Edward C. Geehr, M.D.

Want to prevent asthma in your child? The solution will surprise you.

I've always been impressed by how little asthma I see on medical missions to rural Mexico and Haiti. The generally dusty conditions, combined with inadequate hygiene and poor air quality in homes with open-flame cooking, should provide perfect conditions for asthma in children and adults. Yet, I rarely need to treat asthma. It turns out that this is true for many developing nations.

Why? The answer is the "hygiene hypothesis."

This theory holds that a lack of early-childhood exposure to certain germs and dirt may hinder development of the immune system and increase the risk for allergies. In other words, too much cleanliness may spur a child's allergies.

A new study of urban American children, published in the Journal of Allergy and Clinical Immunology, supports this theory. Researchers found that regular exposure of babies during the first year of life to dirt, certain germs and dander may lesson asthma and other allergic symptoms later in life.

Wheezing illnesses affect 35% to 50% of children by age 3. Not all wheezing children go on to develop asthma. But recurrent wheezing in children who have other signs of allergy – eczema, hay fever and food allergy, for example – puts the child at high risk for asthma. This combination of allergic symptoms is associated with life-threatening anaphylactic reactions when exposed to environmental or food allergens.

Asthma is the leading cause of school absences from a chronic illness, accounting for an annual loss of more than 14 million school days per year and more hospitalizations than any other childhood disease.

To examine the relationship between urban, indoor environments and the development of allergies, researchers from the University of California, San Francisco; Johns Hopkins University; and Washington University studied more than 400 children from birth through age 3. They were assessed based on exposure to environmental allergens and bacteria in house dust collected during the first year of life.

All study children lived in areas of high poverty and had a mother or father with allergic symptoms. The children were tested yearly for allergic sensitivities to milk, egg, peanuts and cockroaches. At ages 2 and 3, additional tests were performed for dust mites and dog, cat and mouse dander.

The results? The immune system's secret recipe for lowering rates of wheezing at age 3 was exposing the children to mouse and cat dander, cockroach droppings and certain bacteria in the first year of life. And it worked better after exposure to each additional allergen.

Those exposed to all three allergens plus the bacteria did better than those exposed to just one, two or none. When all three allergens plus bacteria were present in the home during the first year of life, the rate of wheezing was one-third that of children raised in the absence of the allergens. The group of children without any wheezing or signs of allergic reactions had the highest first-year exposure to the allergens and bacteria.

There is no question that children with established allergic asthma should avoid exposure to allergens. But the study suggests that exposure to high levels of certain allergens and bacteria during a child's first year might be beneficial and could lead to new preventive strategies for wheezing and allergic diseases.

We aren't ready to prescribe a dose of cat and mouse dander along with a pinch of cockroach droppings for infants just yet. But the results of this study will stimulate researchers to take a closer look at how these allergens interact with the immature immune system and whether structured environmental exposures could reduce the incidence of debilitating allergies in the future.