Why do more people have allergies today than in the past? One prevailing—though controversial—theory is known as the hygiene hypothesis, which pertains largely to industrialized countries.
Here’s how it goes: As hygiene practices improved over the 20th century (along with the development of more vaccines and antibiotics), infectious diseases dropped. That, obviously, is a good thing. But being too clean may have a downside: It means infants and children are exposed to fewer microbes and potential allergens early in life, at a time when their immune systems are developing.
And an immune system that faces too few challenges may go awry. Rather than learning to differentiate friend from foe, it views harmless substances (like pollen or peanuts) as dangerous and overreacts, which results in allergic reactions down the road.
Children are also exposed to fewer infectious agents due to declining family size (more siblings means more germs get passed back and forth), and they are less likely to grow up on farms (so fewer are exposed to the allergy-protective effect of animals). The flood of antibacterial products in the last decade or so may further be contributing to a more hygienic home environment.
Along the same lines as the hygiene hypothesis is the microflora hypothesis. It contends that exposure to microbes early in life affects the population of bacteria in the intestines (called the microflora), which, in turn, influences long-term immunity.
This may help explain, for example, why infants born by C-section are more likely to develop certain allergies, as some research has noted. Perhaps the procedure—which prevents a newborn’s exposure to the mother’s vaginal bacteria during delivery—shifts its early microflora and thus its immune development. Similarly, breastfeeding affects a baby’s early microflora, so decreases in that practice through the 1960s may have also contributed to a rise in allergies.
So where does all this leave us? We should not stop vaccinating children against infectious diseases, of course. We don’t want them to be exposed to life-threatening microbes in food, either, such as E. coli in undercooked hamburgers (so keep cooking your meat to the proper temperature). And it’s recommended that women breastfeed whenever possible, since this practice, which has been on the increase in recent years in the West, is good for babies in many ways.
But perhaps there’s no need to be overly zealous when it comes to cleanliness—and antibacterial soap is unnecessary and may actually have adverse effects. A little dirt is okay and perhaps even healthy. As experts advise, let kids be kids.