Grass pollen as a trigger of emergency department presentations and hospital admissions for respiratory conditions in the subtropics: A systematic review

Introduction:
It is unknown if high concentration of airborne grass pollen, where subtropical grasses (Chloridoideae and Panicoideae) dominate, is a risk factor for respiratory health. Here we systematically reviewed the association between airborne grass pollen exposure and asthma emergency department (ED) presentations and hospital admissions in subtropical climates.

Objectives:
A systematic review was performed to identify and summarise studies that reported on respiratory health (asthma ED presentations and hospital admissions) and airborne grass pollen exposure in subtropical climates.

Methods
Searches were conducted in: MEDLINE, Web of Science, Scopus, CINAHL (EBSCO), Embase and Google Scholar databases (1966–2019). Risk of bias was assessed using a validated quality assessment tool. A meta-analysis was planned, however due to the heterogeneity in study design it was determined inappropriate and instead a narrative synthesis was undertaken.

Results:
Nineteen studies were identified for inclusion, with a total of 598,931 asthma ED presentation participants and 36,504 asthma hospital admission participants in six countries (Australia, India, Israel, Italy, Spain, USA). The narrative synthesis found airborne grass pollen appears to have a small and inconsistent increase on asthma ED presentations (judged as: probably little effect n = 5, may have little effect n = 4, no effect n = 2 and uncertain if there is an effect n = 4) and hospital admissions (judged as: probably increase slightly n = 2 probably little effect n = 1, may have a little effect n = 1, no effect n = 3 and we are uncertain if there is an effect n = 4) in the subtropics. Furthermore, the reported effect sizes were small and its clinical relevance may be difficult to discern.

Conclusion:
Exposure to airborne grass pollen appears to have a small and inconsistent increase on asthma ED presentations and hospital admissions in the subtropics. These findings are comparable to reported observations from studies undertaken in temperate regions.