Ask A Scientist© Biology Archive

Question:  A sixth grade student named Katie would like to know why some onions
make your eyes water and sting when you cut them and some do not?
Thank you for your response!

Answer:
Julie/Katie:
 
Onions may cause your eyes to tear for the following reason:
 
When you cut into an onion, you slice through cells which contain
sulfur compounds.  Sulfur is quite a noxious (bad smelling)
element.  The compounds released are highly volatile, meaning they
quickly enter the air in their vicinity.  Your eyes produce tears
because your body wants to wash or dilute the chemicals before they
effect your eyes.  The tears are a natural way the body can wash
the offensive chemicals away.
Since there are a variety of onions, we would expect there to be
a variety of amounts and types of sulfur compounds produced by them.
Think of it as different types of apples: some are red, some yellow,
some green, and they all taste a bit different due to different
chemical compositions.  Onions are different in that way too.
Some are REALLY bad smelling, some only mildly bad smelling.
 
One further bit of information. The chemicals which I described above
are both volatile AND water soluble.  This means if you cut the onion
while holding it under a spray of water, the chemicals will wash down
the drain and will not enter the air in as great an amount to cause
your eyes irritation.
 
Thanks for using NEWTON!
Ric