Science Questions

When I wear my contact lenses, chopping onions doesn’t make my eyes water, does this mean only the part of the eye that’s covered by contact lenses is sensitive to onion vapours? Jennifer, Columbia

Everyone’s noticed this onion effect and people just think ‘oh, there’s something in the onions’ but what’s interesting is next time you’re cutting onions, time how long it takes between cutting into the onion (and the spray you can see coming up under the knife) and the time it takes before your eyes start reacting. 

You’ll see there’s a big delay between the onion being chopped up and you feeling the sting.  The reason is there’s a chemical reaction going on the minute you start breaking into the onion, that starts to trigger the eye smarting effect. 

There are enzymes locked away in the onion called alinases and they’re names after the family you find them in, like onions, garlic, scully and chive.  They’re the alium family, that’s why it’s called alinase.  This breaks down a family of chemicals, which are also in the cells called amino acid sulphoxides.  They’re the smelly things that make onion smell like onion. 

As the enzyme starts chewing into them, it releases something called sulphinic acid and sulphinic acid then breaks down into another chemical which is called sin-propantheal-S-oxide and that’s the irritant.  It comes squirting out of the onion towards your eyes, binds to the corner of your eyes, the cornea, which is very rich in nerve supply (it’s got some of the densest nerve fibres in the body) which signal pain and this is an irritant.  These nerve fibres are wired up to your lacrimal gland which makes tears, because your eye correctly says if there’s something irritating it there’s probably a foreign body in the eye, therefore the way to get rid of it is to activate more tears and wash it out. That’s why you cry.

But why does it take time before that kicks in? because that chemical reaction has to happen? Why does a contact lense make a difference? Well, a contact lense sits on the front of your eye, in front of the cornea where there’s the most nerves, it stops the chemical getting into contact with the nerves and instead, because contact lense wearers probably make more tears as they’ve got a foreign body in their eye all the time, it’s washing the stuff off the contact lense and down your tear duct before it has a chance to irritate the nerves at the front of the eye.

September 2007