Curiosity Corner: Why do onions make us cry?

The chopping of onions is a day-to-day activity that everyone does in their kitchen. However, it is a common phenomenon that while chopping onions, our eyes water and we have to take a tissue paper to dry our eyes. It is a day-to-day activity that everyone experiences in their lives, but they are not aware of the interesting chemistry involved in the chopping of onions. As soon as the onion is chopped into small pieces, a series of chemical reactions takes place that convert a harmless vegetable into a tear-causing culprit. It is not just an emotional experience but rather a protective mechanism carefully designed by nature as well as decoded by chemistry. In the next slides, a more in-depth understanding of why the chopping of onions leads to crying, as well as what happens when the tears flow out of our eyes, will be discussed.

Onions are simple vegetables that have a number of layers inside. However, inside these layers, there are a number of compartments filled with sulfur-based compounds. Till the time the onion is not chopped, these compounds are not mixed with each other. As soon as the onion is chopped into small pieces with the help of a knife, these compartments break apart, and the enzymes get into action to react with the sulfur-based compounds. This instantly starts to produce a gas known as syn-propanethial-S-oxide. This gas rises upwards to our eyes in just a matter of seconds. The onion looks the same, but the chemistry inside the onion just initiated a powerful protective mechanism designed by nature to protect the onion from being eaten by animals.

Human eyes are extremely sensitive and have to be kept moist and protected at all times. The gas that is produced by the onions dissolves in the moisture that is present on the surface of our eyes. This irritates our eyes, as it is a sulfuric irritant. The tear glands immediately go to work and produce more liquid to counter this irritant and remove it from our eyes. The more onions that are chopped, the more gas will be produced, and hence, the more irritant it will be. The tears that we shed while chopping onions are, in fact, our eyes trying to protect themselves.

Onions may not react in the same manner each time that they are chopped or sliced. The amount of sulfur compound that is present in the onions may vary depending on the type and the conditions in which they were grown. The more potent the onions, the more sulfur will be present in them. The onions that have been stored for a longer period may produce more irritant, as they may react faster to produce this irritant, converting the sulfur compounds to this irritant faster. The manner in which the onions are chopped may also affect the production of the irritant, as the more the onions are crushed, the more irritant will be produced.

The more you know about science, the easier it is to cope with those tears! If you keep the onions refrigerated before chopping, it would slow down the reactions, hence minimizing the release of the gas. If you use a sharp knife to chop the onions, it would minimize the damage to the cells, hence minimizing the irritant released. If you chop the onions near a source of ventilation or under a gentle air current, the gas would be dissipated before it reaches your eyes! Some people even chop the onions near a source of running water to dissolve the gas before it rises! Of course, there would always be tears with onions, but a little bit of scientific knowledge would definitely keep those tears at bay!

This article is intended for informational and educational purposes only, explaining the general science behind why onions cause eye irritation. Individual sensitivity to onion vapours may vary, and suggested kitchen tips are common household practices rather than guaranteed medical or safety advice. If you experience severe or persistent eye discomfort while cooking, consider consulting a qualified healthcare or eye-care professional.