This article is a step forward in the definition of the goal and scope of city Life Cycle Assessments (LCA). We contribute to city LCA by proposing procedures (i) to set city boundaries and (ii) for allocating burdens among cities. The study has been based on a systematic literature review of existing guidelines and standards as well as academic work linked to system boundary definition and to allocation procedures setting. Internationally accepted guidelines provide a basis on how current city assessments should be performed. LCA-based and LCA ISO 14040 compliant case studies of other types of systems provide background knowledge to analyse the applicability of existing methods to define the system boundaries and to apply allocation procedures to the case of a city LCA. The analysis and proposal of procedures is complemented with a survey sent to environmental and urban management researchers and specialists. Geographical boundaries and functional boundaries compose the city LCA boundaries. Three methods for setting the boundaries for a city LCA are proposed: (i) administrative-based boundaries, defined by the area delineated for the purpose of local administration; (ii) density-based boundaries, defined by the continuity of the population density of an urban area; and (iii) service-based boundaries, defined by the density-based boundaries plus those services induced by the city. Four allocation methods are proposed: (i) monetary-based allocation, which uses the added value generated at each stage in the life cycle to distribute the burden; (ii) producer-based allocation, which assigns impacts to the city that holds the production of the good; (iii) consumer-based allocation, which assigns impacts to the city that hosts the final user/consumer of the good; and (iv) category-based allocation, proposed by the authors, according to which, global impacts are assigned to the city where consumption takes place, while local impacts are allocated to the city where the impacts occur. Density- and service-based boundaries are the most parsimonious as we see it is the simplest way to simulate the complexity and interdependency of cities. Geographical boundaries of a city LCA should be defined through the density-based procedure, which should include cities, towns and suburbs connected with a continuous density of population. However, the functional boundaries of the city LCA should be defined through service-based procedure, which would help to include, in the city system, those induced activities that happen out of the geographical boundaries of the city. The chosen allocation procedure should follow the causality principle between the impact and the activity performed. The proposed method for allocating impacts to the producer or the consumer, depending on whether the impact category under consideration is local or global, has been found through a survey to be the most preferred way to share responsibilities along the supply chain. Thus, the category-based allocation procedure is the suggested one. The survey results show how density- and service-based boundaries are the preferred by the panel of experts, by giving best marks in lifelikeness and accuracy. They also prefer the category-based allocation procedure, by giving best marks in informativeness, lifelikeness, and accuracy, and second in influence.
