Birch pollen allergen exposure: profiles, sources and characteristics. Settled dust and aeroallergens in indoor and outdoor environments of Finnish homes

Abstract

The main source for indoor birch pollen (BP) allergens is outdoor particles, which are carried indoors mainly by people or pets and less likely via open windows and doors. So far, BP allergens have been shown by ELISA in the indoor air or in the settled dust, but these techniques have not enabled a reliable analysis of the respirable-sized fraction of air particles allergic subjects are exposed to. The aims of this study were to measure the airborne personal BP allergen exposure indoors and outdoors, to study the particle size of inhaled BP allergens and to analyse the allergen concentration of settled dust in relation to personal airborne allergen load. The air samples were collected before, during and after the BP season using a nasal air sampler, and the samples of settled dust were collected by a vacuum cleaner with a special collection device. BP allergens collected by nasal samplers were detected by the HALOgen immunoassay using birch pollen specific human IgE and rabbit IgG antibodies to BP, and the results were compared to IgG-ELISA used for detecting BP allergens in indoor settled dust and outdoor air. The highest concentrations of BP antigenic activity in settled dust were in the entrance corridor (next to the main front door) decreasing substantially from outdoors to indoors. Significant personal exposure to the airborne particles containing BP allergens outdoors was shown by the HALOgen immunostaining, before, during and after the BP season. During the BP season, the large, outdoor airborne particles carrying BP allergens were composed mainly of the pollen grains (75%). However, outside the pollen season, only a few large particles with stained (allergen) halos were found, and only 50% of those were pollen grains. In the indoor air, stained large and small particles appeared mainly during the pollen season and remained detectable until the end of the 6-week follow-up period. The results of the indoor settled dust BP antigenic activity detected by IgG-ELISA showed a good correlation with those of the HALOgen immunostaining. Human IgE detected BP grains with halos containing BP allergens from indoor air during the pollen season but none after the season. Higher sensitivity of the HALOgen™ assay was obtained using rabbit IgG antiserum, which revealed both small (5–20 μm) and large (>20 μm) particles with halos both before, during, and outside the peak BP season. Therefore, it is unlikely that the birch allergy symptoms indoors, after the pollen season, would be due to intact airborne pollen grains, but rather are due to other small particles carrying airborne BP allergens. A probable source of these particles is settled dust, which has been carried indoors by people or pets.