There are generally two schools of thought surrounding the growth of allergies in the twentieth century. Both have evidence in their favor, and neither is complete. Both explain why developed countries have seen such a dramatic increase of allergies, whereas developing nations have not.
_Everyday Chemicals_ – Since the end of World War II, an abundance of chemicals has been introduced into the human environment. Not just the HFCS in our soda pop, but the things in soap and shampoo, the chemical residue on our dishes, leftover Teflon from a pan, or the phthalates that leech out of PVC pipes. It’s true that preservatives and additives all undergo years of testing before they are approved for human consumption, but that handful of years mayn’t be adequate to understand what a lifetime of use can do to a body. No one assumes that any one of our common chemicals is responsible, but all in conjunction are creating an adverse holistic effect.
_The Hygiene Hypothesis_ – Over millions of years the human body has armed itself against a number of ills. It’s had to stockpile immune system weapons for all manner of parasites, germs, and junk, but in our age of daily baths, kitchen sanitizers, and purified water those weapons are somewhat superfluous. A million years of hard-won evolution don’t turn inactive after a mere hundred years of environment. No, instead they go cabin-fever, and freak out attack even benign substances like pollen and dust with a fervor of pent-up frustration.
See Immune System Gone Bad on Damn Interesting.