D'Amato et al. 1996. This is a study that compared outdoor pollen levels with allergen levels both inside and outside.  The study addresses the question, 'Is it useful to recommend that allergic patients remain indoors during pollen allergy season?'.  Investigators looked at the Parietaria season, testing outdoor and indoor air with the balcony door opened and closed.  They found that allergen levels indoors were similar to outdoors when the door was opened, but levels were lower with the door closed and concluded that remaining indoors would be an effective avoidance measure.