Why does chopping onions make you cry?
20 February 2020
The effects of onions on the eyes
We’ve all been there, slicing some onions in the kitchen, when suddenly your eyes start stinging and the tears begin to pour. Whether you’re frying them up to go in a homemade curry, or serving them raw in a cheese and onion sandwich, onions are a staple ingredient for a huge range of dishes. As much as we love them, the effects they have on our eyes can be really annoying – and almost enough to put some people off them entirely! So, what’s the science behind the tears that come with our favourite bulb vegetable?
As onions grow underground, they absorb lots of sulphur which, once inside the onion, rearranges itself into an irritant called syn-propanethial-S-oxide. When you cut into an onion, you are tearing open the cells in which the syn-propanethial-S-oxide is held. This means it is released into the air and is vaporised, wafting up from the chopping board and hitting your eyes. Once the irritant comes into contact with your cornea (the front surface of your eye), your brain signals for your tear glands to release reflex tears, washing it away. It’s thought that onions have this effect as a kind of defence mechanism, to ward off predators and prevent them from eating the bulbs. And, given the impact they have on humans, it’s likely to be very effective!
All about reflex tears
So, what are reflex tears? They’re absolutely essentially for our eye health, not only when trying to battle with the fumes released when chopping onions, but in order to wash out any grit or dust that enters the eyes. These foreign bodies can scratch your cornea, cause infections, and potentially even impair your vision. Making sure all of these are washed away is your body’s natural instinct. That’s why when one of your eyelashes gets stuck in your eye, everything will start to feel a bit watery. The extra lubrication works to clear your eye of the foreign object, eventually – hopefully – washing it out.
It’s a similar effect when the syn-propanethial-S-oxide from an onion makes contact with your cornea; your tear glands go into overdrive in an attempt to wash the irritant gases out of your eyes. However, anyone who’s chopped an onion knows that these uncontrollable tears are possibly even more annoying than the initial stinging sensation!
The type of tear gland that is triggered to produce reflexive tears is called the lachrymal gland. You have one of these glands located above each eye, between the outer edge of your eyelid and eyebrow. Some people’s lachrymal glands are much more sensitive than other people’s causing them to be much more susceptible to shedding an onion-related tear. This also applies to other irritants in the eye, like dust or grit, which have some people’s eyes uncontrollably streaming with reflex tears due to their sensitive tear glands.
How can you ward off the tears?
There are a number of tactics you can employ to prevent the onion tears flowing next time you’re slicing one up in the kitchen:
- Put your onions in the fridge or freezer for around 15 minutes before you cut them. The syn-propanethial-S-oxide isn’t released as quickly when it’s cold as the chill slows it down. This means it doesn’t float up to your tear ducts as easily as when it’s kept at room temperature.
- Use a sharp knife when chopping your onions. When you use a blunt knife, it tears against the onion’s cell wall, which houses the syn-propanethial-S-oxide. This action causes the vapour to be released more quickly and powerfully. A sharp knife creates clean, precise cuts to minimise the amount of damage caused.
- Wear goggles in the kitchen! This suggestion might make you look a bit silly, but it’s certainly a fool-proof method to keep you free from tears. Obviously, if no fumes from the onions actually come into contact with your tear glands, you should be safe from the effects of them!
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