The hygiene hypothesis - are we too clean?


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The hygiene hypothesis – are we too clean?


The hygiene hypothesis suggests that we have evolved to be exposed to a range of different microorganisms, and when we alter this exposure through an excessively sterile environment we can compromise our immune tolerance.

Microbes - friends or foes?

When we discovered a world of living microorganisms, the ‘germ theory of disease’ quickly led to a war against them. They were considered disease-causing agents, and nothing more.

Since that time, we have discovered the human microbiome and recognised the complexities of the microbial world: a mixture of beneficial and pathogenic microbes and those that lie somewhere in between.

The knowledge of microorganisms has helped us to improve hygiene conditions and reduce the transmission of many infectious diseases, dramatically improving our quality and length of life.

But now, years on, the question is being asked: have we gone too far?

The hygiene hypothesis suggests that in some ways, we have.

What is the hygiene hypothesis?

Increased use of antibacterial and antimicrobial cleaning agents and sterilisation of our environment is thought to be increasing the prevalence of allergic diseases, such as asthma, eczema, hay fever and food allergies.

Exposure to a variety of different microbes is critically important for the development of a healthy immune system which can tolerate food-borne antigens (which have the potential to become allergens).

Microbes educate our immune system

Microbes in our gut interact with the immune cells lying just below the surface of intestinal cells, and help to educate the immune system. This exposure helps us to distinguish between friendly beneficial bacteria and food-borne antigens, and potentially harmful pathogenic microbes.

We have evolved to be exposed to a range of different microorganisms, and when we alter this exposure through an excessively sterile environment we can compromise our immune tolerance. When this happens, we can see overreaction of the immune system and development of hypersensitive allergic reactions.

Dogs and dirt protect against allergies

Interestingly, a study published in The Journal of Allergy and Clinical Immunology found that owning pets, specifically dogs, is protective against eczema. The microbiome of our pets is thought to contribute to our own microbial diversity, protecting us against allergies!

Likewise, differences have been found in those raised in the country versus the city where exposures to dirt and the outside world are very different.

A disappearing microbiome

Alongside increasingly sterile environments, the extensive use of antibiotics is also contributing to what some are calling a ‘disappearing microbiome’ - a loss of the diversity and abundance of our microbial communities.

The exact implications of this are still unclear, but we do know that allergic disease is on the rise and a diverse and abundant gut microbiome is a predictor of health.

One thing is clear: while we try to find the balance between clean and too clean, we need tools to support the restoration and maintenance of a healthy gut microbiome.


Blaser MJ. The theory of disappearing microbiota and the epidemics of chronic diseases. Nat Rev Immunol [Internet]. 2017;17(8):461–3.

Degruttola AK, Low D, Mizoguchi A, Mizoguchi E. Current understanding of dysbiosis in disease in human and animal models. 2016;22(5):1137–50.

Pelucchi C, Galeone C, Bach J-F, La Vecchia C, Chatenoud L. Pet exposure and risk of atopic dermatitis at the pediatric age: A meta-analysis of birth cohort studies. J Allergy Clin Immunol [Internet]. 2013 Sep;132(3):616-622.e7.

Risnes KR, Belanger K, Murk W, Bracken MB. Antibiotic Exposure by 6 Months and Asthma and Allergy at 6 Years: Findings in a Cohort of 1,401 US Children. Am J Epidemiol [Internet]. 2011 Feb 1;173(3):310–8.




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