Cal Flyn in Prospect:
While the explosion in food allergies is well documented, there is no agreement on why it’s happened. Theories abound: one of the most enduring has been the so-called “hygiene hypothesis,” put forward by the epidemiologist David Strachan in 1989, which suggested that modern society’s obsession with cleanliness was at the root of the problem. The hypothesis proposed that a lack of exposure to dirt and bugs leads to a badly calibrated immune system prone to misfires. But though the idea feels intuitive, and has thus found a willing public audience, evidence for it has been lacking. Scientists now say that over-enthusiastic hygiene practices are unlikely to be the cause, although exposure to the lungs of particular cleaning products might be.