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June, 2009

Ask the Doctor

Why do I cry when I cut an onion?

Manuel B. Datiles III, M.D.
Manuel B. Datiles III, M.D.
Medical Officer
Senior Clinical Investigator
National Eye Institute

Some country folk say the onions that make you cry were planted by cranky farmers. But NEI's Dr. Manuel Datiles has a more scientific explanation.

Sulfur-containing substances called sulfoxides make their home inside onions, concentrating at the root. "When you cut an onion, enzymes are released that convert the sulfoxides into acids," Dr. Datiles says. Those acids form a volatile gas that floats up from the cutting board and into your eyes, irritating the nerves. Your eyes turn into fountains to wash the gas away.

Some onions are more irritating than others. "Different onions have different contents of the chemical and the enzyme," he notes. "If you have a lot of root, then you have more of the enzyme."

Dr. Datiles has advice for the unwillingly weepy: "If you can prevent the gases from getting to your eyes, you won't cry," he says. Turn on a fan to blow the vapors away, he suggests, or cut the onion under water, so the enzymes are diluted and the water absorbs the gas.

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