The reason why onions make you cry begins with the way in which these plants grow. As is the case with many other plants in the Allium genus, onions absorb a significant quantity of sulfur from the soil, drawing it through their roots and storing much of it in the bulbs that we have come to recognize as a popular ingredient in many dishes. When an onion bulb is intact and undamaged, its cells are able to contain the sulfur the plant has gathered from the earth. Meanwhile, separate cells hold a wide range of other chemical compounds, including enzymes.
When you cut an onion to prepare it for cooking, your knife’s sharp blade slices through the bulb and ruptures its cells. As each cell breaks open, it releases any chemical mixtures it had within it, allowing those chemicals to blend with others that were previously held in other cells. When that mixture happens, new chemical compounds come into being.
Among the chemical compounds that form from onion cutting, one called syn-Propanethial-S-oxide is most responsible for your tears. This airborne compound is a combination of the sulfur that the onion has absorbed and stored with various enzymes present in other onion cells. Known as a lachrymator, syn-Propanethial-S-oxide is a volatile eye irritant capable of stimulating the lacrimal gland that produces the water that makes up your tears. Moments after syn-Propanethial-S-oxide forms, it rises from the cut onion through the air until it reaches your face.
When syn-Propanethial-S-oxide contacts your eye, yet another chemical reaction occurs. This time, the syn-Propanethial-S-oxide reacts with the water present on your eye’s surface to create an acid. In response to the acid, the lacrimal glands in your eye produce reflex tears. Reflex tears are those that arise from irritation rather than as an emotional response, and in the case of cutting onions, they are meant to flush the syn-Propanethial-S-oxide-based acid from your eye. During the entire process, the volatility of the chemical reaction taking place on the surface of your eye can also cause an uncomfortable burning sensation.
This tear-inducing effect is a consistent trait of many types of onions, and some suggest that this phenomenon may be a beneficial survival adaptation. The idea goes that when an animal in the wild attempts to chew the bulb of an onion plant, the resulting tears and burn would be enough to deter them, giving the onion plant a greater chance of living on and reproducing.
Those interested in reducing the amount of crying they do while cutting onions should know that not all onion varieties are equally capable of causing you to cry. For instance, the type of onion commonly known as a yellow onion contains more sulfur than white onions, red onions, and others, making them more capable of irritating your eyes. Regardless of which variety you cut, chilling your onions before you cut them can slow the chemical reactions that lead to tears. However, other somewhat popular methods for reducing onion tears often prove to have minimal effect, if any at all.
Crying is a bothersome aspect of cutting onions that there is not much we can do about. But at the very least, we can understand the real reasons why onions make us tear up. To summarize the process in a straightforward way: Onions store sulfur in their bulbs. The sulfur mixes with other chemicals when an onion is cut, then those new chemical compounds rise through the air and irritate your eye. After that takes place, it’s only natural for your eyes to respond by crying.