September 1, 2011
by John Hardie, BDS, MSc, PhD, FRCDC
Can we be too clean for our own good? That the answer might be yes is so contradictory to current thoughts on hygiene, asepsis and health that I was taken aback on reading that the answer is more likely to be yes than no.
In Katherine Ashenburg’s fascinating book, “The Dirt on Clean, An Unsanitized History,”1 she describes how our current fear of disease has created an obsessive preoccupation with cleanliness of the body, the home environment and the workplace that is unparalleled in history. The author notes that this excessive concentration on hygiene is not without its perils, one of which is that an absence of exposure to certain germs might actually be harmful. This idea is the basis of the Hygiene Hypothesis.
The author expands upon the concept that “A little dirt will do no harm” by describing studies in 1989 which seemed to implicate that unhygienic living conditions might prevent the development of childhood allergies and other conditions such as rheumatoid arthritis, diabetes, Crohn’s disease and multiple sclerosis.1
More recent investigations suggest that a number of the products and techniques used to maintain cleanliness might – through the mechanisms invoked by the hygiene hypothesis – do more harm than good. The purpose of this article is to describe the relevance of the hygiene hypothesis to the practice of dentistry.
The underlying tenets of the hygiene hypothesis are that our clean modern lifestyle combined with a lack of early childhood exposure to dirt, bacteria and other pathogens interferes with and weakens the development of the immune system resulting in increased susceptibility to allergies and asthma.2 This concept arose from the studies by the British epidemiologist D.P. Strachan. In 1989 he published a paper titled, “Hay Fever, Hygiene, and Household Size.”3 He postulated that the unhygienic living conditions and frequent infections existing among large families might be the reason for the relatively lower incidence of allergies among this cohort. Supporting this idea were the 1980s investigations by E. von Mutius of the presence of allergies and asthma among children in West and East Berlin. Surprisingly, she found that these conditions were more common in the relatively clean prosperous environment of West Berlin than in the economically disadvantaged and more unsanitary conditions of East Berlin.1 Epidemiological research in the late 1990s showed that hay fever was less common for farm children than for urban children and for rural children who did not live on farms.4 An analysis of further studies allowed Ashenburg to conclude that children were less prone to allergies if they had older siblings especially brothers, if they had a cat or if they went to daycare during their first year.1 Dr Kugathasan, a gastroenterologist, has noted that while intestinal worms, parasitic infections and childhood infections are common among the inhabitants of many developing countries diseases such as asthma, Crohn’s and multiple sclerosis are remarkably rare. However, the second generation of Asian, Latin American and African children who have migrated to more developed countries and who have not been exposed to such pathogens, have a similar experience of asthma, Crohn’s and multiple sclerosis as those from Europe and North America.5 An English study demonstrated that children who received antibiotics by the age of two were more susceptible to allergies than children who had no antibiotics.6
Exposures to infectious agents from siblings, dirt from farm animals, dander from pets, and environmental pathogens seem to have an epidemiological relationship to the development of asthma, allergies and chronic immune diseases. The hygiene hypothesis extends the relationship beyond the epidemiological to suggest that our modern super clean world with its excessive reliance on antibiotics, vaccinations, the early treatment of childhood infections, personal hygiene, household detergents, antibacterial soaps and air fresheners keeps pathogens and dirt at bay but not without adverse influences on the immune system.
T helper (TH) cells are an integral aspect of the immune system. These TH -cells have a number of direct functions and are so called because of the help they give to other immune system cells such as B lymphocytes and cytotoxic T lymphocytes. Four types of TH cells have been identified. In 2000, Folkerts and colleagues proposed that TH-1 and TH-2 cells play a significant role in the immune disturbances associated with the hygiene hypothesis.7 TH-1 cells have an active role in cell-mediated immunity by helping to destroy intracellular pathogens such as viruses and bacteria. TH-2 cells help B cells and are necessary for the production of IgE antibodies in response to extracellular allergens such as plant pollens, house dust mites, molds, penicillin and peanuts. It is thought that exposure of a young developing immune system to repeated small doses of various viruses and bacteria endemic to a given environment strengthens the TH-1 cell-mediated arm of the immune system while at the same time inhibiting the development of the TH-2 arm. If exposure to dirt and bugs does not occur, the cell-mediated arm weakens leaving the TH-2 cells to overreact to irritants that would otherwise be of minimal significance resulting in the allergies associated with clean living.7 This concept does not explain the rise in TH-1 cell mediated diseases such as inflammatory bowel disease, rheumatoid arthritis, multiple sclerosis and type 1 diabetes that have an epidemiological relationship to improved hygiene and infectious disease control. To explain this association, Bufford in 2005 proposed that the developing immune system requires stimulation from numerous infectious sources such as bacteria, viruses and parasites for the proper development of the entire T helper cell regulatory mechanism. If this does not occur TH-1 and TH-2 responses are repressed and autoimmune diseases and allergic responses are more likely to occur.8
Modern dental offices are kept clean with a variety of disinfectants, sprays, wipes and soaps, the vast majority of which contain antibacterial agents. In 2001, Levy voiced concerns about these antibacterial ingredients.9 The widespread historic overuse and misuse of antibiotics has resulted in a broad spectrum of bacteria becoming resistant to antibiotics. Levy noted that the antibacterial substances contained in a variety of everyday cleaning products can selectively produce strains of bacteria that are resistant to antibiotics leading to a change in the normal microbial flora of the environment in which the cleaning products are used.9 The consequences of this are that dental staff could be exposed to pathogens from patients or the workplace that are no longer sensitive to standard antibiotic regimens. One of these might be the “super” bug methicillin-resistant Satphylococcus aureus especially the community – acquired form of MRSA. This is an increasing global problem with a possible link to antibacterial products.9
The common active ingredient in antibacterial products is triclosan. A major 2007 literature review showed that antibacterial soaps containing triclosan were no more effective at preventing infectious illness symptoms and in reducing bacteria levels on the hands than were plain soaps and water. The review confirmed the ability of triclosan to produce antibiotic resistance among different bacterial species.10 Dental personnel should be aware that triclosan has been found in the plasma and breast milk of nursing mothers.11 To what degree this presence interferes with an infant’s developing immune system remains unknown. In 2009 the Canadian Medical Association called for a ban on all antibacterial household products.12
Almost all antibacterial soaps contain ticlosan, as do many liquid soaps, hand lotions, disinfecting wipes, mouthwashes and even so-called antibacterial toothbru
shes.9 It would behove dental staff to carefully scrutinize the ingredients labels of all antibacterial, antiseptic and germicidal products.
