In a Nutshell : Breaking open onion cells which releases enzymes which in turn react with other released substances to release a volatile gas – which when it reaches our eyes, it reacts with the water that is intended to keep our eyes moist.
A Longer Explanation
The sulfenic acids mix with the tears in our eyes to form sulfuric acid, which is the same toxic stuff that is in our car batteries
Slicing through an onion breaks open a number of cells, which releases enzymes. The escaped enzymes, allinase and lachrymatory factor synthase, decompose some of the other released substances, called amino acid sulfoxides. This reaction forms unstable sulfenic acids, which eventually stabilize into a volatile gas. When this gas reaches our eyes, it reacts with the water that is intended to keep our eyes moist.
The sulfenic acids mix with the tears in our eyes to form sulfuric acid, which is the same toxic stuff that is in our car batteries. Nerve endings in our eyes pick up this irritant and send a message to the brain, which then passes a message to our tear ducts that says, “Dilute that irritating acid to protect my eyes.”
So crying or tearing is used here as a protective measure.
Dealing With It
There are some things you can do to alleviate the problem :
- The best way to deal with this phenomenon is to move away from the sliced onion instead of standing directly over it.
- You can cook the onion first, before slicing it; the cooking process inactivates some of the enzymes.
- You can refrigerate it first.
- You can wear contact lenses or goggles.
- You can use a fan to blow away the fumes.
- You can slice the onion while running tap water over it or cut it underwater, because the water will react with some of the released gas before it reaches your eyes. Steam from a kettle will also do the trick.
Onions making us cry is a powerful reminder of how our body reacts to dangers and how our brain orchestrates chemical changes to protect us.