Like garlic, onions are good for you. Rich in fiber and beneficial sulfurous compounds, they're thought to lower cholesterol, improve circulation and possibly even help prevent colon cancer. The downside: bad breath.
Onions owe their uncanny ability to stick with you to volatile sulfur compounds, which vaporize the instant they hit the air. As your body digests onions, these compounds find their way into the blood and then to the lungs. Breath mints and brushing can only cover up onion breath, not get rid of it.
Onions' stinky side doesn't have to keep you from enjoying them, though. Cooking onions helps to break down the compounds behind onion breath. What's more, onions grown in soil with low sulfur content contain fewer of the offending compounds, and they're sweeter to boot.