Understanding the Chemistry and Biology Behind Onion Tears

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Onions release a gas called syn-propanethial-S-oxide, which irritates the eyes and triggers tearing as a biological response to protect them from the irritant. Various remedies exist to alleviate this reaction, with some suggesting moving away from the onion to allow the gas to disperse. Wearing goggles is also a common recommendation. An alternative method mentioned involves inhaling the onion's smell deeply, which initially causes tears but may help reduce irritation after a few moments, provided the onion remains intact during cutting.
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What is the chemical/s in onions that make your eyes tear? And biologically, why do they tear?
 
Chemistry news on Phys.org
From that web-site:

"There are all kinds of remedies for dealing with this irritating phenomenon, some more effective than others. As a general rule, move your head as far away from the onion as you can, so the gas will mostly disperse before it reaches your eyes. If you really can't stand the tears, the simplest solution might be to wear goggles..."

Actually, there's a method I've tried that works very well: breathe in the smell deeply! It causes your eyes to water a lot right away, but after a few seconds, you're no longer bothered.
 
Just keep the onion intact while you're cutting it :rolleyes:
 
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