Substantial evidence highlights the positive correlations between engaging with green spaces and various mental health outcomes, ranging from alleviating depression and anxiety (Gascon et al., 2018) to mitigating the effects of stressors (Astell-Burt and Feng, 2019; Beyer et al., 2014; Mitchell et al., 2015) and enhancing the mental health of urban residents (Vanaken and Danckaerts, 2018; Wendelboe-Nelson et al., 2019). Nevertheless, a small body of evidence has also found null relationships between the extent of green space and mental well-being (Houlden et al., 2017; Huynh et al., 2013) and has even shown that higher access to green space was associated with poor maternal mental health (Wei et al., 2021). Such discrepancy was also reported in Kondo et al. (2018)'s systematic review, which concluded that higher exposure to green space was linked with improved attention restoration and mood, but not always with lower depression and stress.

It is possible that these inconsistencies in findings can be attributed to the neglect of green space quality aspects (Sivak et al., 2021). That is, the benefits of green spaces may not be limited to their size or quantity; rather green space quality attributes may be important drivers of these associations (Zhang et al., 2017). For instance, high-quality green spaces (attractive and useable) have been shown to influence health through place attachment, passive (visual) enjoyment, and increased engagement in recreational or social activities in those spaces (Zhang et al., 2015). Such studies have developed distinct methods to quantify the various dimensions of green space quality, largely using observational audit tools. However, these audit tools can vary significantly in their design and application depending on specific objectives of each study (Knobel et al., 2019), reflecting varied interpretations of what constitutes a high-quality green space. This diversity in measurement may hinder the achievement of consistent and comparable results across different studies, which has in turn hampered the widespread incorporation of quality assessment in health research (Knobel et al., 2019).

In order to evaluate which characteristics are associated with benefits, it is important to consider the manner in which individuals interact with green spaces – either through active or passive engagement. For example, benefits may be found when actively using the green space for activities, such as running and picnicking, while others might be found through the passive experience of being in view (Maas et al., 2009; Vaeztavakoli et al., 2018) or within earshot (Buxton et al., 2021) of nature. Thus, quality aspects of green space may relate to both active and passive engagement. For example, the presence of amenities such as benches and shelters may promote the social engagement dimensions of green space (Ottoni et al., 2016; Rasidi et al., 2012), while other amenities such as sports fields may promote the physical activity dimensions of green space (Zhang et al., 2019), both of which have been indirectly associated with mental health (Lachowycz and Jones, 2013). Quality aspects such as biodiversity and lack of litter may be more relevant for passive enjoyment of nature. In fact, unsafe, unclean, or not aesthetically pleasing green spaces have been associated with higher stress (Sivak et al., 2021), emphasizing the health importance of (lack of) quality.

With the increasing interest in quality, there has been a shift in focus from quantity to quality of green space (Van den Berg et al., 2015). However, significant gaps remain, which have not been fully addressed in the three existing reviews. First, a recent scoping review of the role of environmental science in mental health research from Roberts et al. (2023) identified a critical gap in quantifying green space exposures – identification of relevant green space qualities most conducive to mental health. Second, a review by Nguyen et al. (2021) identified green space qualities and types associated with health benefits, broadly. This review included perceptions of quality, non-audit techniques, and outcomes beyond mental health. Yet, the importance of addressing mental health, specifically, through quantifiable, auditable qualities has not been fully addressed. Last, Knobel et al. (2019) reviewed observational audit tools, evaluating their validity, replicability, and comparability, as composite measures of quality. Thus, there is a need for further examination of the specific elements of quality within tools used to identify those most frequently associated with mental health benefits (rather than composite measures). Based on these three existing reviews, our study aimed to address the following questions: 1) Which audit tools are relevant to mental health when evaluating green space qualities, and 2) which specific aspects of green space quality, assessed via audit tools, are most consistently and significantly, or potentially, associated with mental health benefits? In answering these questions, we aimed to advance this area of inquiry into the specific components of greenspace quality that yield the greatest and most reliable mental health benefits for future research.