Why Do Onions Cause Teary Eyes?

When my four daughters visited this recent Easter weekend, the oldest suggested I investigate the reason chopping onions makes her cry. I will reveal the answer later since the chemistry now might distract one from the lure of onions.

Onions are consumed in all cultures around the world. The onion enhances food served at all three meals of the day. Just imagine the taste of onions in a breakfast omelet, fried onions on a grilled chicken breast for lunch or raw onions on a grilled hamburger or in a salad with orange slices arranged on spinach leaves for supper.

Onions, originating in Asia and the Middle East, have been cultivated for 5,000 years. Ancient Egyptian history revealed slaves constructing the pyramids went on strike when onions were withheld from their diet.

Onions and garlic are members of the lily family. I found this fact hard to fathom until I unearthed my wilting Easter lily this week discovering root tubers resembling clusters of garlic and onion bulbs. Onions are root swellings called tubers or bulbs which store food for plant growth the next year.

To evaluate the quality of onions in relation to tearing or watering of the eyes, I experimented slicing and chopping all onions including garlic available in a local grocery store. Using a stopwatch, I timed the onset of the onion smell, the onset of my eye irritation, the onset of my nose running (indicating tear formation draining into the nose via the tear duct) and finally the onset of tearing or blurring of vision from an overflow of tears. Interestingly, none of the onion varieties caused tears to overflow my eyelids onto my cheeks as did watching the movie, “A Walk to Remember,” inspired by the author Nicholas Sparks. Since heat produced by peppers is measured in Scoville Units from 0 to 10 with a bell pepper equaling zero and the Habanero pepper rated at 10, I applied units for tearing time and pungency for chopped onions. I ate each onion raw, describing my opinion of taste and pungency on a scale from 1 to 10. A rating on my scale of 10 indicated a strong, biting, highly pungent taste which usually could be rendered mellow by cooking. A rating of 1 indicated a pleasant, sweet taste with minimal eye irritation presenting a desirable flavor when sliced raw for a salad.

Besides tasting good, the onion provides health benefits in the form of minerals such as potassium, iron and calcium. Vitamin C and fiber are in high concentration. Cholesterol and fat are absent. Diets high in onion consumption promote cardiovascular health, reduce inflammation and retard some cancer formation.

Now the information you have been waiting for, what causes tear formation after chopping onions? When an onion is cut, lachrymatory factor synthase (an enzyme) is released which converts amino acids in the onion into sulfenic acid. This acid is unstable, therefore rearranging itself into synpropanethiol-S-oxide which comes into contact with the eye. The lacrimal (tear) duct becomes irritated so it produces tears to wash away the irritating chemical. The yellow color of some onions indicates greater concentrations of tear producing chemicals. Wearing swim goggles or contact lenses and chilling onions before chopping can reduce tearing.

In my opinion, the onion, which is available in exciting varieties and at reasonable prices year round, is a best value. Forget the chemistry, but eat an onion a day to keep the doctor away.