Built environment stocks (buildings and infrastructures) play multiple roles in our socio-economic metabolism: they serve as the backbone of modern societies and human well-being, drive the material cycles throughout the economy, entail temporal and spatial lock-ins on energy use and emissions, and represent an extensive reservoir of secondary materials. This review aims at providing a comprehensive and critical review of the state of the art, progress, and prospects of built environment stocks research which has boomed in the past decades. We include 249 publications published from 1985 to 2018, conducted a bibliometric analysis, and assessed the studies by key characteristics including typology of stocks (status of stock and end-use category), type of measurement (object and unit), spatial boundary and level of resolution, and temporal scope. We also highlighted the strengths and weaknesses of different estimation approaches. A comparability analysis of existing studies shows a clearly higher level of stocks per capita and per area in developed countries and cities, confirming the role of urbanization and industrialization in built environment stock growth. However, more spatially refined case studies (e.g., on developing cities and non-residential buildings) and standardization and improvement of methodology (e.g., with geographic information system and architectural knowledge) and data (e.g., on material intensity and lifetime) would be urgently needed to reveal more robust conclusions on the patterns, drivers, and implications of built environment stocks. Such advanced knowledge on built environment stocks could foster societal and policy agendas such as urban sustainability, circular economy, climate change, and United Nations 2030 Sustainable Development Goals.

In order to fulfill the objectives of environmental protection, today's focus on quantitative recycling rates must be amended by a more qualitative approach. Because modern products represent a mix of numerous and sometimes hazardous substances, ways must be explored to remove detrimental substances during recycling and to establish “clean cycles”. On the one hand, such a “clean cycle” strategy will result in better recycling qualities of secondary products and less dissipation of hazardous substances during further product use. On the other hand, the elimination of hazardous substances during recycling requires sinks for the disposal of the eliminated materials. These topics are presented in general as well as by case studies. In particular, the sink issue is addressed, differentiating between sinks and final sinks and discussing the challenge to supply appropriate final sinks for all materials that cannot be recycled.

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Material management faces a dual challenge: on the one hand satisfying large and increasing demands for goods and on the other hand accommodating wastes and emissions in sinks. Hence, the characterization of material flows and stocks is relevant for both improving resource efficiency and environmental protection. This article focuses on the urban scale, a dimension rarely investigated in past metal flow studies. We compare the copper (Cu) metabolism of two cities in different economic states, namely, Vienna (Europe) and Taipei (Asia). Substance flow analysis is used to calculate urban Cu balances in a comprehensive and transparent form. The main difference between Cu in the two cities appears to be the stock: Vienna seems close to saturation with 180 kilograms per capita (kg/cap) and a growth rate of 2% per year. In contrast, the Taipei stock of 30 kg/cap grows rapidly by 26% per year. Even though most Cu is recycled in both cities, bottom ash from municipal solid waste incineration represents an unused Cu potential accounting for 1% to 5% of annual demand. Nonpoint emissions are predominant; up to 50% of the loadings into the sewer system are from nonpoint sources. The results of this research are instrumental for the design of the Cu metabolism in each city. The outcomes serve as a base for identification and recovery of recyclables as well as for directing nonrecyclables to appropriate sinks, avoiding sensitive environmental pathways. The methodology applied is well suited for city benchmarking if sufficient data are available.

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