The Dangers of Germophobia

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In the case of asthma, however, the data seem clouded. Though some studies appear to support the hygiene hypothesis, there is an overwhelming volume of evidence that asthma is more of a problem in crowded, unsterile environments. According to Mindy Pennybacker in The Green Guide (July 1, 1998), "inner-city children are truly the canaries in the coal mine for this disease"; they are bombarded by allergens—cockroach parts, emissions from gasoline and coal combustion—that aggravate the condition. Pennybacker also points out, however, that asthma is "increasing among Americans of all incomes and races"—and the statistics she presents on inner-city cases show increases in hospitalizations for asthma, not increases in the appearance of the condition itself.

Clearly, the conditions of poverty aggravate asthma, but that fact alone does not disprove the hygiene hypothesis. While supporters of the hypothesis have yet to find indisputable epidemiological evidence of their claims, immunologists have discovered a mechanism within the immune system that jibes with their theory. At birth, we have a certain number of immune cells called Th cells, which are directed by our environment into one of two paths. Either they become Th1 cells, which instruct immune cells infected with bacteria to kill the intruder, or Th2 cells, which send out antibodies, triggering a chain of events that includes the runny nose and wheeziness that plague asthmatics and the allergy-prone. Th cells growing in a flask in laboratory experiments become Th1 cells when they're marinated with a chemical that our immune cells naturally release when under attack.

"In our mucky past, the theory goes, babies swarmed with bacteria and viruses, and this ensured [that] the balance was sufficiently weighted toward Th1 cells," Hamilton writes. "Remove those microbes from the picture, and the immune system is irrevocably pushed toward favoring the allergy-inducing Th2 response."

The most helpful microbes, some scientists argue, are those that lurk in the soil, beyond our increasingly hermetically sealed lives. "Mycobacteria live in soil, and pond and stream water, but not in our bodies, so our changing relationship with the environment is likely to have had a gigantic influence on our contact with them," Hamilton writes.

So sending our children out to make mud pies may soon be seen as preventive medicine.

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