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

  • Ruth

    My husband had a horrible cough after he had the flu. I cut the ends of the onion off. He has not coughed at all today. Seems to be getting his strength back. There must be something in the raw onion.

  • Shannon

    That doesn’t mean the onion myth worked. Those 4 people just didn’t get the flu. We’ve had the flu (confirmed) in our house 3 years in a row. One year it was my oldest, then my second oldest the year after, then my oldest the third year. We didn’t do anything to prevent the rest of us from getting sick and we even had our flu “patients” on the couch in the living room where everyone else was. No one else got sick. Now if we had done your little onion trick I guess we could have said it was that, but the fact was that for whatever reason the flu virus never got passed on.

  • Kris

    I heard the onion story for the first time a few days ago and thought it was ridiculous. I remembered my maternal grandmother making necklaces for my siblings and I to wear. She used clove and an unpeeled garlic to keep our colds from getting worse. She was our guardian for a couple of days. When my parents returned one evening, my father, after grandmother had left the house, proclaimed the necklaces as wives tales and took our them away. I rather liked the clove smell of our fake protection necklaces. If onions and garlic actually protected people from all ills, no one would be sick. However, if a person wants to use these large placebos, I say go for it.

  • nothinggirlJane

    I LOVE that whenever ‘science’ or ‘scientists’ cannot explain or comprehend something, it is proclaimed to be false, and even silly.

    When the coming years CEASE to prove popular science theories of decades Past as false and silly, then I may take these ‘scientists’ word on the subject.

    Until then, I’ll try it, and decide what I will choose to believe myself.

  • jojo sadat

    is true that when you eat onions is dangerous and why if so

  • RavenousWolf

    My very old mesquite trees carry a bacteria that makes poison
    Ivy itch feel good. I have tried everything from gasoline to various
    Ointments on the market and nothing has worked for me until I
    recently put on my skin fresh cut onion juice. I apply it to my eyelids
    at night. I keep 3 dishes of sliced onions in my bedroom and
    sleep like a baby. I haven’t had flu in decades. I love onions.
    Prayer and praise led me to “onions”!!!

  • kate

    The aromatherapy benefits ate what your looking for, and it us quite possible that normal airflow across the room would be enough of a propellant… Scientifically it is completely plausible that minor air currents produced by your furnace fan a nd normal movement would be enough to move the bugs around…

    As for the rest, ill take a flu preventative. That is.nt chemical, doesn’t disturb my biochemistry , won’t wreck my hormonal balance or lower my natural immune system nad is compleltely evironmentally friendly.

  • Jess

    Whether science can prove it does or doesn’t work. I definitely think the power of belief is very strong here. Science can say they’re not finding the onion full of air-absorbed bacteria. But something seems to have been working for hundreds of years. I’d like to think maybe it is the sulfur or odor it’s producing that we breathe in. Or maybe it really is just the power if our minds believing in it. Look at healing magnets, they only work for people who believe in them. Just trying this out is enough to let your brain think it works & that truly is enough to actually have it work. The power of the mind is a great thing. Placebo drugs are enough proof that mind is over matter. I believe in the onion. 🙂

  • Brandy

    I have to say that placing chopped onions around my home did work. 2 of the 6 people that live in my home had the flu. We placed the onions, chopped in bowls, in every room of the house. We left them there for a week or maybe 5 days. The other 4 did not get the flu.
    It made our home smelly but kept the rest of us from getting the flu. I believe there is something in the onion or it’s aroma that kept the rest of us from getting sick.
    Whether it was “absorbing” the virus or the aroma boosting our immune system. It doesn’t matter, it worked for our family and I will do it again.

  • wibble

    I wonder if the onion, by irritating the lining of the nose and throught, produces more liquid that helps protect the body against infection ??

    Isn’t that why we sneeze ?

  • rob mcconnel

    I read this article and thought I would share my experience with onions
    Several years ago when the panic about flus was leading many people to get flu shots ,a co -worker suggested he bring in a onion and cut it in half and put in a bowl in our offices and this was done half each.
    The most interesting thing happened that season our office complex has 5 offices served by a shared heating system not one of us got sick that was served by the common heating system
    But a bit more of myself I have had colds every season someone sneezes I get the virus however for whatever the reason I and four other staff bypassed the colds that year . And for the record all the other staff where sick And I do mean sick so my new co-worker wants to try the onion idea treatment
    Must confess at first when we did the onion in the bowl we were the brunt of all comments and jokes
    So I say what have you got to loose can you imagine the loss of revenue to the doctors and drug stores if onions proved to prevent colds?
    One last comment the professionals are indicating the onions ability to avoid colds for the most part as unproven, Guess got to ask the question show me the proof that the onion doesn’t help until the onion works for me

  • Fiona

    It was interesting to read your article. I also put cut onions in my room and my daughters room as she and her gran, who is sharing the same room have a very bad cough & flu. My daughter seems to be much better today, tho her gran’s voice is a little more hoarse.
    I have tried the onions before as well and found it has helped.

    I had been wondering about the cut onions I have put in the fridge, as I have used them a few weeks later and they seem to be fine.
    I wonder if they are left cut in the open air, they do absorb bacteria?

  • Kathleen

    I must say though, that even with your research I did try the urban tale last night since my little boy has been suffering from a hacking cough and cold for about a week. I placed an cut open onion next to him last night after he was suffering a long period of hacking, after about 30min the hacking stopped and he did not cough the rest of the night, in the moring he stated feeling better and has only coughed a few times. Still I must think there is something behind this.

    • April

      Any aromatic effect an onion may have will be strictly aromatic and not as a result of absorption of any bacteria or virus. While I have not personally tried it I can see how the aromatic properties might benefit a cough, cold, or asthma (which I have) … but the smell would probably drive me up the wall and I know my husband wouldn’t stand for it not to mention the insects it would draw in our house.

    • nate rey

      This is a very persistant folk remedy. I always give ‘old wives’ tales’ respectful consideration. After all, as a demographic, old wives have demonstrated enuff sense to survive to seniority, while raising a family!

      Science simply doesn’t support the supposed bacterial/viral absorption mechanism.

      First, germs, and especially viruses, do not exist as a ‘swampy sea’ in the air of a sick room. They cannot survive in air for any appreciable time. Second, even if they did/could, they have means of propulsion to suck themselves to their doom on the onion, like moths to flame. The onion would have to be a filter in a multi-CFPM air purifier!

      And thirdly, even granting this, it wouldn’t have helped your son. He had to deal with the infection already resident in his system.

      Yet, apparently it works, well enuff to persist in folk medicine.

      So there must be another mechanism, which must derive from the known, potent antimicrobial chemistry of onion, and, for that matter, garlic.

      Is it sufficiently aromatic to function in a ‘salsa’ at bedside?

      If so, I’d go one further, and breathe a puree out of a food mill, several times daily, and place it in front of a small fan in the sickroom.

      Better still, take 3 gm vitamin C, and 6000 IU vitamin D, daily, and don’t get in the sickroom to begin with!

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