In an experimental study, ventilation filters of high quality (F7 & F9) were tested regarding their efficiency in collecting birch pollen allergens in outdoor air. The birch pollen grain concentration in outdoor air was measured at the same time as pollen allergen and particle number concentrations were measured before and after the tested ventilation filters, thus enabling collection efficiency calculations. Simultaneously, the size distribution of birch pollen allergens was measured in outdoor air using a cascade impactor. The study confirms previous indications that pollen allergens may occur in outdoor air in particles much smaller than pollen grains, and can penetrate ventilation filters to a larger extent than might be expected. This entails that although the high quality filters collect most of outdoor air pollen allergens, a significant exposure dose to these allergens can occur in the indoor environment, especially when considering the fact most people stay much more indoors than outdoors. The study also confirms previous similar indications attained with grass pollen allergens, in that the allergenic particles tend to penetrate ventilation filters to a greater extent than other airborne particles.