"It is not the strong odor of the onion that makes us cry, but the gas that the onion releases when we sever this member of the lily family.

The onion itself contains oil, which contains sulfur, an irritant to both our noses and to our eyes. Cutting an onion arouses a gas contained within the onion, propanethiol S-oxide, which then couples with the enzymes in the onion to emit a passive sulfur compound. When this upwardly mobile gas encounters the water produced by the tear ducts in our eyelids, it produces sulfuric acid.

In response to the caustic acid, our eyes automatically blink, and produce tears which irrigate the eye, and which flush out the sulfuric acid.

Another reflex to rid the eyes of a foreign substance, that of rubbing our eyes with our hands, often exacerbates the situation, because our hands are coated with the caustic, sulfuric acid producing oil from cutting the onion, which we then rub directly into our eyes." -- http://www.coolquiz.com/trivia/explain/docs/onion.asp
Source(s):
http://www.coolquiz.com/trivia/explain/docs/onion.asp

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Onions produce the chemical irritant known as syn-propanethial-S-oxide. It stimulates the eyes' lachrymal glands so they release tears. Scientists used to blame the enzyme allinase for the instability of substances in a cut onion. Recent studies from Japan, however, proved that lachrymatory-factor synthase, (a previously undiscovered enzyme) is the culprit (Imani et al, 2002).

The process goes as follows:

Lachrymatory-factor synthase is released into the air when we cut an onion.
Chopping an onion makes us cry, and it all boils down to unstable chemicals. The synthase enzyme converts the sulfoxides (amino acids) of the onion into sulfenic acid. The unstable sulfenic acid rearranges itself into syn-ropanethial-S-oxide. Syn-propanethial-S-oxide gets into the air and comes in contact with our eyes. The lachrymal glands become irritated and produces the tears!
Source(s):
http://www.loc.gov/rr/scitech/mysteries/onion.html

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This is caused by the chemicals that are released by the onion when the skin is cut. Our eyes sense these chemicals and release tears in order to remove them. More can be read on this by going to the source provided (including an excerpt from Scientific America)
Source(s):
http://www.accidentalhedonist.com/index.php/2007/05/03/what_causes_us_to_cr...

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It sounds like you are particularly sensitive to the mild sulfuric acid that is created when the enzymes onions release when cut meet the moisture of your eye.

Because, apparently (according to HowStuffWorks), that's why onions make us cry: the same enzymes that make onions so yummy are released when we cut through the onion cells, and they rearrange to make the gas - sulfenic acid (makes them yummy) - that hits your eyes and reacts with the water to make other gases which irritates them which causes the brain to say "more water to protect you from irritants, important eyes!"

Some of the suggestions about how to deal with this issue include, and I thought this one was interesting although I'm not sure how accurate it is, holding a lemon, piece of bread, or sugar cube in your mouth to absorb the gas. Who knows, but might be worth trying! Also, breathing through your mouth might be worth trying.

If you are cooking them yourself, you can do more traditional things to combat the issue:
-chill it in the fridge after peeling and before cutting (change in temp. alters compounds)
-cook the onion before you slice it (same reason)
-cut the onion under water.

You could also go through your days wearing goggles, to ward off any onion-gases that might be coming from 100-yards away!

http://www.prov.vic.gov.au/images/12903/f/12903-p00001-000517-100.jpg
Source(s):
http://science.howstuffworks.com/vegetable/question539.htm

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You cry because the onion although it has lost it's a-peal, it is becoming more appealing.  Watching this transformation can be very emotional.
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Mild sulfuric acid relaesed makes us cry.

Cut the onion in 2 halves and dip them in water for 5 minutes. When you cut these onions, you won't cry

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