Donovan et al. 1996. Investigators challenge tested patients in an Environmental Exposure Unit with concentrations of ragweed of 2500 to 3000 grains/m3.  The purpose of the study was to test the efficacy of immunotherapy for ragweed.  Three groups of patients were used in the study including ragweed sensitive on immunotherapy >2 years, ragweed sensitive with no immunotherapy, and non-sensitive to ragweed.  It was found that the immunotherapy patients had fewer nasal symptoms.