To the Editor: While the prevalence of asthma
and allergic disease is increasing worldwide,1
both diseases appear to be less common in rural settings in developing countries
and farming communities in industrialized countries.2
Indeed, some locales seem to be almost free of asthma.3
To explain the low prevalence of asthma in these settings, the "hygiene hypothesis"
suggests that early childhood exposure to high levels of bacterial and viral
pathogens leads to a lower risk of asthma and atopy.4,5
The immune response to these infections would presumably inhibit helper T
cell type 2 (TH2)–type allergic responses.
House-dust endotoxin levels are reported in endotoxin units (EU)/mL,
using reference standard endotoxin provided by the US Food and Drug Administration.
Log transformation of endotoxin values normalized the distribution of the
data. Data points with error bars and associated numbers indicate the geometric
mean values and SEM for each location subgroup. Wilcoxon rank sum test, P<.001. Farm barns outnumber farm homes because some
homes had more than 1 associated barn and also because some barns were sampled
with permission from workers when farm homeowners were not home.
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