Parents often believe that the environment that their children live in should be as clean as possible. However in order for a child’s immune system to develop, it needs to be exposed to germs. When a child is in the womb, it is protected by its mother’s immune system. When children are born, they have to develop their own immune system. This is done through exposure to germs so that we can educate our immune system to recognise them. Research has shown that children who are raised in a very clean environment are actually more likely to develop hay fever, asthma and allergies; this is termed the ‘hygiene hypothesis’1-5.
The premise is that – in the developed world at least – we are too clean. From a young age we are protected too much from dirt, bacteria and other germs. Because of this, our bodies and our immune system do not get the education they require to distinguish friend from foe. This can result in the body’s immune system attacking itself leading to autoimmune diseases, or in the case of allergies, the immune system over-reacting to a common substance such as pollen or animal dander.
What is the Hygiene Hypothesis?
In rural and farm environments children are exposed to a wide range of different bacteria, pollens, and animal dander. This early exposure can have positive effects on the education of the immune system. Studies have shown that children who grow up in a rural setting, exposed to these environmental factors from a young age, suffer less from allergies and asthma8,9.
The relationship between the farm effect and the hygiene hypothesis was highlighted this year at the European Academy of Allergy and Clinical Immunology (EAACI) Congress in Muncih, Germanyin a session called “Country roads to allergy prevention” led by Prof. Erika von Mutius, a pediatric allergist and pioneer in the field. The speakers in this session discussed their work in identifying what effects these environmental exposures have on our immune system and how we can take advantage of that knowledge to protect children living in urban environments – click here to read ASL takeaways from EACCI 2018.
Despite the increased awareness of environmental effects on our immune system, there is a genetic element that you can’t escape, and recent research indicates that the hygiene hypothesis is too simplistic10,11. However providing a sterile environment for children to grow up in will not assist them in terms of developing a healthy immune system.
How Clean is Too Clean?
Strict cleanliness is absolutely important in some environments, like using hand steriliser in hospitals. Using the same hand steriliser on a routine basis in the home will actually damage the normal bacteria that inhabit your skin. When you’ve cut raw chicken, clean and sterilise; when you have a wound on your hand, clean and sterilise; however when your children are coming in from the garden before eating, cleaning with normal soap is absolutely sufficient and no anti-bacterial or sterilising soaps are required.
How Does the Hygiene Hypothesis Relate to Asthma and Allergies?
Both allergenic asthma and allergies are triggered by allergens. An allergen is a usually harmless substance that causes an immune response and allergic reaction. A poorly educated immune system identifies an allergen as a foreign or unwanted molecule and sends its immune defense mechanisms to fight off the invasion. The immune system over-reacts to this allergen, leading to allergic symptoms and even asthmatic exacerbations.
The prevalence of asthma and allergies has increased significantly in the past two decades. The Asthma and Allergy Foundation of America reported that allergies may affect as much as 30% of adults and 40% of children in the United States12, and that about 25 million people in the United States suffer from asthma13. This increase is reflected globally, but predominantly in developed countries12. The hygiene hypothesis is a possible explanation for this rise as we move towards more urbanised living with homes and farms are now separated to a greater extent. This does not mean that we should down tools and stop cleaning our homes, invite in farm animals, drink unpasteurized milk or even stop washing ourselves in an effort to educate our immune systems. Importantly sanitation has led to reductions in the cases of typhoid and cholera and other childhood diseases in the developed world. Furthermore, the education of our immune system happens when we are very young, and is more than likely affected by exposures such as vaginal birth, breast feeding and antibiotic use14.
So What To Do for People with Asthma and Allergies?
Following a diagnosis of allergic asthma or development of allergies, your approach to allergen avoidance and cleanliness unfortunately does have to be quite different. At this stage you are already sensitised to a specific allergen, and exposure to those allergens could cause a reaction. In this case, guidance on the “Diagnosis and Management of Asthma” from the national heart, lung and blood institute at the National Institute of Health in the US is to:
Control exposures to allergens and pollutants or irritants that make asthma worse… advise patients to reduce exposure to allergens and pollutants and irritants to which they are sensitive15.
