Objective In the current context of unprecedented urbanization and climate crisis, extreme weather events such as flood, storms and extreme heat occur frequently worldwide, posing a very adverse impact on human health and well-being in the future, and making it urgent to build healthy city. At the same time, with the deepening of urbanization, the phenomenon of urban vacant land is becoming increasingly prominent and spreading to cities in emerging markets and developing countries. The environmental and social problems caused by improper use of urban vacant land will also pose a threat to the health of urban residents. Among multiple models adopted by the international community to address urban vacancy issues, transforming urban vacant land into green infrastructure can not only serve within the “right sizing” strategy to improve the property depreciation and environmental degradation caused by vacancy itself, but also provide ecosystem services as green infrastructure, thus creating scarce ecological space in high-density cities and enhancing the health of urban ecosystems, which has gradually become one of the most mentioned concepts to tackle with urban vacancy issues. This research introduces the One Health framework to explain the health effects of urban vacant land as green infrastructure and the transformation pathway thereof. It further explores the diversified health effects of different types of urban vacant lands and corresponding evaluation systems, as a practical basis for deciding the future green infrastructure model for each type of urban vacant land.

Methods Based on a comprehensive literature review and case studies, this research aims to summarize the existing research and practice of urban vacant land and the transformation thereof worldwide in the past 20 years. Within the framework of One Health, the research elucidates the health effects of urban vacant land as green infrastructure and corresponding working mechanism. It then proposes a typology framework considering development intensity based on existing typology models. An evaluation index system is established, integrating vacant land research, green space evaluation, and green infrastructure planning. Lastly, transformation models for vacant land are summarized based on research results and case studies.

Results This research presents that from the perspective of One Health, the health effects of urban vacant land as green infrastructure mainly include three aspects. 1) Human health: Urban vacant land as green infrastructure can not only improve human mental health by increasing green exposure and creating a positive community atmosphere, but also promote physiological health through the encouragement of physical activity and enhancement of food health. Furthermore, implementing targeted interventions on urban vacant land can effectively mitigate health risks, including the reduction of violent crimes and the elimination of disease-carrying vectors. 2) Biological health: Urban vacant land as green infrastructure can function as ecological corridors to enhance the landscape connectivity of ecosystems, providing dispersal routeways for the migration of multiple species. Furthermore, it can offer valuable habitats for diverse urban species, playing a crucial role in biodiversity conservation. However, human interventions in urban vacant land may pose negative impacts on biodiversity. 3) Environmental health: Urban vacant land as green infrastructure has the capacity to modulate urban climate through the mitigation of air pollution and carbon sequestration. Additionally, it can capture surface runoff, thereby reducing the risk of regional flooding. The pathway for transforming urban vacant land into green infrastructure generally follows three progressive steps. 1) Build an urban vacant land typology model based on development intensity, including undeveloped vacant land, and vacant land with low, medium or high development intensity. This step allows researchers to preliminarily understand appropriate development models and redevelopment difficulties of different types of urban vacant lands. 2) Establish an evaluation index system for urban vacant land within the framework of One Health. This step allows researchers to carry out quantitative research of the potential of transforming each kind of vacant land into green infrastructure according to multifactor evaluation based on geographic information data. 3) Determine the transformation models of different types of urban vacant lands as green infrastructure according to the evaluation results. The transformation models include urban vacant land as stormwater management facility (oriented at environmental and biological health), public space (oriented at human health), and restoration and maintenance (transitional land use model for urban vacant land without high development value at present).

Conclusion The analysis reveals a general lack of awareness in China of reusing urban vacant land, especially the potential health value thereof when transformed into green infrastructure. The research literature and cases abroad account for a relatively large proportion among relevant published works. In addition, the huge differences in land ownership, development model, urban density and form may make the transformation pathway of urban vacant land in China quite differentiated from those in other countries. Therefore, future research and practice on the transformation of urban vacant land into green infrastructure should focus on the particularity of urban space and institutional environment of China, and explore potential health effects and transformation pathways of urban vacant land.