While many people experience watery eyes when cutting onions, some individuals remain tear-free. This variation is not random; it stems from specific biological and environmental factors. Understanding the underlying science reveals why some people are unaffected by onion irritants.
The Chemistry Behind Onion Tears
When an onion is cut, its cells are damaged, initiating chemical reactions. Onions contain sulfur-rich amino acids and enzymes, stored separately within the plant’s cells. Upon cutting, these components mix, and an enzyme called alliinase (also known as alliin lyase) acts on amino acid sulfoxides to produce sulfenic acids.
A second enzyme, lachrymatory-factor synthase (LFS), quickly converts these unstable sulfenic acids into a volatile gas called syn-propanethial S-oxide. This gas, referred to as lachrymatory factor (LF), is responsible for eye irritation. As it becomes airborne and reaches the eyes, it reacts with the water covering the eye’s surface, forming a mild sulfuric acid. This acid irritates sensory nerves in the cornea, prompting tear production to dilute and flush out the irritant.
Individual Differences in Tear Response
The absence of tears for some individuals when cutting onions can be attributed to several factors. These include genetics, cutting techniques, onion characteristics, and environmental conditions.
Genetics
Some individuals have a genetic predisposition that makes them less sensitive to irritants. Research suggests the ALDH1A1 gene may play a role in metabolizing the lachrymatory factor, potentially reducing tear response in those with certain variants. Frequent exposure to onion irritants can also lead to desensitization, making eyes less reactive over time.
Cutting Techniques
The method used to cut an onion significantly influences irritant release. A sharp knife creates cleaner cuts, minimizing cell damage and reducing tear-inducing compounds released into the air. Conversely, a dull knife crushes more cells, releasing a greater concentration of the volatile gas. Cutting speed also matters; slower, more deliberate cuts produce less irritant spray than rapid chopping.
Onion Characteristics
Different onion varieties contain varying levels of tear-inducing compounds. Sweet onions, such as Vidalia or Walla Walla, have lower concentrations of sulfenic acids, making them less potent than pungent varieties like yellow or red onions. An onion’s freshness and storage conditions also affect its potency; fresher onions may cause less irritation than those stored longer.
Environmental Factors
Environmental factors play a role in how irritants disperse. Good air circulation, such as cutting near an open window or under a range hood, helps blow irritant gases away from the eyes. Conversely, cutting onions in an enclosed space concentrates volatile compounds, leading to a stronger reaction. Chilling the onion before cutting can also slow chemical reactions, reducing irritant release.
Eye Sensitivity
Eye sensitivity and contact lenses also affect tear response. Individuals with naturally less sensitive eyes react less to the irritant. Contact lenses provide a physical barrier that limits syn-propanethial S-oxide from reaching the eye’s surface, reducing stinging and tearing for wearers. While contact lenses offer some protection, the gas can still affect unprotected areas around the eyes.