## 5 Environmental Aspects and Health Risks
### Leading Characteristics and Release of MPs from Plastic-Modified Asphalt Mixtures
Plastics that exhibit inadequate compatibility with bitumen may segregate during mixing and application, particularly those characterized by a high molecular weight or low maltene fractions, resulting in the possible leaching of unbound polymers and additives [14]. These incompatibilities elevate the probability of plastic fragments detaching during operation, hence facilitating MP formation [38]. Toxicological issues arise from the release of MPs due to residual additives, including flame retardants, phthalates, and heavy metals, which may leak into surrounding soils and waterways [4; 14; 73]. MPs also serve as carriers for these contaminants and may facilitate their entry into biological systems via ingestion or dermal contact [14; 77]. In ecosystems, these particles can experience bioaccumulation and biom magnification, hence increasing ecological concerns [4; 78]. Although promising results indicate that the encapsulation of MPs in bitumen mastics can diminish release by more than 99% [38], long-term environmental monitoring is essential to validate laboratory-based hypotheses [14]. Recent studies utilizing road dust analysis and batch leaching under simulated rainfall indicate that well-integrated plastic-modified asphalts exhibit performances comparable to those of traditional asphalts for DOC and metal leaching [4; 73; 79; 25]. Recent research has evaluated MP release behaviours for various plastic kinds and inclusion methods. Ref. [80] indicated that MP release from recycled-plastic-modified asphalt was equivalent to that from commercial-polymer-modified asphalt, with plastic content being a significant influencing factor in wet methods, while particle size was more crucial in dry methods. LDPE/LLDPE blends emitted markedly fewer MPs compared to commingled PE/PP plastics, which is attributable to their superior melting compatibility and encapsulation, with the majority of released particles measuring below 38 um. Conversely, PP-rich mixes showed larger MP fragments (>368 um), suggesting the significance of plastic selection for environmental safety. In addition, Ref. [73] assessed airborne microplastic contaminant particles (MCPs) in dry-mixed plastic-modified asphalt composed of polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET), finding no detectable MCP contamination exceeding FTIR similarity thresholds throughout the mixing, laying, or in-service simulation phases. Although PM\({}_{2.5}\) emissions rose during laying, in-service emissions remained insignificant, suggesting no long-term airborne microplastic concerns. These findings underscore the necessity for prolonged field monitoring and aquatic leaching evaluations to comprehensively analyse microplastic release concerns associated with various plastic kinds and modification techniques. Figure 3 illustrates the environmental impacts of incorporating plastic into asphalt mixtures, summarizing the key pathways and mechanisms of leaching, fume emissions, and microplastic release associated with plastic-modified asphalt. Life Cycle Environmental Impact and Life Cycle Cost Characteristics of Plastic-Modified Asphalt Pavement  
The environmental impact of conventional asphalt pavements is primarily influenced by the material production phase, particularly because of the extraction and processing of virgin bitumen and aggregates [81; 82; 83]. Several studies have shown that replacing these materials with recycled plastic polymers such as PET, HDPE, LDPE, and PP can significantly decrease energy usage, greenhouse gas (GHG) emissions, and related environmental impacts [84; 85; 26; 86]. Environmental benefits also encompass diminished dependence on virgin aggregates, diversion of plastic waste from landfills, and a reduced carbon footprint [13; 85; 86; 87; 88]. The lightweight characteristics of plastic aggregates enhance handling and transportation [26]. Furthermore, the encapsulation of plastic within asphalt matrix may diminish the release of MPs, providing a dual environmental advantage [4; 13].