The hygiene hypothesis states that lack of exposure to germs and other immunogenic substances at a young age interferes with natural development of the immune system in the child and consequently an increase in conditions related to the immune system such as asthma. In short, children may be raised in environments that are, paradoxically, too clean. A new study published in the Journal of Allergy and Clinical Immunology found that inner city children exposed specifically to cockroach, mouse and cat allergens during their first year of life had less wheezing at the age of three than children who were not exposed to those allergens.
Researchers from the Johns Hopkins Children s Center in Baltimore looked at data from 467 children from Baltimore, Boston, New York City and St. Louis enrolled in the birth cohort study Urban Environment and Childhood Asthma and followed them from birth to age three. In order to assess exposure to allergens, researchers collected dust samples from the children s homes when they were babies. They found that only 17 percent of children exposed to those three specific allergens developed wheezing problems, while 51 percent of those not exposed to those allergens developed wheezing problems.
Study author and chief of the Division of Allergy and Immunology at the Johns Hopkins Children s Center Dr. Robert Wood says, Our study shows that the timing of initial exposure may be critical. What this tells us is that not only are many of our immune responses shaped in the first year of life, but also that certain bacteria and allergens play an important role in stimulating and training the immune system to behave a certain way.
ACSH s Dr. Gilbert Ross adds, This study is very counterintuitive to what may be generally accepted by parents, but it really does add more support to the hygiene hypothesis. However, the numbers do seem to be a little off, specifically the fact that 51 percent of those children not exposed to these allergens developed wheezing. This does not seem to be in line with the report from the CDC that only 14 percent of children have been diagnosed with asthma in the United States.
But ACSH friend (and blogger at Free Range Kids) Lenore Skenazy puts this study in perspective. Being obsessive about child cleanliness is like being obsessive about 'stranger danger.' When we see all germs -- and humans -- as a threat to our kids and try to keep them at bay, our children end up not being able to distinguish between the normal and the dangerous. Their systems over-react to the whole, great smorgasbord of the world, often reflexively flinching at dander, peanuts and the lady who says 'Hello!' to them as they wait at the bus stop."