What is the Hygiene Hypothesis?

Guest post by Zainab Khan

In most western countries, germs have become synonymous with the idea of something bad that needs to be killed as quickly as possible. However, people have long been questioning the validity of these ideas; a few decades ago it was hypothesized that not enough exposure to germ can and does cause insufficient development of an individuals immune system. New studies have recently shown that this idea of getting rid of all germs, and keeping children exposure to them at an absolute minimum, may possibly cause more harm then good; over cleanliness is suspected to be one of the main reasons that there is such an increased number in asthma and allergy ridden people in western countries. Also, compared to just a generation or two ago, people today have an increased chance of having/developing allergies. Is this all due to society's craze over germs?

It is important when talking about allergies to have some working knowledge on what happens when an individual has allergies or an allergic attack. Allergies are an extreme and inappropriate reaction by an individual's immune system to what typically is a common harmless stimuli found in a normal environment; the body takes something such as hay, food, pollen, etc. and has a hypersensitivity reaction to it. The body ends up activating its white blood cells (these are the cells that defend the body against any foreign bad stimuli), which typically are what help humans ward of virus and bacteria, for example the flu or an infection, which results in an inflammatory response. This inflammatory response manifests itself in different ways: asthma, eczema, hives, runny nose or eyes, coughing etc.

The Hygiene Hypothesis

Over two decades ago, the idea that there is such a thing as too much cleanliness was first proposed by David P. Strachan in his Hygiene Hypothesis. The idea behind this theory is that a lack of early exposure to the types of germs and stimuli that people used to have is the cause of allergies. In developing nations and in earlier time periods families tended to be larger then today. It was uncommon to have just one or two children; the idea behind having more children is that the elder child exposes the younger children to more germs and in turn the children end up having to develop a stronger immune system because the immune system has been fully developed by all the early stimuli [1,2]. This idea of exposure to other children has also held true for children who attend daycare at an early age. Daycare children tend to develop fewer allergies then those who are never in such environments. Research has gone even farther to say that children who are exposed to hepatitis A or the measles are less likely to have certain types of allergies [3].

Arguments against the hygiene hypothesis emerged when statistics were followed about inter city African American children in the United States, who have very high numbers of asthma. A study was done that showed that many of these children had been sensitized to the common allergens found around them; however, they still developed asthma at the same rate as those kids who were not sensitized to the same allergens [4]. Also, it is a scientific fact that some allergies have a genetic component. A child who has two parents with allergies has a 75% chance of also developing allergies. There are genetic links that have been found between certain types of allergic responses which complicates the idea of how much immunity is inherited and how much can be developed [5].

Although the idea of germ exposure has been building momentum within the last few years, the debate and research behind it is certainly not complete. If the hygiene hypothesis is true, this opens up another type of debate on how much and what kinds of bacteria, exposure, and caution should be taken around children. What exactly are the "right" germs, and how many are too many? In a society obsessed with antibacterial hand soaps, disinfectants, and bottled water it is going to be quite a challenge trying to convince people that germs are not all that bad.

Works Cited

1. Am. J. Respir. Crit. Care Med., Volume 164, Number 7, October 2001. The Increase in Asthma Ca Be Ascribed to Cleanliness 1106-1107 Link.

2.Strachan David, Thorax. Family Size, Infection and Atopy: The first Decade of the 'hygiene hypothesis' Link.

3. Matricardi Paolo, Rosmini Franceso, Riondino Silvia, Fortini Michele, Ferrigno Luigina, Rapicetta Maria, Sergio Bonini, BMJ 2000;320 Exposure to foodborne and orofecal microbes versus airborne viruses in relation to atopy and allergic asthma: epidemiological study 412-417. Link

4. R. Call, T. Smith, E. Morris, M. Chapman, T. Platts-Mills, The Journal of Pediatrics, Volume 121, Issue 6 Risk factors for asthma in inner city children, 862-866. Link

