The 'Scope: Potential Benefits of Thumb-sucking and Nail-biting in a Too-Clean World

In a world of Lysol and Purell, it's easy to become all-consumed with keeping clean. And why not? We're on the go more than ever now: we're working longer hours (1, 2), spending more time commuting (3), and we’re under constant pressure to keep up to date on all the available social media networks (4). No one has time to be slowed down with the flu or a cold. So we dab on a little hand sanitizer before we eat, clean our houses regularly with bleach-containing products, and hold our breath when someone sneezes in a crowded elevator (or maybe that's just me).

But is there such a thing as being too clean? Researchers who are focused on testing this so-called "hygiene hypothesis" think there may be.

The hygiene hypothesis proposes that living in a germ-free world is disadvantageous to our health. Studies testing the hygiene hypothesis have shown correlations between our squeaky-clean developed societies and increases in allergic conditions, compared to developing societies lacking modern infrastructures that support public health (5, 6). Some studies even point to differences in the levels of allergic conditions in cities versus rural towns within the same country (7). While such studies only suggest correlations, and don't definitively show clean environments cause a predisposition to allergies, their findings are worth considering.

While the biological explanation for the hygiene hypothesis is still being studied, evidence from such studies so far suggests that when our immune systems aren't regularly challenged by germs normally present in the environments we've been co-evolving with for millennia, the result is an immune system that is predisposed to allergic reactions. Our immune systems rely on a series of specialized cells programmed and primed to respond to different pathogenic and environmental challenges in a coordinated fashion: for example, some cells respond to bacteria and viruses while others respond to parasites. Researchers investigating biological explanations for the hygiene hypothesis have proposed that a lack of exposure to bacteria and viruses in childhood causes a shift in the population of immune cells away from cells primed and ready to attack those germs and instead toward a population of cells programed to respond to allergic stimuli (6).

Of course being clean is a good thing. An awareness of how diseases spread and how to take precautions against them is one of the reasons why modern society has been able to flourish. Hand washing and sterilization techniques introduced in the 1800s by Dr. Ignaz Semmelweis dramatically reduced a common cause of death in maternity wards (9). Modern epidemiology enables us to learn and track how certain diseases can be spread (including the recent outbreak of E. coli in flour) so we can take preventative measures to avoid further spread of diseases. We're careful to cook our food thoroughly to avoid food borne illnesses like salmonellosis. All of these behaviors protect us from unwanted illnesses, and allow us to carry on with our lives. While we certainly don't want to undo all of the advances we've made in limiting the spread of disease, evidence suggests that there needs to be a balance between being too dirty and too clean.

A line of souvenirs at Disney parks last summer included hand sanitizers featuring popular kids characters. Image from https://disneyparks.disney.go.com/blog/2015/08/summer-of-souvenirs-continues-with-new-items-at-disney-parks/

For example, it was recently published in the journal Pediatrics (8) that thumb-sucking and nail-biting, generally thought of as being unsanitary, may help children avoid developing environmental allergies. The results came out of the Dunedin Multidisciplinary Study, in which researchers followed over 1,000 children born in Dunedin, New Zealand between 1972 and 1973 throughout adulthood. For this particular question, children were first examined at ages 5, 7, 9, and 11 and then tested for certain allergies at 13 and 32 years of age. The researchers conducting this study, Stephanie Lynch and Dr. Robert Hancox (from the University of Otago, New Zealand), and Dr. Malcolm Sears (McMaster University and St Joseph’s Healthcare, Ontario, Canada), found that the individuals who had been frequent thumb-suckers or nail-biters as children tested positive for allergic sensitivities less often than those who had not frequently engaged in those habits. More specifically, the researchers report that 49% of participants who had not been frequent thumb-suckers or nail-biters had positivity allergy tests, whereas only 31% of participants who had sucked their thumbs and bit their nails as young children had positive allergy tests.

Granted, this is only one study and it's still probably not a good idea to advocate for children keeping their dirty hands in their mouths all of the time. After all, no one wants their child to be sick. But perhaps thumb-sucking is one thing parents don't have to worry about so much after all. Perhaps instead, we can trust that our bodies are designed to deal with those little germ and dirt exposures, and maybe even benefit from them in the long run.