Children who grow up in environments that afford them a wide range of microbial exposures, such as traditional farms, are protected from childhood asthma and atopy. In previous studies, markers of microbial exposure have been inversely related to these conditions.

This review surveys the data on the increase in the prevalence of asthma in recent decades and finds evidence of a plateau in many Western countries. The authors examine the evidence of possible causal relations to factors such as air pollution, obesity, diet, and exposure to infections, antibiotics, and allergens, including exposures at very young ages. The most strongly supported preventive measure is the avoidance of passive and active exposure to smoke.

How farming protects against allergies: People who grow up on dairy farms only rarely develop asthma or allergies. This is probably because as children, they breathe air containing bacterial components, which reduce the overall reactivity of the immune system. Schuijs et al. chronically exposed mice to bacterial endotoxin before they received an allergic stimulus. The protocol indeed protected them from developing an allergic response. Protection relied on a particular enzyme: A20. In humans, a variant of A20 correlates with increased susceptibility to asthma and allergy in children growing up on farms. Science , this issue p. 1106