Are you familiar with the Hygiene Hypothesis? The HH — or, as some of us call it, the “pound of dirt theory” — is grabbing attention again. A minor medical press feeding frenzy followed the publication in the New England Journal of Medicine of a study based on data from Europe. The summary?
“Children living on farms were exposed to a wider range of microbes than were children in the reference group, and this exposure explains a substantial fraction of the inverse relation between asthma and growing up on a farm.”
This is the Hygiene Hypothesis incarnate. The HH posits that part of our immune system produces an antigen called IgE, which evolved to fight parasites in unhygienic conditions that have prevailed for most of human history, and since we are now cleaner, these antigens attack otherwise harmless proteins in some of us, making us sick, in the form of allergies. Instead of attacking, say, hookworms, the antigen goes after that just-chomped peanut butter sandwich.
Proponents of the HH compare the prevalence of allergies in East and West Germany before and after unification. East Germany had more children growing up on farms and in larger families than West Germany, and much lower rates of allergies and asthma. Now, with its more westernized culture, East German rates of allergies and asthma have nearly caught up with West Germany.
It makes a great story. The whole farm-city thing resonates deep in the American mind. It evokes the mythic hold that farm life has over our national psyche. Farms good; cities bad. Wholesome Jeffersonian America is good for our children not only morally, but physically. The implication is that if we all grew up on farms, asthma wouldn’t be at the epidemic levels we now have. The trouble is that in medical science there are too many variables to draw sweeping conclusions from one set of data, and anyone who would do so is not a serious scientist, or is driven by an agenda (or both).
A case in point is a Forbes blogger who took a pot shot at mold-inspired litigation against landlords, interpreting the study to mean that mold is good for us. The Forbes blogger mentioned the case of Bianca Jagger, who sued her landlord about mold growing in her Park Avenue apartment. Erin Brockovich, Michael Jordan, and Ed McMahon are other celebrities who have coped with mold contamination, along with countless sufferers whose names are not familiar to us.
Some mold is, undoubtedly, good. Without it, we wouldn’t have penicillin or blue cheese. But some mold can kill, particularly stachybotrys chartarum – a toxic black mold – which is often found in buildings with water damage. Other molds, while not immediately life threatening, are still potent allergens, including the ones you find in the woods behind the back 40, in Central Park, and in virtually any basement anywhere. In fairness, it’s not as easy for landlords to decide which molds to allow in their properties as it is, say, to choose between Stilton or Roquefort. As for that wet laundry you left in the washing machine for two days, it may not make you sneeze, let alone kill you, but it does stink.
As objectionable as I find enlisting a specious inference in service of an ideological argument against the American tort bar, there are medical considerations to look at before we let the kids run barefoot through the barnyard as immunotherapy against asthma.
First, these were European farms under study. The European farm population may or may not be a fairly homogeneous group compared to city dwellers, and genetics make a large difference in who develops asthma. It stands to reason that generations of working the family farm may have bred a hearty cohort of kids who can breathe the local air without wheezing.
Second, there may be something about European farming practices that makes their farm/city dynamic different from ours. European farms are regulated very differently from our own, in part because of the health fears of the European commissioners. For example, genetically modified food is much more tightly restricted in Europe, if it is legal at all. This means that Europeans use different fertilizers and pesticides than the ones we use here, which undoubtedly affects the rural health picture.
And European farm asthma may just be lagging behind ours. Typical farms are rampant with chemicals. Add to that the effects of weather on the pollen count and the aromatic plumes from manure lagoons, and no wonder rural America is suffering from an asthma epidemic that rivals the one we're seeing in urban America.
CDC researcher Dr. Teresa Morrison, medical epidemiologist in the Air Pollution and Respiratory Health Branch, was lead author of an article in the Journal of Asthma which concluded that “Asthma prevalence is as high in rural as in urban areas.” The goal of their research is "… to document patterns of asthma symptoms among rural residents in Midwestern states, and learn more about possible environmental exposures that potentially lead to asthma attacks."
David Van Sickle, who has worked with Morrison, holds a doctorate from the University of Wisconsin, and is founder of a Madison-based company called Reciprocal Sciences. In a guest editorial for www.asthmaallergieschildren.com in November, he wrote that studies of farm workers in California showed that exposures to agricultural dusts were associated with the development of persistent wheeze, exposure to pesticides was associated with the development of asthma in women, and that community exposures to airborne waste from large scale animal agriculture might also be associated with exacerbations of asthma. As he also pointed out, this may have remained hidden because it’s hard to study, but that is changing, in no small part because Van Sickle has developed an iPhone app called Asthmopolis, which can transmit information to doctors every time the patient—say a farmer—toots on his inhaler.
No one who has studied immunology, as I have, can ignore the contribution farms have made to the treatment of the human immune system. As every biology student should know, vaccination began because Edward Jenner noticed that milk maids exposed to cow pox gained immunity from small pox. I have my doubts that a similar benefit can be derived with asthma.
The country — where the air is full of all kinds of pollen and chemicals — is probably not the ideal choice for a Fresh Air Fund-style migration of wheezing children. But who knows? Maybe some of those farm microbes do have a salutary effect on kids’ immune systems. I wouldn't recommend sending the kids to the city, either (check out some of the reasons a Bronx neighborhood has the nation's highest asthma rates). If I sound equivocal, it’s because I am. Maybe sneezing, wheezing, and itching are the price we — that's an urban and rural "us" — pay for “progress.”
Dr. Paul Ehrlich is co-author with Dr. Larry Chiaramonte and Henry Ehrlich of Asthma Allergies Children: A Parent's Guide
Photo by Nathan T. Baker: "I might have to get a cooler style for this asthma inhaler."
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