Approximately 70% of the global population is projected to reside in urban areas by 2050, which presents significant challenges in urban development (United Nations, 2018). One of the key goals for achieving sustainable development is building resilient and sustainable cities, with a particular focus on fostering the development of healthy cities (Giles-Corti et al., 2020). Due to the rapid urbanization, uneven socioeconomic status and resources allocation across various areas may lead to health inequalities, which constraints the development of healthy cities (Vlahov et al., 2007). Health inequality is commonly defined as systematic, avoidable, and unfair disparities in health outcomes observed among different sociodemographic groups at both individual and geographic level (McCartney et al., 2019). Researchers have dedicated their efforts to observe and understand health inequalities in cities. For example, a European study identified the socioeconomic inequalities in cause-specific mortality and the disparities in their magnitude across 15 European cities (Marí-Dell’Olmo et al., 2015). A study from China reported that the rapid urbanization resulted in health inequalities in the country due to large-scale migration, pollution, and lifestyle transitions (Yang et al., 2018). Masuda et al. (2012) summarized that health inequalities were associated with differences in social and natural environments between less and more affluent neighborhoods based on their observations in Vancouver, Toronto, and Winnipeg, Canada. All the evidence serves as a stark reminder of the persistence of health inequalities in global cities, highlighting the pressing challenge of addressing these issues.

Social deprivation is a frequently used concept to describe the disadvantages faced by a group of individuals in terms of accessing material and social resources (Wang et al., 2021). The existing body of studies has examined the impacts of different aspects of social deprivation on both physiological and psychological health outcomes to illustrate the causes of health inequalities. Socioeconomic inequality has been identified as a significant factor contributing to health inequalities (Pickett and Wilkinson, 2015). People with lower income or those living in less affluent areas tend to experience poor health and well-being (Babones, 2008; Bor et al., 2017; Chetty et al., 2016; Meijer et al., 2012; Rosengren et al., 2019). Besides, some studies demonstrated that people with poorer housing condition, lower education attainment and occupational grade are more likely to experience negative health outcomes (Downing, 2016; Dugravot et al., 2020; Dunn, 2000; Kivimäki et al., 2020; Pevalin et al., 2017; Raghupathi and Raghupathi, 2020; Ross and Mirowsky, 1999; Xie et al., 2015). Moreover, areas with higher unemployment rate and higher proportion of disadvantaged people are presented to be associated with higher rates of morbidity and mortality in previous studies (Drydakis, 2015; Greves Grow et al., 2010; Weimann et al., 2016).

Notably, single socioeconomic factors (e.g., income) are not able to comprehensively represent the social deprivation status in a neighborhood, which is closely linked to the inequalities in public resource allocation. Hence, there is a need to establish a comprehensive social deprivation index by integrating socioeconomic factors from diverse dimensions. Some studies have constructed social deprivation indices to investigate the association between overall social deprivation at neighborhood level and aggregated health outcomes. They found that increase in social deprivation was commonly associated with higher risks of mortality and other negative health outcomes (O'Farrell et al., 2016; Ribeiro et al., 2018; Sánchez-Santos et al., 2013). These studies primarily originated from the Western context, and there is limited evidence in the Asian context with very different welfare systems and social-cultural traditions, except for two recent studies conducted in Hong Kong. These two studies constructed social deprivation indices using various indicators and found that social deprivation were positively associated with cancer mortality and suicide mortality (Wang et al., 2021; Yeung et al., 2022). However, these two studies did not investigate the effects of social deprivation on all-cause and other cause-specific mortality, nor did they explore variations in these effects over time. Hence, further investigation is necessary to enhance the existing evidence. Moreover, the association between neighborhood social deprivation and mortality may exhibit variation in different periods due to socioeconomic development, particularly over a long time period, which has received limited attention in previous studies. Understanding the temporal dynamics of such associations is significant for gaining a comprehensive understanding of the evolution of health inequalities.

Meanwhile, researchers have made efforts to examine the benefits of urban natural environment on urban health issues. Exposure to urban greenness has been identified as having beneficial effects on health in urban settings in extensive previous studies. Researchers found that usage of urban parks, accessibility to urban green spaces, visible urban greenery, and constructions of urban green facilities probably improved both physical and mental health, as well as social well-being (Ayala-Azcárraga et al., 2019; He et al., 2022; Hunter et al., 2019; Labib et al., 2020; Liu et al., 2020; Remme et al., 2021). Some evidence suggests that exposure to greenness had protective effects on all-cause and cause-specific mortality (Barboza et al., 2021; Bauwelinck et al., 2021; Rojas-Rueda et al., 2019). Furthermore, some studies have examined the potential moderating effects of greenness exposure on health inequalities. Mitchell and Popham were the first to observe that the adverse impacts of income deprivation on mortality were less prominent in areas with higher levels of greenness exposure in UK (Mitchell and Popham, 2008). Furthermore, researchers have also discovered the protective effects of greenness exposure on socioeconomic inequalities in various health outcomes including chronic diseases, all-cause mortality, and mental disorder in countries such as the US, UK, Canada, Belgium, and the Netherlands) (Brown et al., 2016; van den Berg et al., 2015). However, the evidence regarding the moderating effects of urban greenness and health inequalities mainly relied on single socioeconomic factors (e.g., income), without considering the overall status of social deprivation.

To sum up, existing studies provide robust evidence of beneficial effects of greenness exposure and adverse impacts of social deprivation on different health outcomes, as well as the protective effects of greenness exposure on socioeconomic inequalities in health outcomes. However, there are still some gaps that need to be addressed. First, few studies have simultaneously demonstrated the spatiotemporal variations of overall social deprivation, greenness exposure, and mortality. Their associations have yet to be thoroughly discussed under a long-term context in previous studies. Addressing this issue is valuable to reveal a long-term process of changes in social and natural environment and health situations, along with their interplay at the neighborhood level. This contributes significantly to gaining insights for a comprehensive understanding of urban development and implementing tailor-made and sustainable planning strategies to achieve the healthy city goals. Second, studies exploring associations between overall social deprivation and mortality risks are mainly from western countries, with limited exploration focusing on Asian context with different social and welfare conditions, while evidence concerning the extent to which mortality risks arising from overall social deprivation may vary across areas with different levels of greenness exposure remains limited. Addressing this gap not only enhances the evidence of the nexus between overall social deprivation and mortality in the Asian context, but also provides suggestions for policymakers to mitigate health inequalities through effective environmental planning implementations. To fill these gaps, this study aims to (1) construct a comprehensive social deprivation index, evaluate greenness exposure, and calculate all-cause and cause-specific mortality at neighborhood level in Hong Kong, as well as demonstrate their spatiotemporal dynamics; (2) examine independent associations of social deprivation and greenness exposure with all-cause and cause-specific mortality over different periods during 1999–2018; (3) investigate whether greenness exposure could mitigate the adverse impacts of social deprivation on mortality risks in Hong Kong.