From Our Stolen Future:

The prevalence of allergic reactions has been increasing rapidly in the developed world for the last several decades. This has been happening during a period when overall air quality has been improving and during which there is no reason to believe that allergens like dust mites or cockroach dandruff have become more common. These observations have led some scientists to propose that unidentified environmental exposures may be increasing the sensitivity of people’s immune system, leading to heightened allergic responses even though the level of allergens hasn’t increased.

In this paper, Takano et al. provide a clue as to a possible cause of this increase, reporting that exposure to low, environmentally-relevant levels of the phthalate DEHP exacerbates allergic reactions in mice to a dust mite allergen.

They conclude that exposure to DEHP may be contributing to the increased incidence of allergic reactions in developing countries, where phthalate exposure is now ubiquitous.