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Monday’s medical myth: chewing gum stops onion tears

The cultivated onion, Allium cepa, is a savoury staple of cuisines around the world. Yet slicing up onions all too often leads to tears: you peel off the papery outer skin, start chopping and before long…

Breathing through your mouth or chewing gum has no effect: the tear stimulus is in your eyes, not your nose or mouth. Flickr/tarale

The cultivated onion, Allium cepa, is a savoury staple of cuisines around the world. Yet slicing up onions all too often leads to tears: you peel off the papery outer skin, start chopping and before long, your eyes are stinging and watering so much you can hardly see; your nose runs like crazy and you wonder why someone hasn’t found a decent way to prevent this torment.

Suggested solutions abound: chew gum, peel onions under water, use a sharp knife, make sure the onions are cold, light a candle nearby, turn on an exhaust fan, wear goggles, or use good chef’s technique to get the job done as quickly as possible. Best of all, get someone else to do it.

Before deciding which methods work best, let’s examine two fundamental questions: why do onions make you cry? And why do we cry anyway?

We generate tears almost continuously. Tears are made by the lachrymal glands located on the upper, outer surface of each eyeball. Although mostly water, tears contain a complex mixture of salts and organic compounds which together keep the surface of the cornea clean and lubricated.

We produce tears in response to chemical and emotional stimuli. sciain

Every time we blink, the eyelids sweep a film of tears across the cornea. A series of tiny glands (tarsal or meibomian glands) in the eyelids secrete a lubricant to prevent the eyelids sticking to the cornea. This process is known as basal tear secretion and is controlled by parasympathetic nervous pathways.

We don’t usually notice basal tear secretion, since it quickly drains away through a pair of lacrimal ducts in the inner (nasal) corner of each eye into the nasal cavity. If tear production increases much over basal rate, this drainage mechanism cannot cope, and teardrops overflow the eyelids. Your nose also runs, as the ducts drain as much fluid as possible from the eye.

Crying occurs in response to two main types of stimuli: chemical or emotional. In each case, the increased tear secretion is due to greater activity of the relevant parasympathetic nerves, triggered by subconscious neural pathways in the brainstem.

Emotional crying is mostly associated with a subset of extreme emotional states: sadness, elation, anger. Unless you are an actor, tear generation usually is outside conscious control, as are the characteristic facial expressions and vocalisations (sobs, wails, and so on).

Tears also help protect the eye from injury or irritation. Foreign material is detected by fine sensory endings of the trigeminal nerve in the cornea, which activate reflex tear generation, often accompanied by involuntary blinking. Thus, the offending material is washed from the corneal surface or out from under the eyelids.

Goggles are the most effective way to reducing crying when chopping onions. leventali

When we slice onions, damaged cells release enzymes that break down to form a derivative of sulfenic acid. This is rapidly converted into a volatile gas (onion lachrymatory factor) by a further enzyme, lachrymatory factor synthase. The lachrymatory factor reacts with water on the corneal surface to produce a range of noxious compounds, including sulphuric acid and hydrogen sulphide.

Onions and garlic have another sulphur-containing compound, allicin. Along with the onion gas, allicin activates the TRPV1 receptors (also stimulated by noxious heat and hot chillies) and TRPA1 receptors (also stimulated by wasabi). Together, these compounds guarantee your eyes will sting and feel like they are burning. So we cry until the noxious agents are diluted and washed from our eyes.

So which methods work best to reduce tearing up? Sharp knives minimise tissue damage, but volatile irritants are still released. Keeping onions cold reduces vapour formation, whereas cutting onions under water or in a strong airflow from an extractor fan prevent vapours reaching your eyes. Goggles work even better, as long as you don’t mind how you look!

But breathing through your mouth or chewing gum has no effect: the tear stimulus is in your eyes, not your nose or mouth.

In principle, onions could be bred or genetically engineered not to express one of the enzymes that cause you to cry. But where would the challenge be then?

