A short paper first published in the journal BMJ in 1989 touched off a controversy still in progress in contemporary medical literature. In this paper the epidemiologist, David Strachan, wrote about increasing rates of hay fever in the British population since the 1960s. His study showed a positive correlation between increasing rates of hay fever with decreasing family size. Younger children from large families showed the lowest incidence of hay fever.
This study may have not attracted much attention except for some comments that Strachan made in the conclusion of his paper. In the conclusion, Strachan suggested that “unhygienic contact with older siblings or acquired prenatally from a mother infected by contact with her children” may help prevent allergic diseases. He went on to say, “Over the past century declining family size, improvements in household amenities, and higher standards of personal cleanliness have reduced the opportunity for cross infection in young families. This may have resulted in more widespread clinical expression of atopic disease, emerging earlier in wealthier people, as seems to have occurred for hay fever (Strachan).”
Initially the term “hygiene hypothesis” specifically referred to Strachan's paper. Over time, a much more extensive range of ideas have come to be associated with the hygiene hypothesis. For those who look to a more inclusive definition of the hygiene hypothesis, the general concept is that the semi-sterile conditions that exist in post-industrial urban environments are not optimal for promoting normal development of the human immune system. For most of history, humans have lived in much closer association with the natural environment, including a broad array of microorganisms and parasites. It is possible that as humans’ living conditions have changed, the human body has been deprived of important environmental cues that help regulate the proper function of the immune system.
In this Discovery Guide, we not only examine the theory surrounding the hygiene hypothesis but also discuss some of the sociological implications regarding lifestyle and cleanliness both in the industrialized and non-industrialized world. To put the hygiene hypothesis in perspective, we will discuss the history of cleanliness in the industrialized world. As a case study, we will look in some detail at Japan, the “cleanest country in the world.” We will look at how some individuals have interpreted the hygiene hypothesis. We will discuss the difficulties associated with establishing certainty in epidemiological studies. At this point we will continue looking at how the hygiene hypothesis has continued to develop over time since it was first introduced by Strachan.
One thing that even many skeptics of the hygiene hypothesis seem to agree on is that levels of atopic allergies and asthma have been on the rise in industrialized countries since the 1960s. In comparison, less developed countries still experience much lower levels of these allergy-related diseases. For instance “self-reported asthma ranged from 2-3% in developing countries to 20-40% in ... industrialized countries” (Bloomfield). Furthermore, a study in East Berlin showed a sudden increase in incidence of allergic disorders after the wall came down, as people came to adopted Western life styles. Researchers agree that some change to industrialized environments or lifestyles is implicated in the increase of allergies. What is at dispute is the exact role that the hygiene hypothesis has to play in that change (Yazdanbakhsh).
Over the past couple of decades the hygiene hypothesis has come to be associated with an increasing number of sub-hypotheses. A number of review articles explore these sub-hypotheses including Koloski et al. (2008), Martinez (2001), and Bloomfield et al. (2006). The following list of sub-hypotheses condenses a much longer list of related ideas: 1) Young children in smaller families or who are not sent to day care are failing to contract certain illness at a critical developmental stage for the immune system; 2) Increased hygiene in living environment is limiting human exposure to pathogenic or non-pathogenic microorganisms; 3) People living in urban or non-farm settings are failing to receive sufficient exposure to endotoxins or the right combination of microbes to prime the immune system; and 4) People living in post-industrial societies are not being exposed to the parasites that the immune system is accustomed to and are experiencing immune system dysfunction as a result.
Should further research strengthen the case for one or more of these sub-hypotheses in the role of increased incidence of allergies, then it may be found to be associated with a broader range of immune or inflammatory disorders. A review of the association of inflammatory bowel disease with the hygiene hypothesis (Koloski) looks to possible links to such disorders such as Crohn's disease and ulcerative colitis. Other diseases that may be linked are type 1 diabetes, multiple sclerosis (MS), some autoimmune diseases, esophageal cancer, atherosclerosis, Alzheimer's disease, and even depression (Rook).
Other researchers, however, seriously question some of the premises associated with the hygiene hypothesis. The review of the hygiene hypothesis by Bloomfield et al. (2006) points out some of these questions. Are people's living environments really as clean as we think they are? What is known about common cleaning practices suggests that most homes and public spaces are not as sterile as people often believe them to be. Also, the timing in the change of allergy rates and the change in hygiene practices do not necessarily track as closely as researchers might at first have assumed. Further, there could be other lifestyle changes in the industrialized world that might contribute or explain the rise in inflammatory disorders.
From an epidemiological perspective, establishing cause and effect relationships between diseases and their origins can be an extremely complicated process. There is a lot yet to be learned about the human body, the nature of diseases, how human lifestyle and interactions influence the contraction of diseases. Also, there are serious ethical and practical limitations regarding the detection and study of diseases in human populations. Additionally, some diseases are exacerbated by a combination of genetic and environmental factors. The process of untangling such complex relationships can be extremely difficult. For this and other reasons, it is often a long and arduous process determining clear and certain answers to questions regarding human health.
Outside of the medical community, people are often impatient when it comes to waiting for answers regarding their health and well being. Due to impatience, poor knowledge of existing information, or misunderstanding of scientific uncertainty, it is not uncommon for members of the lay audience to jump to their own conclusions regarding the cause of diseases and the best approach to remedying them. In some cases the proposed remedy can be fairly innocuous, and though it may confer no benefit, it at least does no harm. Furthermore, current medical understanding suggests that the placebo effect may often have its own benefits. However, in more extreme instances, self treatment may be harmful.
The hygiene hypothesis poses an interesting sociological problem. On one hand, if it turns out to be true, it constitutes a provocative challenge to the modern life style. Also, it would require a considerable revision of public perception of cleanliness as well as microorganisms and their relationship to the nature of disease. On the other hand, the hygiene hypothesis has already attracted some attention outside the medical community. While some people are already adopting moderate lifestyle changes regarding their relationship to the environment, a small segment of proponents have adopted more extreme methods of self-treatment.
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