The Hygiene Hypothesis: Are Dirty Children Healthy Children?

Is Grime Good For Our Kids? We are a nation obsessed with cleanliness, convinced that dirt is dangerous. We carry hand-sanitizer (“hanitizer” according to my 4-year-old), and we see sanitizer dispensers everywhere: schools, gyms, the library. And that seems reasonable. After all, we transmit fewer bacteria between us when we sanitize our hands on a regular basis. The practice of hand-washing has saved millions of lives, thanks to Dr. Semmelweis.

On the other hand, some doctors are beginning to insist that exposure to a range of microbes is not only safe, it is healthy. “Grime is good for you,” they say.

It’s called the “hygiene hypothesis,” and it goes like this:

Microorganisms like viruses and bacteria that enter the body along with “dirt” boost the development of a healthy immune system. Think of it as “training the immune system,” or immunological “exercise.” That’s right: working-out for your immune system.

According to Dr. Mary Ruebush, an instructor of microbiology and immuncology for Kaplan Medical (pre-med preparation courses), “What a child is doing when he puts things in his mouth, is allowing his immune system to explore his environment.”

Healthy Children, Hygiene Hypothesis

Image: Making Mud Pies

Dirty Children are Healthy Children

Those scientists like Dr. Ruebush who are proponents of the hygiene hypothesis insist that we should not be so quick to wash dirt off our kids. They believe that we are impeding development of their immune systems. Really. “Without exposure, they cannot develop their immune responses,” insists Ruebush.

And Clean Children are Unhealthy?

So what happens if we don’t let our kids explore their environment with their mouths, essentially eating dirt? Well, these scientists believe that our obsession with sterility has caused the increases in allergic disorders – hay fever and eczema – and may be the problem underlying autoimmune disorders including diabetes, multiple sclerosis, and various inflammatory bowel disorders.

Could this be true? What evidence is there to support the notion that grime is good for us?

Since the statement of the hygiene hypothesis in 1989, there have been a number of studies that seem to support the notion that we are being a bit too clean. Here are just two examples:

  • Babies that receive antibiotics within the first year of life have a higher incidence of asthma and other allergic disorders
  • Antibacterial cleaners – the ubiquitous hand sanitizers – have been linked to increasing asthma

Of course, these observations may be true, but not caused by the link. For example, it may be that babies that need antibiotics within their first year of life are generally less healthy than those who don’t need antibiotics; and those less-healthy babies are more likely to develop asthma. And that their development of asthma actually had nothing to do with the fact that they received antibiotics within their first year of life, right? We simply don’t know, and those studies do not help answer this question: they merely make the observation; they don’t tell us the cause.

Cell and Molecular Basis …

What we really need to understand the observed links is to learn the underlying cellular and molecular links. We are just beginning to learn these:

Recent studies may be starting to unravel the molecular basis for these observations that support hygiene hypothesis. For example, reduced exposure to microbes results in an increase in the immune cell type called “natural killer cells;” this, in turn, predisposes to colitis (inflammatory bowel disorders) and asthma. This was nicely reviewed in a brief article by Dr. Umetsu, of Children’s Hospital of Boston, Harvard Medical School, in a recent issue of the journal, Cell Research.

Evolutionary Advantage?

Healthy Children, Hygiene Hypothesis

Image: Exploring the World With the Mouth

Finally, when one considers that most behaviors – especially behaviors of the youngest of a species (including human babies) – have been selected by evolutionary advantage, it seems reasonable that an infant’s exploration of the environment with their mouth must have some advantage. That is, it’s good for a kid to get some grime in their mouth, if not literally eat dirt.

We co-evolved with these microorganisms. They are part of our “microbiome,” and we are co-dependent for our optimal health. The reduction in these microorganisms from our modern urban environments is the cause of our immune dysregulation, according to a modern restatement of the hygiene hypothesis. Dr. Rook articulates this argument very persuasively.

If (and that’s a big “if,” I’m still a bit skeptical) we accept that grime can be good for development of our kids’ immune systems, now what?