Join the conversation

14 Comments sorted by

  1. James Shirvill

    logged in via LinkedIn

    That would explain why I'm fine chopping onions if I'm wearing my contact lenses, but if I'm in glasses my eyes water uncontrollably. Thanks!

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    1. Eric Glare

      HIV public speaker and volunteer

      In reply to James Shirvill

      I too happily chop 6 onions with contact lenses in but can only do 1 without. And I have been treated with complete disdain when I have mentioned this over the years to optomerists, like 'how did you get your PhD?' Finally I'm amongst people who agree with me!

      The article doesn't really explain why partial covering of the eye works - and none of the other ideas really work like contacts including goggles where you eventually breath the vapours into your face.

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    2. Sheena Burnell

      Observer

      In reply to Eric Glare

      I know I haven't been game to mention it until now and in fact wondered if I was imagining it!

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    3. Ian Gibbins

      Professor of Anatomy & Histology at Flinders University

      In reply to Eric Glare

      Partial covering of the eyes helps a bit probably because the total number of sensory nerve endings affected by the onion gas is reduced. Just like on your skin, a smaller burn doesn't hurt as much as a larger one, all other things being equal. Breathing cannot get onion gas to your eyes: however, it could potentially directly irritate the lining of your nose to make it sting and run... hope that helps.

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    4. Eric Glare

      HIV public speaker and volunteer

      In reply to Ian Gibbins

      mmmm - It was a long time ago but I came to the conclusion that, relative to contact lenses, swimming googles and snorkling masks gave rather limited protection from eye irritation - the latter perhaps because some of the gas is expelled through the nose into the face mask. Crying into goggles wasn't a good feeling. A partially blinded study? ;-)

      I do get onion eye irritation with contacts but am able to blink it away.

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  2. Zelong

    logged in via Twitter

    I've tried Goggles and Snorkel as a child, I was the other person in 'Let another person do it'. I still had watery eyes. Than I found if I wash the onion under water than cut on cutting board no tears. I'm thinking the water stops the gas? release. This does work.

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    1. Adam Schembri

      Associate Professor, Linguistics program, Department of Languages, Histories and Cultures at La Trobe University

      In reply to Zelong

      Exactly the same experience as James, but my partner has always been sceptical about my claims - I'll take great delight in pointing out this article to him. As I'm the contact wearer in the relationship, however, I fear that chopping onions may now be forever my household chore.

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    2. Zelong

      logged in via Twitter

      In reply to Adam Schembri

      Thanks Professor you give my reply some clout. I have seen this question around the net before, time to sort out facts from Myths.
      (Wash hands with Lemon to neutralize the Odor too) :)

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    3. Adam Schembri

      Associate Professor, Linguistics program, Department of Languages, Histories and Cultures at La Trobe University

      In reply to Zelong

      Oh dear, I meant to reply to James, not to you Zelong. I'm not sure that my also finding that contact lenses make a difference (as James Shirvill does) can give your reply any more clout: sorry.

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    4. Zelong

      logged in via Twitter

      In reply to Adam Schembri

      LOL... I saw that lol.... Still, Contacts may cover the Eyes enough to stop the tears but I've noticed breathing the air around a kitchen still can get one teary.

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    5. alfred venison

      records manager (public sector)

      In reply to Zelong

      i took "foods & nutrition" in high school and the teacher there taught us to cut the ends off the onion and wash away the white liquid that emerges in running tap water. works for me - i tear up whenever i forget to chop the ends & rinse under water. -a.v.

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  3. Sheena Burnell

    Observer

    Yes, I'm a contact lens wearer too and have noticed what others have said about fewer tears with lenses in, I had always wondered if the lenses formed a physical barrier ands this explains it.

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  4. Sue Ieraci

    Public hospital clinician

    Refrigeration also seems to help - probably less volatiles at a lower temperature.

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  5. Shanna Cowell

    logged in via Facebook

    I hang a piece of bread from my mouth which catches the fumes. Works great in a pinch. Looks really silly.

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