Hoaxes & Rumors

Peeling Back Rumors on Onions, Bacteria, and Dogs

Peeling Back Rumors on Onions, Bacteria, and Dogs

Today we examine online rumors which claim that onions can absorb germs, leftover onions are dangerous, and onions are toxic to dogs. Is this true or false?

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Online Rumor

The text below was floating around Facebook in early 2012, and has been online for several years.

First let’s read the text being circulated:


In 1919 when the flu killed 40 million people there was this Doctor that visited the many farmers to see if he could help them combat the flu…Many of the farmers and their families had contracted it and many died.

The doctor came upon this one farmer and to his surprise, everyone was very healthy. When the doctor asked what the farmer was doing that was different the wife replied that she had placed an unpeeled onion in a dish in the rooms of the home, (probably only two rooms back then). The doctor couldn’t believe it and asked if he could have one of the onions and place it under the microscope. She gave him one and when he did this, he did find the flu virus in the onion. It obviously absorbed the bacteria, therefore, keeping the family healthy.

Now, I heard this story from my hairdresser. She said that several years ago, many of her employees were coming down with the flu, and so were many of her customers. The next year she placed several bowls with onions around in her shop. To her surprise, none of her staff got sick. It must work. Try it and see what happens. We did it last year and we never got the flu.

Now there is a P. S. to this for I sent it to a friend in Oregon who regularly contributes material to me on health issues. She replied with this most interesting experience about onions:

Thanks for the reminder. I don’t know about the farmer’s story…but, I do know that I contacted pneumonia, and, needless to say, I was very ill… I came across an article that said to cut both ends off an onion put it into an empty jar, and place the jar next to the sick patient at night. It said the onion would be black in the morning from the germs…sure enough it happened just like that…the onion was a mess and I began to feel better.

Another thing I read in the article was that onions and garlic placed around the room saved many from the black plague years ago. They have powerful antibacterial, antiseptic properties.

This is the other note.

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 and flu and is the very reason we shouldn’t eat an onion that has been sitting for a time after it has been cut open.


I had the wonderful privilege of touring Mullins Food Products, Makers of mayonnaise. Questions about food poisoning came up, and I wanted to share what I learned from a chemist.

Ed, who was our tour guide, is a food chemistry whiz. During the tour, someone asked if we really needed to worry about mayonnaise. People are always worried that mayonnaise will spoil. Ed’s answer will surprise you. Ed said that all commercially-made mayo is completely safe.

“It doesn’t even have to be refrigerated. No harm in refrigerating it, but it’s not really necessary.” He explained that the pH in mayonnaise is set at a point that bacteria could not survive in that environment. He then talked about the summer picnic, with the bowl of potato salad sitting on the table, and how everyone blames the mayonnaise when someone gets sick.

Ed says that, when food poisoning is reported, the first thing the officials look for is when the ‘victim’ last ate ONIONS and where those onions came from (in the potato salad?). Ed says it’s not the mayonnaise (as long as it’s not homemade mayo) that spoils in the outdoors. It’s probably the ONIONS, and if not the onions, it’s the POTATOES.

He explained onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion.. He says it’s not even safe if you put it in a zip-lock bag and put it in your refrigerator.

It’s already contaminated enough just by being cut open and out for a bit, that it can be a danger to you (and doubly watch out for those onions you put in your hotdogs at the baseball park!). Ed says if you take the leftover onion and cook it like crazy you’ll probably be okay, but if you slice that leftover onion and put on your sandwich, you’re asking for trouble. Both the onions and the moist potato in a potato salad, will attract and grow bacteria faster than any commercial mayonnaise will even begin to break down.

Also, dogs should never eat onions. Their stomachs cannot metabolize onions. Please remember it is dangerous to cut an onion and try to use it to cook 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.

Please pass this on to all you love and care about.

Three Claims About Onions

The quoted rumor above makes many claims, but we mainly have three distinct issues here:

  • Can an unpeeled onion absorb germs in the air?
  • Are leftover onions dangerous?
  • Are onions dangerous to dogs?

The above text appears to have combined multiple onion-related stories into a sort of “consensus” of onion-based rumors. The first section – relating to leaving an unpeeled onion to fight germs – has many of the characteristics of a classic urban legend, with the vague “doctor” and the hairdresser “friend” corroborating the story. It’s interesting to try to reconcile the “onion absorbing germs” part of the story with the “eating leftover onions is unsafe” part. There is a flimsy attempt to imply that both are related to the way an onion absorbs germs. If we read the above text, we’re to believe that because an onion attracts germs, it keeps them out of the air, but eating an onion is bad because of this absorption.

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What is also implied, however is that when we have an unpeeled onion absorbing germs out of the air, those germs remain alive on the onion indefinitely. Didn’t the first part of the text state that onions have powerful antibacterial, antiseptic properties? These properties should kill the germs, right? Studies have shown that onions are in fact antimicrobial in nature, along with a host of other health benefits. That doesn’t mean that an onion sitting on a table will attract these microbes, nor would it retain them indefinitely.

Can an Unpeeled Onion Absorb Germs in the Air?

As mentioned above, this claim has many of the hallmarks of a typical urban legend, and there is no scientific evidence to support such a statement.  A November 2009 article in the Wall Street Journal directly addresses this online rumor, and states that it is unreasonable to believe onions attract viruses out of the air. It should also be noted that the author of the above text doesn’t appear to know the difference between bacteria and viruses, as they are used interchangeably. Viruses have no mechanism for mobility and rely on external sources such as being propelled by a cough or sneeze. Even if the air were full of flying viruses, it doesn’t stand to reason that they would all find an onion before a human or another living host.

If an unpeeled onion sitting out had any benefits, wouldn’t those working around onions have noticed these benefits? Millions of people work around produce such as onions every day. If simply being around them in their raw form provided such miraculous benefits, these workers should all be thriving. No studies have ever supported this.

Are Leftover Onions Dangerous?

I’ve found no evidence that leftover onions are any more dangerous than any other food of its kind. Certainly all food spoils at some point, but the onion doesn’t display unique properties in this regard as it relates to human safety. This isn’t to say that onions, or any other food item, can’t contain pathogens, but outbreaks traced back to onions often end up relating to viruses transferred onto them in the handling process.

Are Onions Dangerous to Dogs?

This part of the claim is somewhat true. In fact, the American Society for the Prevention of Cruelty to Animals (ASPCA) reports that onions and garlic are toxic to both dogs and cats, as it can damage their red blood cells which may result in the development of hemolytic anemia. For some scientific information on this subject, view actual science studies here and here.

The fact that onions are dangerous to dogs and cats is related to their inability to tolerate toxins within onions, and not the onion’s interaction with germs. Human beings sweat these toxins out which is why we sometimes smell like onions or garlic long after eating them. It has also been established that chocolate isn’t good for dogs, yet we don’t see humans avoiding chocolate or assuming it’s unsafe to eat for this reason.

Bottom Line

The benefits of onions are not accurately described in the first part of the quoted claim, and many of the “dangers” of onions are completely false. Onions do not attract germs out of the air, and it is completely safe to eat leftover onions as long as they haven’t spoiled. However, it is true that onions can be toxic to dogs due to their inefficiency in eliminating certain toxins.

Updated October 10, 2014
Originally published February 2012

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