Maybe you're a masochist and like the sulfuric acid in your eyes?

https://en.wikipedia.org/wiki/Syn-Propanethial-S-oxide

Let it spit in your eye, for it is doomed. Soon you will consume its essence and that is Good.

Or maybe it's not a big deal for just a couple of onions?

If it's really bothering you, make sure your knife is sharp enough. A dull knife makes dicing much slower and releases many, many more compounds.