This humble cousin of the stately lily can make a strong hero shed streams of salty tears. The juicy leaves which form the bulb of the onion contain a strong chemical oil which seeps out of the plant cells and escapes into the air when the leaves are cut. Sensitive nerve endings in your nose and maybe also around your eyes find this onion chemical very irritating.
When you slice an onion, it fights back with chemical warfare and, in a few minutes, your eyes and nose flood with tears, just as they do when you cry. This onion oil is very volatile, which means it tends to turn to vapor and mix with the air. The assorted gas molecules of the air measure about 600 million to an inch, though we cannot line them up in a row.
In a thimbleful of air, the number of gas molecules is estimated ,to be about three, followed by 19 zeros. If a sliced onion is near by, about 30,000 molecules of the strong chemical are mixed in each thimbleful of air. Each gas molecule has room to turn about ten cartwheels before bumping the next. And they speed around in all directions at more than 1,000 feet a second.
The number of collisions is fantastic, though not fatal.
At room temperature, gas and vapor molecules behave like very elastic little flying tennis balls. Each collides with a neighbor and bounces off about five billion times a second.
In this helter skelter, the molecules of vapor from the onion are mixed and spread through the air. In a few minutes the air around the kitchen carries a strong smell of onion. This air touches your skin and eyes and some is taken into your nose. There it touches the sensitive nerve endings of the olfactory organs of smell which are near to other sensitive nerves from the eyes.
Some experts say that different odors cause chemical changes in the organs of smell, others suggest that they work by sifting ultra violet heat. At present we are not sure why vapor from onion juice irritates these nerves. We cannot explain how this vapor causes a stuffy liquid in the nose and tears in the eyes.
If you close your eyes and hold your breath, you may be able to peel an onion without weeping. This, for a while, may prevent the air, carrying its tiny fraction of onion vapor, from reaching the sensitive nerve endings. You will find less trouble if you do the job outdoors in plenty of air where the vapor will be spread more thinly. Another trick is to slice the tasty bulb under water. This slows down the mixing of the volatile oil with the air in the kitchen.