Why do eyes burn when slicing onions

Onions are a popular kitchen vegetable that is used almost all over the world. In addition to food preparation, it is also used in medicine for various digestive or inflammatory diseases, as well as for arteriosclerosis and high blood pressure. But why does it cause burning eyes when slicing?

Bulb onion is one of the oldest crops used as a spice. Due to its penetrating taste, it has spread all over the world. However, it is considered to be of Central Asian origin. Onions are also mentioned in a number of the oldest surviving documents, such as Sumerian clay tablets, the Bible, ancient Egyptian papyri, and Indian and Chinese books.^[1]

Freshly chopped onions cause a pungent feeling in the eyes and uncontrollable tearing. Behind it is a substance called thiopropanal S-oxide, which irritates the optic nerve in the form of an aerosol. Thiopropanal S-oxide is formed by a cascade of enzymatic processes by which the plant is protecting itself against mechanical damage. Upon contact with the eye, the lacrimal glands begin to release tears so that the pungent substance is diluted as much as possible and washed out of the eye.^[2]

Eye irritation can be alleviated or completely avoided by simple tricks. Slicing the onion in water will not release the pungent gas at all. Mitigation can then be achieved by slicing the onion with a very sharp knife, with good ventilation, cooling the onion in the refrigerator or slicing the onion towards the root. This is because the root contains a large amount of sulfur compounds, which are behind the formation of thiopropanal S-oxide.^[2]