Why Do Onions Make You Cry

If you’re like most people, your eyes may water at some point during the day because of something as simple as onions touching them. This chemical reaction happens because onions contain two essential chemicals — enzymes called allinase and myrosinase. When these enzymes are exposed to oxygen in the air (like they would be if you cut into or through an onion’s peel), they release sulfuric acid and ethylene. The combination of these three ingredients produces one of the strongest known irritants on Earth: propanethial S-oxide.
The word “lachrymatory” comes from the Latin lacrima, which means “tears.” Propanethial S-oxide causes tears to form by burning holes in the cornea of the eye. It also reacts with mucous membranes such as those found in the nose, mouth and throat, resulting in swelling, inflammation and coughing. Because of this effect, it can cause someone who eats raw onions to start crying involuntarily within minutes after eating them.
This unfortunate side effect has made onions one of the world’s oldest cultivated vegetables. Ancient Egyptian mummies have been discovered with traces of onions still clinging to their eyelids. And Greek historian Herodotus noted that Egyptians believed onions could drive people mad. While we know today that onions don’t make you crazy, there are other myths about this vegetable. One common belief is that onions cause blindness. But why?
It turns out that while onions do contain enough allinase to trigger an allergic response, the amount isn’t strong enough to actually damage the eye. If someone ate a very large number of onions over a short period of time, however, then eating any more onions might cause problems. In fact, consuming even small amounts of onion juice can result in burning, stinging and tearing. Since onions are full of pungent oils, the effects will be exacerbated for anyone suffering from allergies or asthma.
Even though onions aren’t inherently harmful, they can definitely hurt your eyes. So what else should you avoid before going to dinner?
Onion Allergy Symptoms
When onions touch your eyes, they react quickly and dramatically. As soon as the contact is made, you’ll feel pain along the upper eyelid, followed by redness and irritation. Within minutes, you may notice swelling around the affected area. A few hours later, you may see fluid buildup in your eyelashes. By the next morning, you should begin feeling better but probably won’t want to eat any onions for several days.
There are many different types of reactions that occur when you consume onions, so it’s important to determine exactly what kind you experienced. For example, you might have noticed that your eyes watered when you touched them, but you weren’t sure whether you were reacting to saltiness, heat or something else altogether. If you suffered from an allergy to onions, you’d likely experience similar symptoms.
Allergic reactions happen when substances enter the body that shouldn’t normally be present. An allergic reaction doesn’t mean that anything bad happened; rather, it indicates that certain cells became inflamed. To understand how this works, let’s look at how your immune system protects you against foreign invaders. Your immune system consists of white blood cells, antibodies and proteins. White blood cells fight off bacteria, viruses and other pathogens using phagocytic action. Antibodies attack specific targets, neutralizing toxins and preventing infection. Finally, proteins called cytokines help the immune system communicate with cells and tissues.
During an allergic reaction, the immune system releases histamine and another protein called leukotrienes. Histamine increases blood pressure and constricts blood vessels, helping to bring down swelling and reduce inflammation. Leukotriens act similarly, increasing circulation and promoting the growth of new tissue. Both of these chemicals are responsible for causing inflammation and itching.
As mentioned earlier, onions contain molecules called allinases and myrosinases. They activate these proteins whenever their outer layers are damaged. Once activated, both types of enzyme convert alliin (the compound that makes onions smell) into S-allyl cysteine. Myrosinases also break down glucosinolates, a type of amino acid that gives fresh garlic its characteristic bite. Glucosinolates give onions their bitter taste, and they’re also linked to cancer prevention. Without the presence of glucosinolates, we wouldn’t get our familiar sweet flavor from onions.
Sulfenic acids are a family of organic compounds that come from either allinase or myrosinase activity. Like the other compounds discussed here, they burn through the skin of plants, releasing hydrogen sulfide and propanethial S-oxide. The latter of these compounds is the culprit behind onion tears.
Because of this ability to destroy cell walls, doctors often prescribe drugs containing sulfites to treat patients who suffer from severe allergies. However, sulfites can also prevent the body from properly absorbing iron and calcium. Therefore, it’s best to limit your intake of onions, garlic and chives.

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