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Hygiene hypothesis: When it comes to preventing asthma and other autoimmune diseases, can "clean" be "too clean"?

Asthma affects 35% - 50% of American kids, making it one of our most common chronic childhood illnesses. So, obviously, anything that can be done to help prevent asthma would benefit millions of us and save lives of thousands each year.

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Question: Who would you guess is most likely to suffer from asthma by age 3:


A) A child, raised to the age of 1 in a very clean home.
B) That same child, raised their first year in a home with high levels of dust, cockroaches, cat dander and mouse droppings.

Perhaps you are familiar with the Hygiene Hypothesis, that by raising young children in "too clean" of an environment (providing clean food and water, vaccines, antibiotics when appropriate, good sanitation, etc.) we may inadvertently be preventing the child's immune system from developing tolerance to irritants and, the thought goes, that this may be related with the dramatic increase since the 1960's in cases of asthma, allergies and an assortment of autoimmune diseases related to inflammation.

To test this argument, Johns Hopkins Children's Center in Baltimore recently followed 467 children from pre-birth to age 3. What was of greatest interest to the researchers was how bacteria and allergen exposure during the first year of life might translate into the children later developing allergies and asthma. What they found surprised even them. Infants raised in households that were abundant in dust (source of bacteria), cockroach and mouse droppings and cat dander (allergen sources) suffered dramatically lower incidents of wheezing and allergies (which can be precursors to asthma) as well as asthma. In fact, in this study, those infants living their first year in the homes with the highest presence of all three allergens and bacteria were allergy and asthma free at age 3.

So, why do inner city children (who are probably exposed to higher levels of bacteria and allergens) report some of the highest rates of asthma?

Other factors that can contribute to the development of asthma include: 1) exposure to a variety of allergens and bacteria after age 1 (since the protective properties of bacteria and allergen exposure does not appear to benefit older children and adults) and 2) inner city children tend to have higher exposure to other risk factors such as household stress, tobacco smoke, and pollutants.)

In addition to dust bacteria and the three allergens highlighted in the Johns Hopkins study, there can be many possible triggers of asthma flare-ups:

dust mites
animal dander (dandruff)
food additives

paint fumes
aerosol sprays
infection like bronchitis, pneumonia a cold or flu
extremely dry, wet or cold air


The most common explanation for the hygiene hypotheses is that, a toddler's immune system, which has been suppressed before birth to avoid rejecting maternal tissue, must be educated to recognize and respond to germs and viruses, allergens, intestinal parasites and other infectious organisms. The forgotten friends theory is another possibility, however, stating that the absence of friendly bacteria, that would normally populate our gastrointestinal tract and airway mucosal surfaces, are also being eliminated with the improved hygiene practices. The human gut contains over 100,000,000,000,000 (1014) bacteria from over 1000 different species which play an important role in immune development and providing intestinal protection. A comforting aspect of this theory is that it allows us to keep our homes and daycare facilities clean without compromising our children's health.

Studies involving farm vs. city children, the sudden modernization of East Germany and others support the Hygiene Hypothesis -- but many questions still remain. Almost universally, however, researchers agree that, now that medical advancements have helped to prevent many major infectious diseases, contaminating children or adults with infectious agents is obviously not recommended. Until science better understands the relationship between allergies, asthma and immune development, preventable exposure to germs and parasite is likely to do more harm than good.

Update Feb. 1, 2016: Related to this topic is a recently published study in Nature Medicine. Because babies born by C-section are at higher risk for allergies, obesity and other health problems, researchers suspected that the delivery process may be "too clean." They found that, by immediately slathering babies born by C-section with their mother's birth-canal fluids, they were able to restore microbe populations to more closely resemble that of natural-birth babies. The hope is, that this bacterial exposure may help regulate normal metabolism and train the immune system for C-section babies.

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Join in the conversations
Showing comment(s)
October 18, 2014
Very interesting. I wonder if probiotics, like in yogurt, would be beneficial?
James at WisconsinHealth.org
October 19, 2014
Thank you for your comment, Jessica. This was considered in one of the studies we cited to in the article above (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2841828/).

Since probiotics have an anti-inflammatory effect, they could account for "the relief of symptoms without changes in disease progression, as implicated in the hygiene hypothesis." They went on to say that "there are data suggesting that probiotics may have a favorable role in immune disorders, but the case is far from proven."
February 23, 2015
You explained this very well. You may enjoy reading this related article in the NY Times which supports the hygiene hypothesis: http://well.blogs.nytimes.com/2015/02/23/allergy-risk-may-be-tied-to-how-you-wash-your-dishes/
James at WisconsinHealth.org
February 23, 2015
Wonderful article, Jake. Thank you for sharing it. And how coincidental that it touches on the probiotics (fermented foods) issue that Jessica mentioned in the previous comment!
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