This advice has recently been corroborated by a comprehensive review of the literature published in the journal of allergy and clinical immunology16. The study confirmed that multicomponent interventions that include HEPA vacuuming and pest control with other interventions such as air cleaners or mattress covers can reduce asthma exacerbations and improve quality of life.
The hygiene hypothesis is still a working theory, however there is clear evidence that our immune system and our general health is affected by our environment as well as our genetics. Future research will look to take advantage of these findings and help young children to develop stronger, well-educated immune systems. In the meantime, cases of asthma and allergies are increasing globally and for these individuals effective environmental controls in the home can help to reduce exposure to allergens and improve their quality of life15.
About the author
Thanks to Dr. Tim Yeomans for this insightful article.
Dr. Tim Yeomans is the Centre Manager for Shannon Applied Biotechnology Centre, a collaboration between two third level colleges in Ireland. Tim holds a PhD in Microbiology and postgraduate qualifications in Technology Commercialisation and Innovation Management. Tim has worked in research and development for 20 years, both in industry and academia. In his role in Shannon ABC, Tim is responsible for the scientific direction of the Centre, intellectual property management and business and technology development.
Hygiene Hypothesis, cleanliness, farm effect, asthma, allergies, immune system, allergen, indoor environment, environment
1.Krämer U, Heinrich J, Wjst M, Wichmann HE. Age of entry to day nursery and allergy in later childhood. Lancet. 1999;353:450-454.
2.Ball TM, Castro-Rodriguez JA, Griffith KA, Holberg CJ, Martinez FD, Wright AL. Siblings, day-care attendance, and the risk of asthma and wheezing during childhood. N Engl J Med. 2000;343:538-543.
3.Braun-Fahrländer C, Riedler J, Herz U, et al; Allergy and Endotoxin Study Team. Environmental exposure to endotoxin and its relation to asthma in school-age children. N Engl J Med. 2002;347:869-877.
4.Ownby DR, Johnson CC, Peterson EL. Exposure to dogs and cats in the first year of life and risk of allergic sensitization at 6 to 7 years of age. JAMA. 2002;288:963-972.
5.Ege MJ, Mayer M, Normand AC, et al; GABRIELA Transregio 22 Study Group. Exposure to environmental microorganisms and childhood asthma. N Engl J Med. 2011;364:701-709.
6. Skripak, J and Wood, RA. Exposure to Pets, and the Association With Hay Fever, Asthma, and Atopic Sensitization in Rural Children. American Academy of Pediatrics. August 2006, VOLUME 118 / ISSUE Supplement 1
7. Nicolaou, N, Siddique, N and Custovic, A. Allergic disease in urban and rural populations: increasing prevalence with increasing urbanization. Allergy 2005: 60: 1357–1360
8. Scientificamerican.com (2018) Why Are Asthma Rates Soaring? [online]. Also available at: Why are asthma rates soaring?
9. Bloomfield, SF, Rook, GAW, Scott, EA, Shanahan, F, Stanwell-Smith, R and Turner, P. Time to abandon the hygiene hypothesis: new perspectives on allergic disease, the human microbiome, infectious disease prevention and the role of targeted hygiene. Perspectives in Public Health July 2016 Vol 136 No 4
10. AAFA.org (2018) Allergies [online]. Available at: AAFA Allergies
11. CDC.gov. (2018). CDC – Asthma – Data and Surveillance – Asthma Surveillance Data. [online] Available at: CDC Asthma Data [Accessed 16 Feb. 2018]
12. Xiwei Zheng, Cong Bi, Marissa Brooks, and D. S. H. (2015). HHS Public Access. Anal Chem., 25(4), 368–379.
13. Health Topics, guidelines for diagnosis management of asthma
14. Leas, B. F., D’Anci, K. E., Apter, A. J., Bryant-Stephens, T., Lynch, M. P., Kaczmarek, J. L., & Umscheid, C. A. (2018). Effectiveness of indoor allergen reduction in asthma management: A systematic review. Journal of Allergy and Clinical Immunology,141(5), 1854–1869. Effectiveness of indoor allergen reduction in asthma management