I was basing that on the large number of sites and articles that claim otherwise:
www.pbs.org/newshour/rundown/why-do-onions-make-us-cry/
When we cut into an onion, we break its cells, releasing the contents inside. This allows chemicals that were previously separated by a cell membrane to combine with each other and with the air. Enzymes and “amino acid sulfoxide” chemicals from inside the cells react to produce a volatile sulfur gas. This gas wafts up from the onion and reacts with the natural water in your eye to form sulfuric acid, which brings about the familiar stinging sensation.
http://www.chm.bris.ac.uk/motm/pso/psov.htm
It is well known that people ‘cry’ when chopping onions but why is this so? The answer is that propanethial S-oxide (often referred to as thiopropanal S-oxide) is released into the air during chopping. Propanethial S-oxide is a lachrymator, an irritant that causes the eyes to fill with tears without damaging them. When a lachrymator comes into contact with the surface of the eye, the cornea, it is detected by the nervous system and triggers a response from the lachrymal (tear) glands. Tears are then produced in order to dilute the irritant. Propanethial S-oxide is relatively volatile and when its vapours come into contact with the eye a small amount reacts to form sulfuric acid, causing the burning and itching sensations that accompany the tears.
https://www.scienceabc.com/nature/onions-make-cry.html
The molecules of syn-propanethial S-oxide are lighter than air, so they waft up towards our unsuspecting eyes. Once they come into contact with the basal tears in our eyes, which already exist due to the lacrimal gland, they once again change their chemical nature. This time around, the chemical reaction results in a far more severe chemical being created – sulfuric acid.
So ... according to various university websites and the like, the burning sensation is in fact caused by the formation of sulfuric acid, therefore that's the main defensive mechanism we're talking about here. Bacteria need water, and if they get onto the cut onion and there's enough water for them to function they're going to be affected by the sulfuric acid.
As a general rule, Reelya, you can find "a large number of sites and articles" that say anything you want, particularly lies-to-the-public about science. That doesn't mean it's right. If you want right, go read peer-reviewed articles in scientific journals with respectable impact factors. Posting links to PBS to contradict actual science is just silly.
Now, on to why you are completely wrong here: for sulfuric acid -- or any acid -- to do anything, it needs to lower the pH of the solution enough to actually trigger the eye's chemoreceptors (or, you know, kill bacteria). Let's humor PBS and pretend that syn-propanethial S-oxide instantly becomes sulfuric acid with 100% efficiency when it contacts water.
There's a few micromoles of syn-propanethial S-oxide per milliliter of onion juice. That means onion juice is at around pH 5.15 at best, and more likely 6, if you were to pour it into your eyes directly. That's somewhere between milk and bananas in acidity. There are chemoreceptors in your mouth, too; do bananas burn when you eat them? Regardless: the few microliters (if that; I'm being generous here) of exposed onion juice giving off the gas that gets into your eyes are not going to do anything meaningful to the pH of your tears, which are
actually well buffered against acids. But we don't even need to care about that.
Sulfines don't dissolve in water to give sulfuric acid. They do give sulfur dioxide under certain conditions (and ketones) but sulfur dioxide must be catalyzed to oxidize quickly into the trioxide required to form sulfuric acid. Tell me, Reelya: how long have you been crying vanadium(V) oxide and alkali metals to act as a catalyst? If you don't know off the top of your head, it would be around the time your tears started to catch fire in air. syn-propanethial S-oxide just doesn't make sulfuric acid.
And beyond even that, think about this logically: if there were enough acid in onion gas to kill anything in the vicinity, and this acid formed upon contact with water, what do you think it is going to do to the water in the onion cytosol -- and why on Earth would onions evolve to burn themselves if you hurt them?
So no, no, and again, NO. syn-propanethial S-oxide doesn't turn into sulfuric acid in your tears, and even if it did, there isn't enough of it for the quantities getting into your eyes to make a difference.
As for what actually happens:
syn-propanethial S-oxide is a known agonist for transient receptor potential ankyrin 1. TRPA-1 leads to an inflammatory response which includes tearing up. That is literally all there is to it. No acid or junk pseudoscience involved.
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