Here are some tips:

  1. Use plain old soap and water. Dispense with the “antibacterial” disinfectants, and especially eliminate products with Triclosan. These experts say that over-killing microbes in our environment (including on us and in us) can result in growth of bad pathogens. Think of it as killing off the good guys, allowing the bad guys to move in. “But what about super-bugs like MRSA,” you ask? When it comes to fighting off dangerous bugs, nothing beats a strong immune system and good-ole soap and water.
  2. Ban the Triclosan. Cleaner hands do transmit fewer bacteria, but keep in mind that the vast majority of common illnesses are caused by viruses. Note that anti-bacterial soaps, disinfectants, sanitizers, and other anti-bacterial products that include Triclosan will have no effect on viruses. And: have you ever heard the phrase, “what doesn’t kill you makes you stronger”? Well, that’s true for bacteria, too. Using these anti-bacterial agents everywhere in our environment, along with prescribing antibiotics for every little sniffle or ear ache, has resulted in a world full of superbugs! Those few bugs that weren’t killed became stronger.
  3. Allow minor illness to run its course! Of course, as parents we want to do everything possible to relieve our child’s misery when they are ill. But that course of antibiotics that we brow-beat the pediatrician into prescribing for Pooky’s ear ache only makes us feel better; it doesn’t change the course of the viral infection. It doesn’t actually make your child feel any better. It does, however, result in more antibiotic-resistant bacteria (see #2, above).

Me? I am guilty: I am obsessed with my children’s cleanliness. I blame my surgical training. But I am trying. If you believe the hygiene hypothesis, perhaps you should too?

Healthy Children, Hygiene Hypothesis

Image: International Mud Day

Please leave a comment and let me know your thoughts.


For further reading, see the links below.


International Mud Day:

Marra, et al. 2006: Does antibiotic exposure during infancy lead to development of asthma? A systematic review and metaanalysis. Chest 129: 610-618:

Zock, et al. 2007: The use of household cleaning sprays and adult asthma: an international longitudinal study. American Journal of Respiratory and Critical Care Medicine 176: 735-741:

Hygiene Hypothesis Reviewed in 2012, in the journal, Nature Immunology vol 13(5), pg 437.

Recent expression of the hygiene hypothesis is well articulated by Dr. Graham A. W. Rook, Professor of Medical Microbiology at the University College London:

If you want some great reading on this topic, check out the book by Jessica Sachs, Good Germs, Bad Germs, from 2008:


Hi, I’m Russell Faust, author of this medical education blog.

Russell Faust, PhD, MD boogordoctor / healthy children

Image: Dr. Faust and Friend

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  1. How do you keep a nose from swelling due to cold or allergies? My 4 year old went to an ENT, and he said he had allergic rhinitis. He also said that he had caught a sinus infection from someone. We have been clear for 7 weeks, and now he said he doesn’t have anything wrong with him. We have been battling with his nose infections for over 2 years now. When he gets a cold his nose never runs, how do I prevent a cold from going into a sinus infection?

    • Russell A. Faust, PhD, MD says:

      Hi Kelly,
      Sounds painfully familiar – that was my own condition for decades: it seemed that every viral ‘cold’ that I got just extended into a full-blown sinus infection. And my baseline is allergic rhinitis. How did I get over that cycle of chronic rhino-sinusitis?
      Note that nearly all of these products are affiliate links through my Amazon Store – not because I make so much money from them (the price is the same to you either way), but because they are all products that I use myself, for our own children, and that I recommend to my patients, so I put them into one place – my Amazon Store.

      Here is my approach:
      (1) A few things, but number-one is nasal saline rinses! Pretty challenging for a 4-year-old (I know, have been struggling with my two boys from ages 3-7). Worth the effort. Be sure to check out for help with rinsing little ones. Dr. Hana is an expert, check out her videos! This is THE MOST effective step, but also the most challenging.
      (2) include Xylitol in the nasal saline rinse, or use as a nasal saline spray that contains Xylitol (check out Available through my Amazon Store:
      (3) consider adding a high-quality probiotic to your son’s daily regimen. We give our kids (and ourselves) the Probiotic Pearls by Integrative Therapeutics. They are enteric coated, and make it through stomach acid; most probiotics for kids are killed by stomach acid, and therefore provide no benefit. Probiotic Pearls are through my Amazon store affiliate link:
      (4) finally, consider adding a natural anti-inflammatory agent to his regimen, like Quercetin. We use the D-Hist-Jr by Ortho Molecular (also on my Amazon Store:

      So, those are the steps that helped me finally stop the cycle of viral-cold-sinusitis-viral-cold-sinusitis: saline rinses / xylitol / probiotics / anti-inflammatory.

      For truly ornery chronic rhinosinusitis, there are some further tricks for the saline rinses. But this should do the trick for the vast majority of children with CRS!
      Thanks so much for visiting, and for sharing.
      Best success!

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