Dirt and You: The Hygiene Hypothesis

by Joe Martin on March 23, 2012

Pictured: Me, fighting the development of allergies. All Credit to my exceedingly excellent mother.

From what I have garnered from generations before mine, things used to be different. Not just different, but better. (Or worse, when it meant the grizzled old man telling the story was manlier than I, and had to put up with more. Don’t you know we used to have to walk to school in the snow, uphill both ways? And we had no shoes so we wrapped barbed wire around our feet for traction?). But when it comes to health, it appears that this might a little true. Some diseases appear to be on the rise, such as autism. And while there is ample discussion on why we as a society are experiencing higher rates of this disease, (or if we in fact are experiencing higher rates, as opposed to simply diagnosing it more often), there is no debate on whether allergy rates are on the rise.

The Hygiene Hypothesis is one answer that has been proposed to explain the rise in allergy rates. Quickly, the Hygiene Hypothesis states that if children are kept in too clean of an environment through their developmental years, they are more prone to develop life-long allergies. To explain it a little more in-depth, children who are maintained in a less-than-spotless environment are exposed to many different biological agents, like bacteria suspended in dust and dirt. In a way, this acts like exercise for the immune system, helping it to develop properly. If a child is not exposed to such an agent, their immune system, which is primed to develop resistance, is more likely to focus on common allergens, like dust mites or pet dander. Similarly, it may be that the immune system has a limited working capacity, and while cutting its teeth on the common germs present in our environment, does not have time to focus on allergens like pollen.

The epidemiology of this phenomenon is well demonstrated. The trouble with the Hygiene Hypothesis comes from our inability to explain why some exposures lead to sensitization and some lead to immunity without allergic reactions. In some cases, exposure is directly related to sensitization, as is common with household mold, and such sensitization occur commonly even into adulthood. Until the efforts of countless diligent scientists and allergists come to fruition, the link between the observed effect and the biochemical processes will remain.

But there is one important thing I’d like to point out. It isn’t a simple question of living in a dirty or clean environment. The term “hygiene hypothesis” suggests that being overly-clean is bad for children’s health, and perhaps used to disparage parents who are fastidious about making sure their children or homes are clean. But this isn’t necessarily true. You’ll notice under the link in the paragraph above (under “epidemiology”) that the paper cited doesn’t actually use the words “Hygiene Hypothesis”. Instead it uses “farm effect”. What the paper notes is that the benefits were seen among children raised on farms, with benefits extending even to individuals 75 years old. But the paper does not examine hygiene practices like hand washing or bath times or even antibiotic exposure. The effect was observed in people who lived on farms, versus those who lived in urban or suburban areas.

The point, then, is this. Restricting children from being exposed to the ubiquitous biological agents which are present throughout the world may have a detrimental effect on their life, and make them more likely to suffer the ill-effects of allergic disease like hay fever. So let kids play in the dirt, dig around to find bugs and stomp in mud puddles. Perhaps let them sample the fine flavors of an dried oak twig. Make sure they run around and breathe in some summer air that isn’t loaded with car fumes. But its okay to demand that they wash their hands before dinner and clean the melted popsicle from their face (and hands and neck and hair and shirt and pants and somehow toes even though they were wearing shoes). Its okay that the likelihood of a bath rises in direct proportion to the amount of time spent playing in the dirt. Especially since developing a habit of washing your hands can be one of the best ways to prevent illness later in life.

In the next two decades or so our understanding of the development of our immune systems may be at a place where we can better control our exposures to maximize immunity to infection and minimize allergies. Until then, kids should be kids in all their sticky, messy, mud-loving glory. But the hygiene hypothesis should not be followed like a prescription from a health professional. So here’s to eating dirt, but also to washing hands.

Thanks to everyone who has stopped by to read, and comment, over the course of this class and blog. I appreciate your advice, and hope to have the opportunity to further refine my skills and maintain public health and science communication as a major facet of my future career.

Jennifer March 23, 2012 at 1:13 pm

This post was a good read and a good description of the hygiene hypothesis. The first sentence was a tad awkward though…?

Joe Martin March 23, 2012 at 2:35 pm

That first sentence would be awkward, as “If” was a typo, and not deleted from a first draft of the introduction. Also, thanks for reading!

PF Anderson March 23, 2012 at 2:07 pm

I love this one! I like that it clarifies the issue of oversimplification of complex matters into misunderstood soundbites. I do wish there was more evidence provided on both sides. I also really wish that citations were given for the links, or that at least enough information was given to track down the article later.

J. Eriksson, L. Ekerljung, J. Lötvall, T. Pullerits, G. Wennergren, E. Rönmark, K. Torén, B. Lundbäck. Growing up on a farm leads to lifelong protection against allergic rhinitis. Allergy November 2010 65(11):1397–1403.

My greatest concern with this post is, that while I happen to personally agree with the suggestions provided here, there really isn’t much provided in the way of science or evidence to support those positions. I went to all three of the links you provided. One was a news article that did not include a link to the research (but did give info for me to find it), the second was a link to a patient education page by a professional medical organization who themselves did not provide evidence or citations to support their statements. This is a very tricky topic to search. I expect there are probably other articles about it, but would have loved it if you had referred to some of them to help people learn more. Or perhaps to have used one that is more recent, like this one.

Torsten Olszak, Dingding An, Sebastian Zeissig, Miguel Pinilla Vera, Julia Richter, Andre Franke, Jonathan N. Glickman, Reiner Siebert, Rebecca M. Baron, Dennis L. Kasper, Richard S. Blumberg. Microbial Exposure During Early Life Has Persistent Effects on Natural Killer T Cell Function. Science DOI: 10.1126/science.1219328


Helen Thompson. Early exposure to germs has lasting benefits: Findings help to explain how microbes programme a developing immune system. Nature 22 March 2012

Very very interesting topic!

Paula Johnson, PhD, MPH March 23, 2012 at 2:20 pm

I agree that the first sentence is awkward, and doesn’t seem to make sense. But the entire piece seems awkwardly written and contains grammatical errors. Please edit more before posting. I was also confused about what article you were reviewing. You say, “The point is this…” and proceed to describe the hygiene hypothesis, and not the epidemiology article on farm kids. Was this simply YOUR point? That was not clear. Maybe I’m misunderstanding the purpose of these posts. I would have liked more translation of the science, but this seems more like an opinion piece.

Angela March 23, 2012 at 10:49 pm

Thanks for the post – and thread! Was not quite clear on what you meant by ‘sensitisation’ (what process sensitises you to what), but otherwise I understood everything. Epidemiology might need a one-word explanation in brackets. Finding the shift from Hygiene Hypothesis to farm effect interesting and am curious about future allergy research, as doctors seem to be in the dark about much of it. Wow – is this really the last post?!

Jeff March 25, 2012 at 6:08 pm

I’ve enjoyed your posts over the last couple months. You’ve tackled some pretty heady stuff and have done so with humor and insight even if much of it was beyond this old man’s intellect. I’ve read many of the comments from folks and it seems you’ve taken their advice to heart. Your writing has improved but you’ve kept your sense of humor, which I’ve always appreciated. Well done. I’m proud of you! See you soon! LY -P

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