Chopping onions is a common starting point for countless recipes, yet it reliably leads to a stinging sensation that forces people to tear up in the kitchen. The tears are not a sign of emotional distress but a natural defense mechanism activated by a specific airborne chemical. When the knife breaks the plant’s cellular structure, it triggers a rapid chemical sequence that produces a volatile irritant. This compound quickly floats up from the cutting surface, engaging the body’s sensory system and resulting in an involuntary flood of tears.
The Chemical Chain Reaction
The process that causes eye irritation begins the moment the onion’s cell walls are ruptured during cutting. Inside the onion, sulfur-rich amino acid sulfoxides and enzymes are kept separate within different cellular compartments. Once the barrier is broken, an enzyme called alliinase quickly mixes with the sulfoxides, initiating the chemical breakdown.
This initial reaction converts the sulfur compounds into unstable molecules known as sulfenic acids. These acids are then rapidly acted upon by a second enzyme called lachrymatory factor synthase (LFS). The discovery of this enzyme was a significant step in understanding the chemistry of the tear-inducing process.
Lachrymatory factor synthase rearranges the sulfenic acids into the final, highly volatile irritant: syn-Propanethial S-oxide. This compound is a gas that easily vaporizes into the air. It is this airborne molecule that acts as the primary agent responsible for the stinging sensation and subsequent tearing response.
The Body’s Defense Mechanism
Once the syn-Propanethial S-oxide is released into the air, it diffuses rapidly and makes contact with the eyes. The volatile gas immediately dissolves in the thin layer of moisture covering the cornea and conjunctiva. It reacts with the water to form a small amount of sulfuric acid. This acid is the actual stinging agent that triggers the body’s protective response.
The eye is equipped with an extensive network of sensory nerves, which are branches of the ophthalmic division of the trigeminal nerve. These nerves are sensitive to chemical irritants and transmit a pain signal to the brainstem almost instantly. The brain interprets this signal as a threat to the eye’s integrity.
In response to the irritant, the brain signals the lacrimal glands, the body’s tear producers, to activate. These glands release a flood of reflex tears, which are chemically distinct from emotional tears. The purpose of this sudden wash of fluid is to dilute the sulfuric acid and flush the irritant away from the sensitive eye tissues, protecting the cornea from damage.
Simple Ways to Prevent Tearing
Minimizing the release of syn-Propanethial S-oxide or preventing its contact with the eyes are the most effective ways to avoid tearing.
- Use a very sharp knife, as a clean slice ruptures fewer cells than a dull blade that crushes the tissue. Research shows that duller blades can cause a violent ejection of droplets and up to 40 times more particles into the air compared to a sharp knife.
- Use a source of moving air, such as cutting near an open window or beneath an active kitchen exhaust fan. This airflow captures the volatile gas and redirects it away from the face before it can reach the eyes.
- Chill the onion in the refrigerator or freezer for about 15 minutes before cutting. The cold temperature slows the enzymatic activity of the lachrymatory factor synthase, limiting the rate at which the volatile irritant is produced and released.
- Cut the onion near or under running cold water, as the irritant is highly water-soluble. The water dissolves the syn-Propanethial S-oxide before it can vaporize and reach the eyes. Leaving the root end intact until the last cut is also beneficial, as this part contains the highest concentration of sulfur compounds.