Recent times have seen an increase in the prevalence of allergic diseases due in part to the alterations and exacerbations of the global pollen cycle resulting from climate change. Climate change related temperature increases are associated with increased pollen production and an earlier and longer pollen season. A broad body of research has emerged demonstrating that green spaces in urban areas have mental health benefits. Yet, whether the relationship between green space and mental health may be altered by the presence of allergenic pollen remains underexamined. Using Scopus and Web of Science as search engines, this paper systematically reviews the emerging body of research exploring how allergenic pollen in green spaces may alter mental health, behavioural and perceptual outcomes. A total of 26 papers were identified. We found relatively consistent evidence that people living near green spaces with higher levels of allergenic pollen (real or perceived) experienced worse mental health than those living near green spaces with lower levels. However, for those currently visiting green space, the mental health effects of allergenic pollen were less clear. There was strong evidence that those who perceived the presence of pollen within a green space had adverse perceptions of that space. Green spaces with higher objective pollen counts also received less visitors. Finally, a small number of studies identified that green spaces that contained mainly hazel, alder, birch, cypress, linden, poplar, plane, maple and juniper trees were all associated with higher perceived pollen levels, avoidance, and adverse mental health effects, although more research is required to confirm this. We conclude with prospects for future research and also highlight the need to closely monitor and carefully select the species that are planted within green spaces
