Do Onions Release Toxins? Debunking the Myth of Leftover Onions

December 12, 2025 •

4 min read

According to the National Onion Association, a widely circulated email claiming that cut onions become highly poisonous overnight is completely false. This article will delve into the science behind this claim and definitively answer the question: do onions release toxins?

Quick Summary

Leftover cut onions do not become poisonous or toxic after sitting out. The myth, often tied to a 1919 flu folk remedy, is scientifically debunked. Instead, onions contain compounds that are naturally antimicrobial, inhibiting pathogen growth when handled properly.

Key Points

The myth is false: The claim that cut onions absorb bacteria and become poisonous overnight is a widely debunked urban legend.
Onions are naturally antimicrobial: The sulfur compounds in onions create an acidic, low-protein environment that inhibits bacterial growth, rather than attracting it.
Proper storage is key: To prevent cross-contamination and spoilage, cut onions should be stored in a sealed container in the refrigerator and are safe for up to seven days.
Risk for pets: While safe for humans, onions are highly toxic to dogs and cats, even in small amounts, and can cause hemolytic anemia.
Origin of the myth: The folk remedy likely stems from the 1919 flu pandemic era, based on the now-disproven miasma theory of disease spread.
Watch for external contamination: The real food safety danger for cut onions is cross-contamination from dirty hands or surfaces, not internal toxicity.

Separating Fact from Fiction: The Onion Toxicity Myth

For years, an alarming claim has circulated via email and social media: a leftover cut onion is a magnet for bacteria and can become poisonous if consumed later. This falsehood often cites a story from the 1919 influenza epidemic, suggesting onions placed around a sick person could absorb the illness-causing germs, thereby purifying the air. The story concludes that these 'toxic' onions must be discarded, a logic extended to all leftover cut onions. Food safety experts and organizations, including the National Onion Association, have thoroughly debunked this urban legend. The truth is that the chemical properties of an onion make it an exceptionally poor breeding ground for pathogens.

The Science Behind the Onion's Safety

When you cut an onion, you damage its cells, which releases sulfur compounds responsible for the vegetable's distinctive odor and tear-jerking effect. This very chemical reaction is what makes the onion hostile to bacteria, not a 'magnet' for it. Research has shown that juice released from a cut onion can actually inhibit the growth of certain microorganisms, including some responsible for food poisoning. The low pH (acidity) and low protein content of onions further contribute to an unfavorable environment for bacteria. Therefore, a properly stored, cut onion is no more of a food safety risk than any other piece of produce.

Proper Handling and Storage of Cut Onions

While onions do not magically become toxic, standard food safety practices are still crucial to prevent contamination from external sources. The main risk to a cut onion comes from cross-contamination in the kitchen, such as using an unwashed cutting board that previously held raw meat. Proper storage is also key to maintaining freshness and safety. For instance, storing cut onions in an airtight container in the refrigerator will prevent them from drying out or absorbing odors from other foods. The National Onion Association advises that properly stored cut onions can last up to seven days in the fridge. If you notice any signs of spoilage, such as mold or a slimy texture, you should discard the onion, just as you would with any other food.

Best Practices for Onion Preparation and Storage

Use a clean, sanitized cutting board and knife to prevent cross-contamination.
Store leftover cut onions in an airtight container to maintain freshness and prevent odor transfer.
Refrigerate cut onions and use them within 7-10 days.
Wash whole onions under running water before cutting to remove any dirt or surface bacteria.
Discard any onion that shows visible signs of rot, mold, or an unusually strong odor.

Potential Downsides and Safety Concerns (for Humans and Pets)

While not toxic in the mythical sense, onions do have a few considerations for human consumption. For individuals with Irritable Bowel Syndrome (IBS), onions are high in FODMAPs (fermentable oligo-, di-, mono-saccharides and polyols) and can trigger digestive issues like bloating, gas, and discomfort. Some people also have an intolerance or rare allergy to onions, with symptoms including skin rashes or gastrointestinal upset. For pets, the story is entirely different. Onions, along with garlic and other alliums, are highly toxic to dogs and cats, causing a type of anemia by damaging their red blood cells. Pet owners should be diligent about keeping onions and onion-containing foods away from their animals.

Myth vs. Reality: A Comparison of Cut Onions


Aspect	The Myth	The Scientific Reality
Mechanism of Harm	Onions absorb germs and become a breeding ground for toxic bacteria.	The low pH and antimicrobial compounds in onions inhibit bacterial growth.
Toxicity Over Time	A cut onion turns poisonous if left overnight.	A properly stored cut onion is safe to eat for up to a week.
Food Safety Risk	The onion itself becomes contaminated by absorbing germs.	The primary food safety risk is external cross-contamination, not the onion's properties.
Folk Remedies	Placing cut onions around the house absorbs viruses like the flu.	There is zero scientific evidence that onions purify the air or absorb airborne germs.

Conclusion: Onions Are Safe, When Handled Correctly

The widespread myth that cut onions release toxins and become poisonous has been conclusively debunked by food safety experts and scientific research. Far from being a breeding ground for bacteria, the compounds within onions actually provide a natural defense against microorganisms. The true risk, as with any food, lies in improper handling and storage, leading to potential cross-contamination. As long as you practice good kitchen hygiene and store leftovers correctly, you can safely continue to enjoy onions for their flavor and nutritional benefits. The next time you hear this old wives' tale, you can confidently explain why it's simply an urban legend. For more authoritative information on this topic, consider visiting the National Onion Association's website.

Frequently Asked Questions

No, you cannot get food poisoning from a properly stored cut onion that was left in the fridge overnight. In fact, onions contain antimicrobial compounds that help inhibit bacterial growth. The risk of food poisoning comes from external contamination, not the onion itself.

No, the myth about onions absorbing germs during the 1919 flu epidemic is not true. This idea was based on a misunderstanding of how diseases spread and has no scientific basis. Cold and flu viruses are spread through contact, not by floating in the air for an onion to absorb.

To store a cut onion safely, place it in an airtight container or a sealed plastic bag in the refrigerator. This will prevent it from drying out, absorbing odors, and getting contaminated. It is best to use it within seven days.

If you leave a cut onion uncovered on the counter, it won't become 'poisonous,' but it will likely lose its flavor, dry out, and potentially transfer its odor to other foods. It may also eventually develop mold if left for too long.

Yes, some people may experience issues with onions. Individuals with IBS may be sensitive to the FODMAPs in onions, causing digestive upset. Allergic reactions, though rare, can also occur. Onions are not inherently toxic to humans in a food context.

Onions contain compounds called thiosulphates that are toxic to dogs and cats. These compounds cause oxidative damage to their red blood cells, leading to a serious and potentially fatal condition called hemolytic anemia. Never feed onions to pets.

If you find mold on a leftover cut onion, you should discard it immediately. While the onion itself isn't toxic in the way the myth suggests, mold indicates spoilage and the potential presence of harmful bacteria, just like any other food.