Short-term exposure to pollen and the risk of allergic and asthmatic manifestations: a systematic review and meta-analysis

BACKGROUND: Several studies have assessed effects of short-term exposure to pollen on allergic and asthmatic manifestations. The evidence is inconclusive, and no meta-analysis has been published. OBJECTIVE: To synthesise the evidence on the relations between short-term pollen exposure and the risk of allergic and asthmatic manifestations. METHODS: We performed a systematic literature search of PubMed and Scopus databases up to the end of August 2018. In addition, we reviewed the reference lists of relevant articles. Two authors independently evaluated the eligible articles and extracted relevant information in a structured form. We calculated summary effect estimates (EE) based on the study-specific ORs and regression coefficients (β) by applying both fixed-effects and random-effects models. RESULTS: 26 studies met the a priori eligibility criteria, and 12 of them provided sufficient information for the meta-analysis. The summary EE related to 10 grains per m³ increase in pollen exposure showed an 1% increase (EE 1.01, 95% CI 1.00 to 1.02) in the risk of lower respiratory symptoms and a 2% increase (EE 1.02, 95% CI 1.01 to 1.03) in the risk of any allergic or asthmatic symptom. Correspondingly, the risk of upper respiratory symptoms and ocular symptoms increased 7% (EE 1.07, 95% CI 1.04 to 1.09) and 11% (EE 1.11, 95% CI 1.05 to 1.17), respectively, in relation to such pollen exposure. Short-term exposure to pollen did not show any significant effect on daily lung function levels. CONCLUSION: Our results provide new evidence that short-term pollen exposure significantly increases the risks of allergic and asthmatic symptoms.