Too Clean? A Potential Down Side of Our Germ-Phobia

Asthma and allergy rates are rising rapidly in the U.S. Though scientists do not yet agree on a definitive explanation, one popular theory is known as the “Hygiene Hypothesis” which suggests that cleanliness, the reduction in family size, and diminishing microbial exposure could explain the increase in allergy and asthma rates in the U.S. and many other countries worldwide.


The Hygiene Hypothesis states that childhood exposure to germs and infectious diseases helps the immune system develop by affording it ample opportunities to learn to differentiate between harmful and harmless substances. Without this experience in discerning, the body can mistakenly perceive harmless elements in the environment (such as pollens, pet dander, etc.) as harmful elements. While it is a positive thing when the body marshals its defenses against true villains like germs, it is very hard on the body to launch a full-scale reaction against no-threat items such as pollens. When the body engages in “warfare” against allergens, it results in a release of histamines that wear down the body and lead to troubling allergy symptoms.

The Hygiene Hypothesis was proposed by David Strachan more than twenty years ago in an attempt to explain the striking increase in the prevalence of allergic diseases and asthma that has been observed in the past few decades. Strachan noted that younger children in big families suffered less from asthma and allergy. This is based on the assumption that younger children are more exposed to infections brought home by older brothers and sisters. Moreover, children who live in rural settings and/or in developing countries tend to have a lower incidence of allergy than those who live in cities and developed countries.

Several studies and experimental models have been developed to test the validity of the Hygiene Hypothesis. For instance, researchers at the Brigham and Women’s Hospital conducted a study on mice to come up with a comparison of the immune system between mice living in sterile conditions and mice living in a normal environment with moderate levels of germs and bacteria.

Researchers concluded that the mice that were not exposed to microbes and bacteria in their early life had an increase in problems similar to asthma and colitis. In contrast, those that were exposed to germs and bacteria during their youth had better immune systems. Their protection against diseases was also longer-lasting.

Though some dispute the validity of the Hygiene Hypothesis, there is strong evidence that it may contribute in at least a small way to the burgeoning allergy and asthma epidemic.