“Crying is cleansing”, or why onions are no reason to weep

Why do we cry while cutting onions?

We all know those sto­ic types who nev­er cry. But there is still one thing that can bring them to tears – onions. Why do peo­ple cry while cut­ting onions? Are onions harm­ful or can they be a rem­e­dy?

(Creative commons by Mike Mozart is licensed under CC BY 2.0) [Flickr]

In the fa­mous Ital­ian chil­dren’s sto­ry by Gi­an­ni Ro­dari, the onion boy Cipolli­no de­feat­ed the chief vil­lain by mak­ing him cry. No won­der, since we all cry ev­ery time we cut onions in the kitchen. More­over, the smell is so strong that it we of­ten have to ven­ti­late the room.

What is “wrong” with onions?!

The sub­stance that makes us cry is syn-Propanethial S-ox­ide (СН3-СН2– СН=S=О). It has a strong ir­ri­tant ef­fect.¹

(syn-Propanethial S-oxide) Black = carbon, grey = hydrogen, yellow = sulfur, red = oxygen [Wikimedia]

This sub­stance is not the only one which can make us cry. Cu­ri­ous­ly enough, there is even a rel­a­tive­ly large group of com­pounds with sim­i­lar ir­ri­tant prop­er­ties. They are all called lachry­ma­tors (from the Latin word for a tear, lacrima). Some of them are used by the po­lice or the mil­i­tary.

The mech­a­nism of ir­ri­ta­tion is the fol­low­ing: lachry­ma­tors ir­ri­tate the sen­si­tive ends of the trigem­i­nal nerve, which stim­u­lates the mus­cles of the eye­lids and lacrimal glands. This re­ac­tion pro­tects our eyes from more se­vere le­sions.

(Creative Commons by Meme Binge is licensed under CC BY 2.0) [Flickr]

The smell is caused by sul­fu­ric or­gan­ic com­pounds, which are re­leased by cut­ting onions. The main com­pound is al­lyl propyl disul­fide (see the pic­ture be­low) which is re­spon­si­ble for the spe­cif­ic onion smell. In fact, these com­pounds ap­pear only af­ter cut­ting. They emerge from the amino acid cys­teine af­ter spe­cial en­zymes trans­form them to disul­fides.²

The smell of onions is al­most im­pos­si­ble to get rid of; nei­ther brush­ing your teeth nor rins­ing out your mouth can help. The rea­son is that these com­pounds are formed in the lungs! Disul­fides en­ter the blood and spread through­out the body, reach­ing the lungs. From there they are re­leased with ex­haled air.

Onions are your friends

In spite of all the in­con­ve­niences, onions are very healthy, nu­tri­tious and es­sen­tial, es­pe­cial­ly in win­ter.

Creative Commons by Darwin Bell is licensed under CC BY 2.0 [Flickr]

Onions are a great source of vi­ta­mins C, E, B1-B3, B5-B6, and B9. Ad­di­tion­al­ly, they are rich in mi­croele­ments such as potas­si­um, iron, sul­fur, phos­phor, cal­ci­um, and mag­ne­sium. All these nu­tri­ents en­hance the im­mune sys­tem and make us more re­sis­tant to in­fec­tions.

Es­sen­tial oils and phy­ton­cides of onions have an­tibac­te­ri­al prop­er­ties and can be ef­fec­tive against harm­ful germs such as strep­to­coc­cus, agents of dysen­tery and diph­the­ria, my­cobac­teri­um tu­ber­cu­lo­sis, etc.

Onions are good for the di­ges­tive sys­tem, they ac­ti­vate the me­tab­o­lism and cleanse the blood, and elim­i­nate con­sti­pa­tion. How­ev­er, onions are ir­ri­tant to mu­cosa, so they are con­traindi­cat­ed for peo­ple with gas­tri­tis or gas­tric ul­cers.³

To sum up

Now it is clear that onions not only tor­ture peo­ple, mak­ing them cry be­cause of disul­fides. Apart from or­gan­ic sul­fides, onions con­tain many nu­tri­ents that help us to stay healthy. So is it worth weep­ing a lit­tle bit with onions, or be­ing ill with­out them?


1. Zwa­nen­burg, B. (2004). "Thioalde­hyde and Thioke­tone S-Ox­ides and S-Imides (Sulfines and De­riv­a­tives)". In Pad­wa, A. Het­eroatom Ana­logues of Alde­hy­des and Ke­tones. Sci­ence of Syn­the­sis 27. pp. 135–176. ISBN 9781588902047.

2. Block, E. (2010). Gar­lic and Oth­er Al­li­ums: The Lore and the Sci­ence. Roy­al So­ci­ety of Chem­istry. ISBN 0-85404-190-7.

3. J. C. Har­ris, S. Cot­trell, S. Plum­mer, D. Lloyd. An­timi­cro­bial prop­er­ties of Al­li­um sativum. Ap­plied Mi­cro­bi­ol­o­gy and Biotech­nol­o­gy. Oc­to­ber 2001, Vol­ume 57, Is­sue 3, pp 282-286.