The incidence of allergies and related conditions such as asthma and food allergies is increasing around the world. It is estimated that up to 1 in 3 people are allergic to one or more substances (allergens) at some point in their lives.
Hayfever, which was first recognized in England over a century ago, used to be an uncommon disorder, affecting less than 10% of the population. Allergic rhinitis, which is the medical name for hayfever, now occurs in at least 25% of the population.
Significant increases have also been noted in the incidence of bronchial asthma and of food allergies, such as peanut allergy.No one knows for sure why allergies are on the increase, but there are two leading theories.
The “hygiene hypothesis” suggests that a lack of childhood infections skews the immune system so that it is more likely to recognize common substances such as pollens as dangerous. Vaccinations, antibiotics, and improved sanitation have all changed the environment in which a child’s immune system develops.
Children living in more developed countries, in urban areas, with fewer siblings, who do not attend day care, and who are not around farm animals or cats are all more likely to develop allergies and asthma.
The other theory suggests that air pollution, especially diesel exhaust particles, plays a role. For example, the prevalence of allergies seems to parallel industrialization in many countries. There is research that diesel exhaust particles and other pollutants can act directly to affect the immune system. In either case, it is important to recognize and treat allergic diseases early before serious reactions, tissue damage, or chronic infections occur.