Onions use sulfur in the soil to create amino acid sulfides which are sulfur compounds to produce the chemical irritant known as syn-propanethial-S-oxide. Because onions grow underground this complex chemical reaction helps to deter critters that want to feed on the onion.
When the onion is cut, this irritant, sulfuric acid, is released into the air. This stimulates the eyes lachrymal glands, so they release tears to flush out the irritation from the eyes.
Chopping onions can cause some burning and irritation, but there is no serious long-term threat to your vision.
White, yellow and red onions create higher amounts of the gas than do sweet onions, green onions and scallions.
Some people have a higher sensitivity to the gas than others.
Tips to help with dealing with the discomfort of chopping onions:
- Googles can prevent the gas from reaching the eyes while cutting onions,
- Cut the onions in a bowl of water.
- Use eye drops to lubricate or rinse the eyes and dilute the gas exposure to the eyes.
- Chill the onions 10 to 15 minutes before cutting.
- Spread a little citrus juice on the cutting board.
- Chew mint chewing gum while chopping onion.