Ask A Scientist

Why do onions make us cry?

Asked by: Charlotte Coker
School: Tioga Hills Elementary
Grade: 1
Teacher: Mrs. England

Answer from Stephanie Craig

Teaching Assistant and Ph.D. Candidate, Binghamton University

Research area: Nitrogen biogeochemistry in the Binghamton community
Interests/hobbies: Ballet, yoga, reading, hiking, cooking and traveling

When you cut or crush an onion your eyes quickly become painful, irritated and then produce tears. This happens because the cells of the onion have broken open and chemicals that are normally contained within the cells are released. Once they are in the air, a series of reactions create many new chemicals. Of these chemicals, one gives onions their strong smell and another is irritating to your eyes. The one that makes eyes irritated and produces tears is known as the "crying factor." It travels through the air and when it reaches your eyes, it meets the corneas where "pain fibers" are triggered. The "pain fibers" cause the eyes to feel pain and they also cause the formation of tears, which wash the chemical away. When you cut an onion and begin to cry, your first reaction may be to wipe away the tears—but be careful! The "crying factor" is not only in the air, but it is also on your hands and if you touch your eyes you will make the irritation worse. It’s nothing harmful but after you handle a raw onion, be sure to wash your hands before you touch your eyes and your face. Onions and other members of the Allium plant family (which includes garlic and leeks) most likely evolved to have chemicals like these to defend against predators. The strong smells and eye irritation caused by the plants keep animals away and insects that could harm them, allowing onions to survive and reproduce for generations. 

Last Updated: 3/1/17