*3.3. Life Cycle Inventory—LCI*

The life cycle inventory analysis (LCI) constituted the second phase of LCA, which is the analysis of the set of inputs and outputs. This model represents the product system structure, and its smallest elements are unit processes. A unit process is the smallest component of the product system for which data are collected. What is more, the processes are connected by material and energy flows. All individual unit processes represented in the model were subjected to a completeness check and evaluation, performed using the material and energy balancing. The data were acquired according to inventory types and collected in the inventory table. Data collection provides additional background information regarding the source of origin, geographic range, age, etc. Therefore, the acquired data help determine the spatial and temporal location of the problems. However, the key classification

criterion at this stage was the fact of belonging to the inventory of the studied system. In consequence, the problems were not assigned to a time or place, which, furthermore, is the reason why LCA is considered to be a so-called general technique. During analysis, the collection of data is adapted to model requirements and the impact category indicator (Figure 3) [43–45].

**Figure 3.** Inputs and outputs throughout the car tire life cycle. Own work based on [28].

The research conducted within this study was carried out with the use of SimaPro 8.4.1 Software. The data were mostly acquired from tire manufacturers, however, the data related to processes and materials, less relevant for the assessment of environmental burden, were derived from the Ecoinvent 3.4 database (a component of SimaPro product).

The aggregation of data concludes this phase and introduces the subsequent stage—LCIA [45].

#### *3.4. Life Cycle Impact Assessment—LCIA*

In essence, the primary function of the Life Cycle Impact Assessment (LCIA) stage is to determine the impact of the lifecycle of a given product on the environment. The differences in the methodologies of various approaches to LCA are principally concerned with the LCIA phase. Inventory analysis is focused on quantifying the environmental burden, and its reliability is relative to data quality and the choice of criteria and assumptions for building the product system [42,46].

LCIA consists of mandatory and optional elements. Mandatory elements include defining impact categories, category indicators, characterization models as well as classification and characterization, whereas among optional elements there are normalization, grouping, weighting, and data quality analysis. While the sequence of obligatory elements is strictly defined and should be followed as is during the analysis, with regards to the optional elements, the question is whether and which steps are to be taken. If the results from the analysis are to be publicized, normalization and weighting are typically omitted, and the interpretation is performed at the level of the LCIA environmental profile (impact categories). Although this results in the loss of transparency and increases the difficulty of the analysis, it does carry a positive outcome, specifically, the results are deprived of subjectivity [47,48].

The results from the LCIA phase are presented in Section 4.
