But when you cut into it, you release a gaseous sulfur-containing chemical called the lachrymatory factor (propanthial S-oxide) that spreads in the air and comes into contact with your eyes.





"The lachrymatory factor activates the nerve endings of pain fibres in the top layer of the cornea," explains Professor Jonathan Dostrovsky, director of U of T's collaborative program in neuroscience and a professor in the Department of Physiology. "When these pain fibres become active, they send signals to the brain and give rise to the sensation of pain in the eye," he says. "At the same time, these signals also activate the part of the autonomic system [the system responsible for the optimal functioning of the body] that controls the lacrimal gland [the gland that produces tears] leading to increased production and release of tears."