The incidence of actual allergies is rising in adults and children around the world, for reasons that aren't totally understood. A popular theory is that modern hygiene has reduced the number of germs children are exposed to, so that their immune systems don't develop fully and attack harmless substances. Allergies may develop late in life, or seem to do so, in adults who had mild seasonal allergies that went unnoticed when they were children; the allergies may have become more severe as pollen counts have worsened.
Some environmental triggers are getting worse. In northern U. S. and Canada, pollen season in 2009 lasted 27 days longer than in 1995, a recent study found. Ragweed season in the New York-New Jersey area started last week, 10 days earlier than usual, says Leonard Bielory, an allergy specialist at Rutgers University and co-author of the study in the Proceedings of the National Academy of Science. "My prediction is that next week, it will just explode."