush the irritant away, a person with impaired tear production may not generate the volume of fluid needed for this defensive flush. Additionally, the sensory nerves that initiate the tear reflex are also connected to the nasal passages. A compromised sense of smell or other neural pathway differences may interfere with the signal that typically causes lachrymation.