Dirt-Asthma Link Needs Scrubbing

Cleanliness may be causing children to become asthmatic, researchers suggested this week.

It’s a wacky idea that seems to be getting attention thanks to the oft-parroted factoid that childhood asthma has soared over the last 20 years.

A study in the Sept. 19 New England Journal of Medicine reports that exposure to bacterial substances in dust from mattresses was weakly correlated with reduced rates of asthma and wheezing among 812 children living in rural areas of Europe.

The editorial accompanying the study, "Eat Dirt -- The Hygiene Hypothesis and Allergic Diseases," set the tone for media headlines, such as the New York Times’ "Environment Rich in Germs May Reduce Risk of Asthma."

The so-called "hygiene hypothesis" tries to explain supposedly higher rates of allergic responses -- such as asthma -- among "cleaner," affluent nations as compared to rates in "less clean," poorer nations. The idea is that greater childhood exposure to germs leads to fewer subsequent allergies.

Based on this notion, the editorial describes the "introductions of indoor plumbing in the 19th century, antibiotics in the middle of the 20th century, and the cleaner more energy-efficient homes at the end of the 20th century" as possible culprits of the supposed "epidemic" of allergies in developed societies.

But let’s look closer at the alleged "epidemic" of childhood asthma before reverting to outhouses, bloodletting and log cabins.

The Centers for Disease Control and Prevention recently reported the prevalence of childhood asthma increased 86 percent from 1980 to 1996 (from about 37 cases to 69 cases per 1,000 children).

At face value, the statistics seem to validate the existence of an epidemic. A more detailed examination, however, exposes the folly of such superficial analysis.

The CDC data were collected by researchers who conducted telephone surveys merely asking, "During the past 12 months, did your child have asthma?"

But asthma is a complex medical condition that survey respondents weren’t qualified to diagnose. There was no medical confirmation for the claimed cases of asthma. So the data can’t be viewed as reliable.

You don’t have to take my word for it, though. Even the CDC lost confidence in its line of questioning.

In 1997, the CDC altered the survey to ask, "Has a doctor or other health professional ever told you that your child had asthma?" and "During the past 12 months, has your child had an episode of asthma or an asthma attack?"

Since the new survey questions, the reported prevalence of asthma has leveled off.

An explanation for the reported rise in childhood asthma from 1980 to 1996 lies in the fine print of the CDC’s report.

The CDC says the asthma prevalence data are subject to the phenomenon of "diagnostic transfer" -- essentially misdiagnosing bronchitis as asthma.

The CDC researchers said reports of asthma increases are mirrored by reports of decreases in bronchitis, bronchiolotis and pneumonia. "These patterns suggest that diagnostic transfer has contributed to the increasing trend in asthma health care utilization," they acknowledged.

Since it’s not clear there’s been any epidemic of childhood asthma -- or any other allergic condition for that matter -- it’s not clear a "hygiene hypothesis" is needed to explain anything.

The origin of the hygiene hypothesis -- i.e., apparent differences in disease rates between first-and third-world populations -- reminds me of the myth about a high fiber diet reducing the risk of colon cancer.

A British medical missionary observed in the 1970s that African populations seemed to have less colon cancer than Western populations. He attributed this difference -- casually, and not in any scientific way -- to higher fiber African diets.

The intuitive appeal of the alleged dietary fiber-colon cancer connection helped institutionalize the unproven idea in our society. Now cereal companies are allowed to advertise that high fiber cereals "may reduce the risk of certain cancers."

Thirty years after the birth of the hypothesis, however, research has yet to confirm that high fiber diets reduce colon cancer risk.

The hygiene hypothesis could very well belong in the same scrap heap. Its major underpinning -- the reputed childhood asthma epidemic -- almost certainly belongs there.

Steven Milloy is the publisher of JunkScience.com , an adjunct scholar at the Cato Institute and the author of Junk Science Judo: Self-defense Against Health Scares and Scams (Cato Institute, 2001).

Respond to the Writer