Over the past 2 decades, a significant portion of the world, particularly the United States, western and north-ern Europe, Canada, Australia, and New Zealand, has witnessed a substantial increase in allergic diseases. Approximately 2 decades ago, 3% of the American pop-ulation had asthma; today the prevalence is closer to 7%. Similarly, during the same time span, the prevalence of allergic rhinitis has increased from 10% to 20%; for atopic dermatitis, prevalence has increased from 5% to 8%; and for anaphylaxis, prevalence has increased from 1% to 3%.
This point is brought home even more dra-matically by a large national study, called the National Health and Nutrition Examination Survey (NHANES); in 2008, 58% of Americans tested positive for at least one airborne allergen.