Chopping onions is a common kitchen task, but it often comes with an unwelcome side effect: tears. The tears aren’t about emotion; they’re a reflexive response to a volatile irritant released when the onion’s cells are damaged. By looking at what happens in the onion and what happens in the eye, you can understand why onions make you cry and how to reduce it without sacrificing flavor.
The chemistry behind onion tears
What happens inside the onion when you cut it
Onions store sulfur-containing compounds in their cells, along with enzymes that keep these compounds separate. When you slice, crush, or bite into an onion, you rupture cell walls. That collision of chemicals allows an enzyme (often called alliinase) to meet sulfur-containing precursors. The enzyme catalyzes a transformation, turning them into reactive sulfenic acids. Those sulfenic acids are then rearranged by another enzyme to form syn-propanethial-S-oxide, a volatile gas that acts as the onion’s tear-inducing agent—the so-called lachrymatory factor.
The tear-causing gas and how it irritates the eye
The syn-propanethial-S-oxide gas rises into the air and makes its way to the surface of the eye. In the tear film, it irritates the sensory nerves of the cornea and conjunctiva. The nerves signal the brain that something is amiss, and the lacrimal glands respond by producing tears. Tears serve to dilute and flush away the irritant, so tearing is a normal protective reaction rather than a sign of sadness.
Why some onions trigger more tears
Varieties and their sulfur compounds
Onion varieties differ in how much of these sulfur compounds they contain and in how active the enzymes are. Some onions release more of the tear-inducing gas when cut, while others are milder. The result is that certain varieties—often described as sweeter or milder—tend to provoke less tearing than sharper, more pungent onions.
Individual sensitivity
People vary in sensitivity to the lachrymatory factor. Genetic differences, how dry or moist the eyes are, and even how deeply one breathes while chopping can influence the intensity of tearing. In practice, two people can work with the same onion and experience notably different levels of tearing.
How to reduce tears
Practical tips you can try
- Chill the onion for about 30 minutes before cutting. Cold slows the chemical reactions that produce the tear gas.
- Use a sharp knife. A clean, precise slice minimizes unnecessary cell damage and slows the release of irritants.
- Cut the root end last. The root contains a higher concentration of sulfur compounds, so saving it for last can reduce tearing.
- Create air movement. A fan, open window, or range hood can push the gas away from your face.
- Consider cutting onions under running water or near a stream of water. Washing away some of the emitted gas can help, though it may not eliminate tearing entirely.
- Cook onions when you can. Heating denatures the enzymes involved, which reduces the production of the tear-inducing gas.
- If needed, protective eyewear can offer the most reliable barrier against irritation.
Conclusion
Tears while chopping onions come from a small but well-orchestrated chemical process inside the plant and a quick response from our eyes. By understanding the steps—from the onion’s sulfur compounds and enzymes to the eye’s protective tearing—you can tailor simple techniques to keep tearing to a minimum. Whether you prefer a crisper cut with a sharp knife, a cooler onion, or a little extra airflow, you can enjoy onions’ bright flavor without as much watery eyes.