Crying when cutting onions is a result of the release of sulfur compounds from the onion cells, which can irritate the eyes and trigger tear production. Here’s a more detailed explanation of why this happens and how it can be prevented:
Why do I cry when cutting onions?

1. Sulfur compounds: Onions contain enzymes, such as alliinase, that convert a compound called alliin into sulfenic acid when the onion cells are damaged. This process is accelerated when onions are cut or chopped.
2. Odor and irritation: Sulfenic acid and other sulfur-containing compounds have a strong odor and can irritate the eyes, nose, and throat. The eyes respond by producing tears to flush out the irritants.
3. Tear production: The tears help to dilute and wash away the irritants, protecting the eyes from potential damage.

How can it be prevented?
Here are some strategies to minimize or prevent the tearful response when cutting onions:

1. Chill the onions: Before cutting, place the onions in the refrigerator for about 15-20 minutes. The cold temperature slows down the activity of the enzymes, reducing the amount of sulfur compounds released.
2. Cut under running water: Use a sharp knife to cut the onion under running water. The water washes away some of the released compounds as they are produced.
3. Use a food processor: Grating or mincing the onion in a food processor can also reduce the amount of sulfur compounds released into the air.
4. Wear goggles: While this may seem excessive, wearing goggles can protect your eyes from the irritants if you find yourself very sensitive to them.
5. Cut quickly and efficiently: The less time you spend cutting the onion, the less opportunity there is for the enzymes to release irritants.
6. Consider genetically modified onions: Some varieties of onions have been genetically modified to produce fewer sulfur compounds, which may result in less tearfulness.
7. Acidification: Adding a little acidity, such as lemon juice or vinegar, to the cutting board can help inhibit the activity of the enzymes responsible for releasing the sulfur compounds.

Remember, this is a perfectly normal physiological response and is not caused by any emotional sensitivity to the act of cutting onions. If you find the tearful response to be particularly problematic, trying one or more of these prevention methods may help ease your discomfort.