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Background: Experimental challenge studies have shown that pollen can have early and delayed effects on the lungs and airways. Here, we qualitatively and quantitatively synthesize the evidence of outdoor pollen exposure on various lung function and airway inflammation markers in community‐based studies. Methods: Four online databases were searched: Medline, Web of Science, CINAHL and Google Scholar. The search strategy included terms relating to both exposure and outcomes. Inclusion criteria were human‐based studies published in English that were representative of the community. Additionally, we only considered cross‐sectional or short‐term longitudinal studies which investigated pollen exposure by levels or season. Study quality assessment was performed using the Newcastle‐Ottawa scale. Meta‐analysis was conducted using random‐effects models. Results: We included 27 of 6551 studies identified from the search. Qualitative synthesis indicated associations between pollen exposure and predominantly type‐2 inflammation in both the upper and lower airways, but little evidence for lung function changes. People with ever asthma and/or seasonal allergic rhinitis (SAR) were at higher risk of such airway inflammation. Meta‐analysis confirmed a positive relationship between pollen season, eosinophilia and eosinophil cationic protein (ECP) in people with ever SAR but the results between studies were highly variable. Heterogeneity was reduced after further subgrouping by age, and the forest plots indicated that eosinophilic airway inflammation to outdoor pollen exposure increased with age. Conclusion: Among people with ever asthma and ever SAR, exposure to increased ambient pollen triggers type‐2 upper and lower airway inflammation rather than a non‐specific or innate inflammation. These findings can lead to the formulation of specific pollen immunotherapy for susceptible individuals. Future research should be directed towards investigating lagged associations and effect modifications using larger and more generalized populations. Systematic review registration: CRD42020146981 (PROSPERO). [ABSTRACT FROM AUTHOR]

Thunderstorm asthma is defined as epidemics of asthma occurring shortly after a thunderstorm. While grass pollen has been implicated in thunderstorm asthma events, little is known about the role of fungi and studies have not been synthesised. This systematic review aims to evaluate whether grass pollen is necessary in thunderstorm asthma events and whether fungi also play a part in these associations. We conducted a systematic search using six electronic databases (i.e. CINAHL, Medline (Ovid), Web of Science, ProQuest Central, EMBASE and Google Scholar) and checked reference lists. The search terms used were pollen AND thunderstorm* AND asthma. The inclusion criteria were studies published in English with original human data relating to outdoor pollen and thunderstorm asthma. Twenty of 2198 studies were eligible. Reported findings differed due to variation in methodological approaches and a meta-analysis was not possible. Nonetheless, of the 20 studies included, 15 demonstrated some relationship with nine demonstrating lagged effects up to four days for increasing grass pollen counts associated with increased risk of thunderstorm asthma. Of the 10 studies that examined fungi, nine demonstrated a positive relationship with thunderstorm asthma. The fungal taxa involved varied, depending on whether measurements were recorded before, during or after the thunderstorm. Nevertheless, none of the studies considered fungi as a potential effect modifier for the pollen-thunderstorm asthma association. We found evidence to suggest that grass pollen was a necessary factor for thunderstorm asthma but there are other as yet unrecognised environmental factors that may also be important. Further research is required to examine the role of fungi and other environmental factors such as air quality as potential effect modifiers of the association. • Grass pollen is most commonly associated with thunderstorm asthma. • There is a lagged effect of up to four days. • Grass pollen on its own is not enough to cause thunderstorm asthma. • Fungi is also important, but more studies are required. [ABSTRACT FROM AUTHOR]