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While the seeming increase in allergies may reflect greater media publicity, more public interest and better diagnosis, many experts suggest that there is probably a real rise in allergy incidence as well.
The reason isn’t known, but air pollution (especially vehicle and diesel fumes) and other environmental changes may aggravate allergic symptoms. Epidemiological evidence linking air pollution with an increase in allergy comes from Japan, where hay fever was almost unknown before World War II but is now common. 
It’s mainly triggered by tree pollens, so if hay fever were merely a matter of breathing in cedar and other tree pollen it would probably be most prevalent in rural areas.
However, hay fever is far more widespread in Japanese cities, near highways, suggesting that there are city air components that increase vulnerability. Allergic disease is also generally more prevalent in developed areas such as Europe and North America than in underdeveloped areas, suggesting that there may be something about the modern urban lifestyle that promotes it.
Another proposed explanation for the rise in allergic disease is the “tight building theory.” According to this concept, today’s energy-efficient urban buildings, with their sealed windows, have created the perfect microclimate for accumulating allergens such as dust mites, animal dander, fungal (mold) spores and insect parts. Supporters of this theory point out that many of us spend much time indoors, breathing in allergens, which exacerbates our respiratory allergies.
Other possible allergy-inducing aspects of our Western lifestyle include the rising popularity of pets, and changes in dietary patterns — for example, less breastfeeding, and earlier introduction of cow’s milk and solid foods that can sensitize an infant’s immune system.
Commercial products may also play a role. The frequency of allergic reactions is directly related to the number of people exposed (“population exposure”) to a given allergen. For example, exposure to latex, a natural product from trees used in surgical gloves and medical devices, has spawned an ever-growing crop of latex allergies.
When new consumer items or drugs appear on the market, product allergies often develop. Once penicillin became readily available after World War II, the frequency of allergic reactions to this medication jumped, until today penicillin is one of the most frequent causes of severe anaphylactic shock.
Similarly, the increasing diversity of peanut products, their presence in many prepared foods, their growing popularity as snack items — particularly for young children — and their presence in restaurant meals is producing what some call an epidemic of peanut allergies.