Airborne birch pollen dispersal in an urban setting – case study

An increase in pollen related allergies is anticipated as a result of climate change. This effect is expected to be more pronounced in urban areas that act as heat islands. Although green spaces are intended to mitigate the impact of urban warming, thoughtful greening strategies are required to limit hay fever related symptoms. To integrate pollen exposure of citizens into green space designs and enhance forecasting models and risk maps with finer-scale effects, it is essential to understand the short-range dispersal of allergenic pollen. The aim of this study is to quantify the short-range dispersal of the highly allergenic pollen from the birch (Betula spp.) tree and to compare pollen levels in urban situations with varying numbers of birch trees. To assess these aspects, airborne birch pollen levels were collected with handheld samplers. Pollen levels were compared between three urban localities with none to several birch trees using light microscopy. Additionally, pollen levels were sampled at different distances from a birch tree. The effect of tree distance was tested while correcting for metereological factors. DNA analysis were used to discriminate pollen from the source tree (Betula utilis) from background birch pollen (mainly other Betula species). Our findings indicate that pollen levels are significantly higher in streets with multiple birch trees compared to streets with none or a single birch and decrease between 0 and 25 m from the source. Although this is a case study including few locations and it is unknown how these results may be generalized to other urban situations, these results highlight the value of incorporating local pollen sources into fine-scale forecasting models rather than relying solely on pollen concentrations currently taken at rooftop levels. Moreover, given the known adverse health effects of allergenic pollen, our findings suggest that birch trees should be planted at least 25 m away from unavoidable public places such as hospitals and schools while avoiding high densities of birch trees.