Several years ago we shared a cabin for the weekend with a few other couples I went to college with. We split the cooking duties among the couples so each couple was responsible for at least one meal for all of us.
One of the husbands is a terrific cook. He was in the kitchen one evening dicing onions when I came in to get a drink out of the refrigerator. I commented that it must be a mild onion. I hadn't noticed the onion smell at all. He said said "Nope, it's a pretty strong onion," and pointed to a candle burning beside the cutting board.
I didn't get it, so I asked what the candle had to do with it. My friend explained that the candle helped burn off the vapors from the onion that caused a person to cry.
The other thing I've found that helps prevent crying is to refrigerate the onion before cutting it.
So when I purchase onions, I put them in the fridge straight away, and I burn a candle while dicing onions. No more tears.