Why do onions make us cry?

"It is hard to imagine a civilisation without onions," the home-cook hero Julia Childs is reported to have said. From a mirepoix to a soffritto, from stocks to stews, onions are the building blocks of savoury flavour foundations. After a hard day at the office, the sweet smell of an onion being sautéed in a pan can provide stress relief almost on a par with a loving hug from an old friend. But our tear ducts often pay the price on the road to unleashing the comfortingly familiar smell and flavour of a cooked onion. Why is it that onions make us cry? And is there anything we can do about it?

Well, it turns out that onions contain a chemical irritant called syn-propanethial-S-oxide. In his 2010 book Garlic and Other Alliums: The Lore and the Science, chemist Eric Block shares his insights on the onion. "See, the onion is a perennial bulb that lives in the ground with lots of critters who are looking for a snack," he says. "So it has evolved a chemical defence system." When we cut into an onion, we release enzymes which mix with other chemicals inside the onion cells. This creates a chemical reaction which produces molecules of syn-propanethial-S-oxide which float on up into the air, gradually making their way to the eyes of the person cutting the onion.

Is there a way to protect our ducts from these tear-inducing molecules? The American National Onion Association (onions-usa.org) recommends chilling the onions for 30 minutes before peeling the outer layers while keeping the root end, which has the highest concentration of the sulphuric compounds, intact. But that’s not even half of much craic as wearing goggles, right? And my swimming goggles pass the test at home; I slice a white onion in its entirety without even a tickle to my tear ducts.

I even take the goggles off and get my eyes up close and personal with the sliced onion to make sure it wasn’t just an onion low in enzymes. I can report, while shedding a tear, that it was indeed the goggles that prevented me from tearing up.