Cutting onions can turn even the simplest cooking task into a tearful experience. But why does chopping an ordinary onion make your eyes burn and produce tears?
The answer lies in a chemical defense mechanism inside the onion.
What Happens When You Cut an Onion?
An intact onion stores sulfur-containing compounds inside its cells. When you cut into the onion, you break those cells apart and allow different substances and enzymes to react.
These reactions eventually produce a volatile chemical called the lachrymatory factor, commonly identified as propanethial S-oxide.
Because the compound is volatile, it travels through the air and reaches your eyes.
Why Does It Make You Cry?
When the chemical reaches the moist surface of your eyes, it irritates the nerves responsible for detecting potentially harmful substances. Your eyes respond by producing more tears.
Those tears are essentially your body’s cleaning and protective response. They help dilute and wash away the irritating chemical from the surface of the eyes.
So when you cry while chopping onions, you’re not experiencing an emotional reaction—your eyes are simply trying to protect themselves.
Why Are Some Onions Worse Than Others?
The amount of the tear-producing chemical can vary between onions. The formation of the lachrymatory factor depends on the onion’s sulfur compounds and specific enzymes involved in the biochemical reaction.
Scientists have even investigated ways of producing onions that generate much less of this irritating compound.
How Can You Reduce Onion Tears?
A few practical tricks may help reduce irritation:
- Chill the onion before cutting it.
- Use a sharp knife to reduce unnecessary tissue damage.
- Cut the onion in a well-ventilated area.
- Avoid placing your face directly over the freshly cut onion.
- Wearing protective kitchen goggles can provide a physical barrier.
The next time onions make your eyes water, remember that those tears are actually a clever defense mechanism protecting your eyes.