A basic aspect of the hygiene hypothesis focuses on the developing immune system achieving the correct balance between TH-1 cells providing cellular immunity and TH-2 cells promoting antibody production. Nevertheless, the potential ability of cleaning products to induce harm through the creation of bacterial resistance to antibiotics falls within the remit of the hypothesis as it helps to explain why enthusiastic cleaning of our clinics might cause us harm. It also serves as a notice to review the types of domestic cleaning products that dental personnel are using. If these are employed to excess, they might reduce the home bacterial flora and have an adverse effect on the immune system of infants through a lack of stimulation of TH-1 cells. Alternately, if they contain triclosan the potential exists for household pathogens to become resistant to antibiotics, thus exacerbating conditions that previously might have succumbed to routine antibiotic therapy.
Another concern for the dental staff is increasing epidemiological evidence that many professional cleaning products in a spray format (bleach, ammonia, glass-cleaners, air-fresheners, perfumed or scented products) are associated with adult onset asthma.13 It is not known how these sprays influence the mature immune system. The hygiene hypothesis might play a role. Possibly, this is another example of how excessive cleanliness might explain why some dental personnel are becoming ill, although their use of similar products in the home might be significant. Masks to prevent the inhalation of the sprays might ameliorate their adverse effects.
In recent years the dental profession has become obsessed with cleanliness. Recommendations have been made and numerous techniques adopted to satisfy this passion with little thought being given to the possibility of unintended consequences. In consideration of the findings reported by Levy which are supported by the US Food and Drug Administration, a return to cleaning surfaces with plain soap and water and using evaporating alcohol based wipes will create not only a clean but a much healthier working environment.
As this article was being completed another twist to the cleanliness issue occurred. A report was released at the April 2011 annual meeting of the Society for Healthcare Epidemiology of America regarding hands-free electronic water faucets. The study was undertaken at the John Hopkins Hospital. It showed that these faucets were more likely to be contaminated with bacteria especially Legionella species than were the old fashioned fixtures with separate handles for hot and cold.14 The results forced hospital officials to replace all hands-free electronic faucets throughout the entire hospital. The exact reasons for the unexpected but considerable bacterial contamination are unknown although it is thought that the sophisticated valves within the faucets encourage bacterial growth.14
It is not known to what degree this is or could be a hazard in dental offices. Perhaps it is an area worthy of research. Nevertheless, this unintended consequence is yet another example of newer not necessarily being better.
Limitations of Hypothesis
If dental personnel are suffering from workplace related illnesses, the hygiene hypothesis might provide the reasons. However, it must be remembered that it is simply a hypothesis. In other words, it is a theory to account for something that is not completely understood. Until there is further clarification, the significance of the hygiene hypothesis to dentistry is perhaps best described by Dr. J-F Bach, Professor of Immunology at the Necker Research Institute in Paris who noted, “That is not to say that dirt is good, but too much cleanliness is not really necessary.”15 OH
Dr. Hardie was intimately involved in the development of the RCDSO 1996 evidence-based guidelines. Since then he has maintained an interest in this topic and how it and related diseases have influenced dental infection control recommendations.
Oral Health welcomes this original article.
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2. Martinez FD. The coming-of-age of the hygiene hypothesis. Respiratory Research 2001; 2: 129-132.
3. Strachan DP. Hay fever, hygiene, and household size. Br Med J 1989; 299: 1259-1260.
4. Braun-Fahrlander C et al. Prevalence of hay fever and allergic sensitization in farmer’s children and their peers living in the same rural community. Clin Exp Allergy 1999; 29: 28-34.
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14. Science Daily. 2011 Hands-Free Electronic Water Faucets Found to Be Hindrance in Infection Control; Manual Faucets Work Better, Study Shows. Available: http://www.sciencedaily.com/releases/2011/03/110331081849.htm.
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