Birch and grass pollen are the main cause of seasonal allergies in Northern Europe. There is clinical confirmation that allergic symptoms may also occur outside the local pollen season. The objective of this study was to find possible explanations to these `late` season symptoms. One cause for these symptoms was thought to be indoor settled dust. Airborne birch and grass pollen concentrations were not analysed in this study. Indoor and outdoor dust samples obtained by vacuuming were analysed by the ELISA (enzyme-linked immunosorbent assay) method. The persistence of birch and grass pollen allergens was tested by ELISA and that of birch pollen also by IgE-immunoblotting. In order to obtain a quick immunochemical analysing method, airborne grass-pollen samples were analysed by immunostaining image analysis, and the ELISA method.

Birch and grass pollen were shown to maintain their allergenic activity until the following pollen season in experimental conditions at 20 ^oC and at different relative humidities (23 %, 33 %, 43 % and 54 %). The relative humidities tested, the range of which coincides with normal values in Finnish homes, did not have any particular effect on the persistence and variation of pollen allergenic activity.

The present study demonstrates one explanation for post-season pollen allergenic symptoms. Materials originating from birch and grass pollen have been shown to maintain their allergenic activity in indoor and outdoor settled dust, during  and after the pollen period, in both urban and rural areas. Outdoor antigen concentrations were higher than indoor ones. Allergenic activity in indoor settled dust peaked weeks later than the maxima of the the pollen periods. The correlation between the antigenicity of outdoor an indoor settled dust and the lag of about 3 weeks in indoor settled dust compared to that outdoors gave support for the finding that allergens are carried indoors by the movement of people and pets. The concentrations seemed to be slightly higher in urban homes, but the number of samples was not large enough to draw any definite conclusions. It seems evident that the allergenic activity of grass pollen in Finnish homes never reaches the level of birch pollen allergen concentrations, but even grass pollen allergenic activity may be high enough to cause allergic symptoms also after the pollen period.