Edit: Swim goggles should work too if you don’t have to worry about glasses.
I know this will be obvious to some, but I’ve never seen anyone in my family wear them. It’s so obvious once you think about it, and yet I’d wager most people (especially ones who don’t cook as a hobby) have never tried this.
The reason you cry is because slicing onions produces an organosulfur called syn-Propanethial-S-oxide. Lab goggles – as designed – keep the chemical irritant from reaching your eyes. I’ve used them hundreds of times now, and I think there was one time it got into my eyes when I didn’t have the goggles situated right (not difficult; I was just being a moron).
My 3M anti-fog pair were about $5 USD when I got them, and it looks like they’re about $7.50 USD now. For that price, I never have to dread cutting up onions again. It’s not magic; it’s just basic PPE, and it works. You can even wear them over eyeglasses (I’m sure some huge, circular frames won’t fit, but most should).
Even if you forget them and remember them midway through slicing, it can still help somewhat. So even if you’re as absent-minded as I am, you can benefit from trying this.
These (below) were the ones I got personally, but feel free to try what you already have if you already have a pair on-hand for e.g. cleaning. I’d assume the important thing is just that they’re goggles, not glasses.

Why YSK: owie, oof, ouchie, my eyes. Cooking is just DIY organic chemistry.


I heard burning a candle nearby while cutting also helps but idk