The hygiene hypothesis is the idea that allergies, such as hay fever and asthma, are the result of a lack of early childhood exposure to germs and parasites. Looking at this idea from another angle, we can say that allergies are the result of living in a highly sanitised and clean environment (since sanitation involves wiping out infectious agents). So, all these advertisements about products that can rid your home of germs (Detol claims to eliminate 99.9% of all germs) might actually do more harm than good.
The reasoning behind the hygiene hypothesis is that a lack of early childhood exposure to germs and parasites sets off an autoimmune reaction. An autoimmune reaction is when your immune system turns against your body, recognising an agent as harmful that it would otherwise consider benign. AIDS is an example of an autoimmune disease. Proponents of the hygiene hypothesis claim that we need to be exposed to infectious agents at an early age because this allows our immune system to develop properly.
The idea is not a very recent one. It dates back to a paper by David P. Strachan entitled Hay Fever, Hygiene and Household Size (1989) published in the British Medical Journal. He observed that hay fever and eczema, both allergies, were less common in children from larger families, who we can presume were exposed to more infectious agents than children from smaller families. Marc McMorris, a paediatrician from the University of Michigan, has the following to say:
We’ve developed a cleanlier lifestyle, and our bodies no longer need to fight germs as much as they did in the past. As a result, the immune system has shifted away from fighting infection to developing more allergic tendencies.
Airtight doors and windows mean that bacteria, viruses and parasites are kept out, but allergens (such as dust) are kept in. The body’s immune system has evolved to fight infection, but without many or any infectious agents to fight, our immune system will fight against any other foreign agent that it recognises. Allergies are essentially an inappropriate reaction to harmless substances such as pollen, dust and food.
Humans did not evolve to live in super clean, sanitised conditions. We have co-existed with germs and parasites for most of our evolutionary history. We have a sort of love-hate relationship with them, whereby they can make us ill or kill us, while at the same time we depend on them for a regulated and balanced immune system. Our obsession with being hygienic (e.g. with anti-bacterial soaps), the elimination of childhood diseases and the widespread use of antibiotics are all factors that have greatly reduced our exposure to germs and parasites. If the hygiene hypothesis is backed up by supporting data, then it may be wise to let kids play in the dirt and to re-think our attitudes towards cleanliness.
So what is the supporting evidence for the hygiene hypothesis, if any? The idea is supported by a great deal of epidemiological data (data that comes from the study of the patterns, causes and effects of health and disease on a given population). Studies have shown again and again that autoimmune diseases are much less common in developing countries, where sanitation is not as prevalent as in more developed countries. The same applies to allergies. Western, industrialised countries have much higher rates of hay fever and eczema than do less developed countries. The rise in allergies is also very much a general trend – in October 2008, the Centers for Disease Control and Prevention issued a report highlighting that food allergies increased 18% over a 10-year period in children under the age of 18.
As a response to the support for this hypothesis, parasitic worms (known as helminths) are being studied as a way to treat immunological and autoimmune diseases (which would include allergies). Many studies point to the potential benefits of this treatment, especially in the context of Crohn’s disease, multiple sclerosis (MS), and asthma. One criticism of the hygiene hypothesis is that it does not take into account genetic factors. For example, it is a well-established fact that a child who has two parents with the same allergy has a 75% chance of also developing that allergy. It is therefore difficult to tell how much of immunity can be explained by genetics, and how much can be explained by environmental factors.