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

Abstract

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.

Introduction

The prevalence of asthma and allergic disease appears to be increasing globally, with an estimated 300 and 400 million people suffering from asthma and rhinitis respectively (Lai et al., 2009; To et al., 2012). The common coexistence of allergic disease in people with asthma, further complicates the burden of disease. Both asthma and allergic rhinitis appear more prevalent in developed countries (Lai et al., 2009; To et al., 2012). The United Kingdom (UK) shares among the highest burden of asthma in Europe, with an estimated 10–13% of the population affected (Lai et al., 2009; To et al., 2012). While the UK is wholly located in a temperate region, other developed countries span temperate as well as subtropical climatic zones; United States of America (USA) and Australia suffer high asthma morbidity of 8 and 10% respectively (Australian Bureau of Statistics, 2018; Lai et al., 2009; To et al., 2012). A systematic review of observational studies reported high concentration of airborne grass pollen is a risk factor for child and adolescent asthma emergency department (ED) hospital presentations in numerous countries: USA, Canada, Australia, Spain and Israel (Erbas et al., 2018). The Melbourne thunderstorm asthma event of 21 November 2016, further showed an association between high concentration of airborne grass pollen and respiratory health (Lee et al., 2017; Rangamuwa et al., 2017; Sutherland et al., 2017; Thien et al., 2018). During the event, thunderstorm asthma was associated with an estimated 42% (95%CI: 40%, 44%) increase in the number of cases requiring emergency medical service and a 432% increase in emergency medical attendances for acute respiratory distress symptoms (Andrew et al., 2017). Research on risk of airborne grass pollen and asthma exacerbation, has primarily emanated from cooler climates where temperate grasses (Pooideae) dominate. The risk of airborne grass pollen on asthma ED presentations and hospital admissions may differ in regions where subtropical grasses (Chloridoideae and Panicoideae) dominate, such as parts of Americas, Africa, Australia, Asia and Europe (Osborne et al., 2014; Still et al., 2003). A knowledge gap exists in our understanding of the risk of airborne grass pollen on asthma ED presentations and hospital admissions, in populations from subtropical climates. To address this gap in knowledge, this review examined the effect of airborne grass pollen exposure on asthma ED presentations and hospital admissions in subtropical climates.