Background: Allergy to tree pollen is common and aeroallergens have been associated with severe asthma exacerbations in the community setting. To determine the impact of different trees on asthma, we tested the association between daily hospitalizations for asthma and daily concentrations of different tree pollens in 10 large Canadian cities. Methods: Daily time-series analyses were employed to remove unwanted temporal trends. For each family or genus, results were adjusted for day of the week, temperature, barometric pressure and relative humidity. Results were expressed as the percentage increase in asthma hospitalizations related to an increase in tree pollen concentration equivalent in magnitude to its interquartile range. Results: For an interquartile increase in daily tree pollen concentration, percent increases in daily hospitalization for asthma were: 2.63% (95% CI 1.19–4.07) for Ulmus (elm), 2.45% (1.12–3.78) for the group containing Pinaceae (pine, fir, spruce), Tsuga (hemlock) and Larix (larch, tamarack); 2.32% (0.93–3.71) for the group containing Quercus (oak) and Castanea (chestnut), and 2.16% (0.70–3.62) for Acer (boxelder and maple). Statistically significant (p < 0.05) but small (<2%) effects were observed for Fraxinus (ash), Populus (aspen, poplar), Alnus (alder), Betula (birch) and Corylus (hazelnut). Conclusions: Several common tree pollens are an important cause of acute exacerbations of asthma severe enough to require hospitalization.

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