Health Tips: How Onions Can Turn Into Toxic & Poisonous

You might read websites doling out warnings like this: “Please remember it is dangerous to cut an onion and try to cook it the next day, it becomes highly poisonous for even a single night and creates toxic bacteria which may cause adverse stomach infections because of excess bile secretions and even food poisoning.”

And this: “Onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion. It’s not even safe if you put it in a zip-lock bag and put it in your refrigerator.”

Also: “Lots of times when we have stomach problems we don’t know what to blame. Maybe it’s the onions that are to blame. Onions ABSORB BACTERIA is the reason they are so good at preventing us from getting colds & flu and is the very reason we should NOT eat an onion that has been sitting for a time after it has been cut open.”

The sentiments vary in wording, but the overall gist is clear: stay far, far away from cut, raw onions. They suck up bacteria from the air and become poisonous, dangerous, illness-inducing orbs.

The “science” can start to seem somewhat legitimate, and the fact that there are so many different sources spewing this claim adds to its credibility. On the flip side, like many of my friends, have been eating onions practically my entire life and have yet to become terminally ill, or even sick. I decided to do some digging.

Are cut, raw onions poisonous or toxic?

For starters, the chemical makeup of onions just doesn’t support bacteria growth, she explained. Their low pH (i.e. acidic nature) and low protein content mean they are not an ideal breeding ground for germs, viruses or other pathogens.

In fact, the opposite is true: onions contain compounds that have antibacterial properties.

“When cut, onions release compounds that do not promote pathogen growth,” the National Onion Association said in a statement published on their website. “The Center for Food Safety at the University of Georgia states, juice released from cut onion is known to kill or inhibit the growth of several types of microorganisms, including some of those capable of causing food poisoning in humans.”

According to the National Onion Association, this onions-are-poisonous myth originates from several different sources, including a folk belief from the 1919 influenza epidemic that claims cut onions left around the house will combat the flu virus, and a 2008 blog post via Dinner With Zola that purports onions and potatoes cause more food poisoning than spoiled mayonnaise. The blog post, which has since been deleted, triggered Chicken Little-esque chain emails and sensationalized warnings that although officially debunked, still exist on the internet today.

So when it comes to raw onions—and onions in general—is there anything humans should be worried about?

“There is nothing unique about the poor little onion,” Jeff Nelkin, food safety specialist, told me. “The only introduction of poison and bacteria would be strictly environmental [e.g. from contaminated soil or unhygienic food prep.]”

Like any other type of food grown in soil, there is the (very rare) potential for onions to have E.coli, salmonella or other viruses that can be spread via toxic fertilizer, Nelkin explained. But this risk is quite low, and not greater in onions than in any other type of food pulled from the ground.

How long do cut onions last? 

Overall, there are “no safety precautions that are unique to an onion that wouldn’t hold true to apples, carrots or anything other type of produce,” confirmed Steinberg. “All of same rules apply for anything else you would get out of a garden.”

These rules include thoroughly washing your hands and kitchen tools and wiping down counters with sanitizer before beginning the food prep process so that you don’t introduce outside contamination, recommended Nelkin.