Why is it when you cut onions your eyes sting?

Onions contain a chemical compound called syn-propanethial-S-oxide (SPSO). When you cut an onion, this compound is released into the air and comes into contact with your eyes. SPSO is a powerful irritant that causes the eyes to water and sting.

The reason why SPSO is such an effective irritant is because it binds to the TRPA1 receptor in the eye. This receptor is responsible for detecting harmful substances and triggering the body's pain response. When SPSO binds to the TRPA1 receptor, it sends a signal to the brain that the eye is in danger. This causes the eyes to water and sting in an effort to flush out the irritant.

There are a few things you can do to reduce the amount of eye irritation you experience when cutting onions. First, you can try cutting the onion under running water. This will help to wash away the SPSO before it has a chance to come into contact with your eyes. You can also try wearing goggles or safety glasses when cutting onions. This will help to protect your eyes from the irritant.

If you do happen to get SPSO in your eyes, there are a few things you can do to relieve the irritation. First, you can try flushing your eyes with cool water. You can also try using a cold compress or applying artificial tears. If the irritation is severe, you may need to see a doctor.