Just Ask: Why Do Onions Make Us Cry?

We all know the stinging sensation and welling up of tears that comes from chopping onions. But why does this happen?

In fact, the reason onions make us cry can be traced back to the soil. Onions are part of the “Genus Allium” plant family, along with garlic, leeks and chives, and they absorb sulfur from the soil.

When we cut into an onion, we break its cells, releasing the contents inside. This allows chemicals that were previously separated by a cell membrane to combine with each other and with the air. Enzymes and “amino acid sulfoxide” chemicals from inside the cells react to produce a volatile sulfur gas. This gas wafts up from the onion and reacts with the natural water in your eye to form sulfuric acid, which brings about the familiar stinging sensation.

“One of the reasons we cry is to rinse harmful things such as dust and in this case acid out of our eyes,” said Bruce Bryant, senior research associate at the Monell Chemical Senses Center. Pain receptors send a message to our tear ducts, prompting a flow of tears to dilute the acid.

But there are strategies to temper the tears. Cooling the onion for a few minutes in the freezer helps, according to Alyson Mitchell, chair of the American Chemical Society division of Agriculture and Food. “The enzymes are less reactive when they are cold,” she said. Cooking also lessens the activity of the enzyme.

And take note, onion sufferers. Scientists are now breeding a new strain of onions with lower levels of the offending enzymes. Could tear-inducing onions become a thing of the past?

If you have a question on science or technology for Just Ask, send an e-mail to science@newshour.org with “science question” in the subject line or leave it in the comments section below.