Examining the hygiene hypothesis

When you’re growing up, adults, whom you assume know what’s good for you tell you to practice proper hygiene. By the time you’re six years old, the phrases “Wash your hands,” “Brush your teeth,” “Don’t let the dog lick your face,” and “Stay out of the mud,” play like Zen mantras in your head. Perhaps our obsession with cleanliness is at the root of the increase in allergies. This theory, referred to as the hygiene hypothesis, is the most popular explanation for the rise in allergies. Simply stated, the hygiene hypothesis proposes that the less the immune system is exposed to germs and bacterial by-products the more energy it has to unleash on allergens. If this sounds kooky to you, examine the evidence before dismissing this theory:

• Allergies are much more common in developed countries, and the prevalence in allergies rises pretty much in direct proportion to the rise in development.
• Allergies are less common in children who grow up on farms, who attend daycare in early life, and who have multiple older siblings.
• Allergies may be less common in families that have pets, perhaps because pets increase exposure to bacteria and bacterial by-products.

Before you move out to the country, surround your family with livestock, and start rolling around in the pig pen, realize that the hygiene hypothesis has some holes in it, suggesting that other factors may play a role. The inner-city environment, for example, defies the hygiene hypothesis. Children growing up in the inner-city should reap all the benefits of poor hygiene in warding off the onset of allergies, but inner-city kids have some of the highest rates of asthma and allergy anywhere. This suggests that other factors, such as air pollution or environmental allergen exposure, may trump the hygiene hypothesis.