5. Mackay, Rosen, Volume 344, January 2001. Allergy and Allergic Diseases 30-37


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An excellent post, that for the most part seems rather un-biased! I just wanted to comment on one thing - there are many types of hypersensitivity reactions, and of those, only certain white blood cells are activated in the type tha tyou are talking about (asthma, runny eyes, etc). These are all caused by immunoglobulin E being excessively produced. I think it is important to note that the mobilization of these specialized white blood cells does not happen after every exposure - in fact this is not what causes the allergy. Instead, free IgE in the blood plasma is responsible for the allergic reaction. Of course, for this to happen, B cells must begin churning IgE out upon exposure in the first place, but after that happens, the actual reaction is caused by IgE.
That was probably longer than needed... ;) Just wanted to clarify things!

Do you know of the PARSIFAL study? If so, would you mind giving your opinion when you have the time?


Nice guest post. Interesting content, but find a grammar cop friend next time for proofreading!

Good choice of topic, but it's a very complicated one.

There are a couple of additional issues here:

1) The development of the human gut flora, and its role in providing vitamins and aiding in food digestion. (Taking antibiotics to fight infectious diseases also wipes out your natural gut flora; eating live yogurt cultures helps replace them). These are not "germs" however, but they can become infectious - as with some strains of E.coli (O1H57).

2) Asthma in inner cities tends to correlate with air pollution levels, which points towards fuel combustion aerosols as the main culprit.

A more illustrative example might be autoimmune studies focused on workers exposed to pesticides, cleaning solvents, etc. Here's an example that gets at the issue:

"There is good evidence for an association of chlorinated ethylenes such as vinyl chloride, trichloroethylene, and tetrachloroethylene in the causation of a life-threatening autoimmune disorder known as systemic sclerosis-like syndrome, or scleroderma; the mechanism by which chlorinated ethylenes cause this sclerosis-like syndrome is unknown. It is our hypothesis that the development of an autoimmune response in certain susceptible individuals may be precipitated by the metabolic activation of chloroethylene's to reactive intermediates..." - Mechanism(s) of Chloroethylene-Induced Autoimmunity.

In those cases, we are dealing with chlorinated chemicals never seen by the immune system during evolutionary history. Hence, there is a big difference between playing in clean dirt and exposing your immune system to natural chemicals, and playing in dirt contaminated with chorinated industrial solvents and heavy metals!

3) Similar issues apply to vaccinations - for example, autoimmune disorders have been known to develop among people who get a great many vaccinations for a wide variety of diseases (this has been noted in scientists employed in biowarfare research). Even Darwin argued that vaccination would end up weakening the public's resistance to disease by preventing the elimination of weak genetic types!

It's worth remembering that infectious diseases - tuberculosis, yellow fever, smallpox, septic infections - used to be the #1 killers, dreaded by all. Exposure to these diseases would result in some immunity, but often survivors were heavily scarred and had shortened lifespans. They were only defeated by a combination of antibiotics, vaccination programs, and hygiene.

So, it's a good essay, but covers a lot of complicated grounds. For something that short, you might have wanted to stick to one subjects - how the hygiene hypothesis relates to infectious disease OR to allergens & autoimmune disorders. Those are complicated enough subjects all on their own!

P.S. Instead of using "germs" throughout, consider using "microbes" or "bacteria" as well.

Being exposed to bacteria and viruses allows our immune system to develop. When the Europeans gave blankets to the Native Indians (knowing they were used by people who were infected with the measles), it wasn't to keep them warm. The Native Indians had no previous exposure to the Measles. The Measles killed the Native Indians.
Another idea behind the increase in allergies, one should look at food choices and antibiotics. Taking rounds of antibiotics kills off our own natural bacteria allowing potential food particles to irritate our gastrointestinal lining and possibly pass through the barrier. This would allow an unknown antigen be picked up by our immune system. Thus triggering a reaction. Not to mention how we are genetically modifying our crops to produce herbicide and pesticide in every single cell of that organism. That just can't be healthy to consume. Sources: houston chiropractor

By Dr. Vanderloop, D.C. (not verified) on 19 Feb 2011 #permalink