Moving on From the Hygiene Hypothesis

Authored by: Nicole Loy

Art by: Grace Liu

Various health-related myths love to circulate in our common culture. One popular example is lovingly, but incorrectly, quoted by moms everywhere: “One glass of red wine a day is good for your health." While many have shaken off this myth as correlation and not causation, another secretly pervasive myth remains: the idea that cleaning too much has increased allergies and asthma in children. 

This myth is based on the "hygiene hypothesis” spread by David P. Strachan in 1989. Strachan noticed that hayfever was less common in children with older siblings. Therefore, Strachan believed that early microbial exposure might protect against allergic diseases [1]. Over time, this hypothesis has expanded. This new hygiene hypothesis claims that modern declines in “cleanliness” via vaccines, sanitation, antibiotics, and urban design have amplified rates of allergies, asthma, and autoimmune diseases [2]. 

However, the more scientists researched the hygiene hypothesis, the more flawed they realized it is. Dirtier cities show higher rates of asthma, not less. And exposure to some infections, such as measles or respiratory illness, don’t seem to protect against allergies at all. 

One alteration of the hygiene hypothesis proposes that the increase in autoimmune sensitivity is not because of a decrease in exposure to viruses or bacteria, but due to a decrease in parasitic worm exposure. According to this theory, the human immune system evolved alongside worms, becoming used to their constant threat and exposure [3]. In our cleaner modern world, our immune systems notice the lack of parasitic invaders and overreact, causing allergies and autoimmune diseases. While this theory isn’t perfect, it is supported by the fact that countries with higher parasitic worm infections also have a much lower prevalence of autoimmune diseases [4]. 

Another proposed direction points to the importance of microbial diversity. Newborn babies get their first bacterial exposure from their mother, which later becomes their own healthy bacterial microenvironment. However, altering the composition of this microbiome can cause changes to their immune system. High antibiotic use in children can alter their new, fragile microbiome and increase their chances of allergic diseases. As a result, adults with these allergic and autoimmune diseases tend to have different microbial makeup than the rest of their non-affected peers [5]. This finding suggests that while bacterial exposure is important to the human immune system, cleanliness may not be the causative factor. 

While scientists may not currently have all the answers to autoimmune diseases, we do know that the public perception of the hygiene hypothesis is not medically accurate or productive. Personal cleanliness, such as washing hands, cleaning food, and sanitizing households, is a good thing – it decreases the spread of viruses and keeps people healthy. While it is likely that increases in antibiotic diseases are because of changes to our modern environment and healthcare, these changes were essential in protecting us against viruses and diseases that would otherwise severely impact our health. Abandoning hygiene practices that protect against the spread of disease is not the solution to fixing our allergies.

References:

1. Strachan, D. P. (1989). Hay fever, hygiene, and household size. BMJ, 299(6710), 1259–1260. https://doi.org/10.1136/bmj.299.6710.1259

2. Stiemsma, L. T., Reynolds, L. A., Turvey, S. E., & Finlay, B. B. (2015). The hygiene hypothesis: Current perspectives and future therapies. ImmunoTargets and Therapy, 4, 143–157. https://doi.org/10.2147/ITT.S61528

3. Yazdanbakhsh, M., Kremsner, P. G., & Van Ree, R. (2002). Allergy, Parasites, and the Hygiene Hypothesis. Science, 296(5567), 490–494. https://doi.org/10.1126/science.296.5567.490 

4. Rook, G. A. W. (2010). 99th Dahlem Conference on Infection, Inflammation and Chronic Inflammatory Disorders: Darwinian medicine and the ‘hygiene’ or ‘old friends’ hypothesis. Clinical and Experimental Immunology, 160(1), 70–79. https://doi.org/10.1111/j.1365-2249.2010.04133.x

5. Strzępa, A., Lobo, F. M., Majewska-Szczepanik, M., & Szczepanik, M. (2018). Antibiotics and autoimmune and allergy diseases: Causative factor or treatment? International Immunopharmacology, 65, 328–341. https://doi.org/10.1016/j.intimp.2018.10.021