Why Do Onions Make Us Cry?


Chemical Reactions in Onions

When we cut an onion, we release enzymes such as alliinase. These enzymes react with sulfur compounds present in the onion tissues, leading to the formation of volatile compounds like syn-Propanethial S-oxide. This particular compound is responsible for the characteristic eye irritation we experience when cutting onions.

Additionally, the enzymatic reactions that occur upon onion cutting also result in the formation of other volatile compounds that contribute to the overall pungent aroma associated with onions.

Eye Irritation Mechanism

Upon coming into contact with our eyes, syn-Propanethial S-oxide stimulates a reaction with the tear duct glands. These glands then kick into action, producing tears in an attempt to dilute and wash away the irritant. This process is a natural defense mechanism of our body to protect the sensitive tissues of the eyes from potential harm.

The production of tears, specifically in response to irritants like syn-Propanethial S-oxide, results in the stinging and tearing sensation that we commonly attribute to cutting onions.

Varieties of Onions and Tear Production

The tear-inducing effect of onions can vary depending on the onion variety being used. Sweet onions, such as Vidalia or Walla Walla onions, contain lower levels of sulfur compounds compared to their more pungent counterparts like yellow onions. Consequently, cutting sweet onions may result in milder eye irritation due to the reduced presence of volatile irritants.

Cooking onions also plays a role in mitigating the tear-inducing potential of the vegetable. Heat applied during cooking can break down enzymes and reduce the formation of syn-Propanethial S-oxide, consequently minimizing the irritants released into the air during food preparation.

Minimizing Onion-Induced Tears

To alleviate the eye irritation caused by cutting onions, there are several strategies that can be employed. One common method is to chill the onion before slicing, as lower temperatures can slow down the activity of the enzymes responsible for triggering the chemical reactions leading to tear production.

Another technique involves cutting onions under running water. This method can help wash away the volatile compounds as they are released, preventing them from reaching the eyes and causing irritation. Using a sharp knife to slice onions is also recommended, as it causes less cell damage and reduces the release of irritants.


Although the tears induced by cutting onions are often an inevitable part of cooking, understanding the chemical processes underlying this phenomenon can empower us to take proactive measures to minimize their effects. By implementing simple yet effective strategies like chilling onions, cutting them under water, or using sharp knives, we can significantly reduce the discomfort associated with onion-induced tears, making the culinary experience more enjoyable and tear-free.

Nate Douglas

Nate has worked as a nutritionist for over 14 years. He holds a Master's Degree in dietetics from the University of Texas. His passions include working out, traveling and podcasting.