Why do onions make you cry?

There are some common techniques to help you avoid crying.

Chilling the Onion

Chilling will reduce how fast the enzymes convert the sulfur compounds into the tear-inducing chemicals. You can put the whole uncut onion in the fridge for about 30 minutes prior to using it. Alternatively, you can place the onion in the freezer for around 15 minutes. Both of these actions should result in fewer enzymes being able to convert the sulfur compounds.
Cut Under Cold Running Water

If you cut the onion underwater, or use a fan to blow the gas away from your face, it will dissolve in the water before it gets to your eyes.
Cut the Onion Using a Sharp Knife

When you cut an onion with a sharp knife, you are going to damage fewer of the onion cells than when you cut an onion with a dull knife. Since each damaged cell releases additional amounts of tear-inducing gas, the number of damaged cells affects how severe the reaction will be.
Cut From the Top Downward

It has been shown that the part of the onion closest to the roots contains the most concentrated forms of the lachrymatory factor. Therefore, you may want to peel and chop the upper parts of the onion first and leave the root until last.
Wearing Protective Eyewear

Wearing protective eyewear such as goggles can be one of the best ways to prevent your eyes from becoming irritated. Protective eyewear creates a barrier that prevents the gas from getting close to your eyes.

Although the temporary discomfort of tearing while chopping onions can be frustrating, onions are full of nutrients that promote overall health and well-being. They contain antioxidants, fiber, vitamins, minerals, and other beneficial components. The tear-causing gas is also partially responsible for many of the onion’s beneficial health aspects. Research shows that eating onions can provide protection against heart disease and cancer.

ONION TIPS AND TRICKS FOR LESS TEARS:

Chill your onion for 10–15 minutes before cutting. The chill slows down the enzymatic process which leads to fewer tears.
Use cold running water while cutting. The water helps keep your eyes moist and prevents the gas from entering them.
Cut the onion under a vent hood or near an open window. This helps disperse the gas away from your face.
Cut the onion using a super-sharp knife. Cutting an onion with a dull knife will cause more cells to rupture releasing more of the tear-producing gases.
Cut the onion from top-to-bottom rather than bottom-up. Research indicates that the bottom portion of the onion is higher in sulfur compounds and therefore produces more of the tear-producing gas.
Wear goggles while chopping onions. Although uncomfortable, goggles will completely block out any gas produced during the cutting process.

Q&A

Will I eventually get used to chopping onions and stop crying?

You will never actually become immune to the chemical reactions caused by cutting onions. Your body does adapt to the sensation of chopping onions however and some individuals may find it less bothersome over time.

Does chewing gum really help prevent onion-induced tearing?

Chewing gum and bread are both said to help alleviate symptoms of tearing associated with chopping onions. The reasoning behind this is that chewing gum increases saliva production causing air flow away from your tear ducts or possibly distracting your senses so you don’t notice as much. There is no research to support this claim and it is purely anecdotal.

Do “tearless” onions exist?

Yes, some types of onions were genetically engineered or cross-bred to produce fewer lachrymatory-factor synthase (the enzyme responsible for turning isoalliin into syn-propanethial S-oxides). While you will still experience some level of irritation from cutting, you may experience less than usual.

Why do my eyes sometimes burn long after I finish chopping?

The reason you occasionally get burning in your eyes after stopping chopping onions is because of lingering syn-propanethial S-oxide molecules in the air. Even though you’ve ceased chopping for several minutes, small amounts of gas can still escape and cause eye irritation for a few minutes longer depending on ventilation in the room you’re working in and amount chopped.