## 4 Critical Discussion of Environmental and Economic Indicators
Cu- and Zn-based NMs can also induce high toxicity, causing cellular toxicity via multiple mechanisms (e.g., the disruption of cell walls, nucleic acid damage, and the release of toxic metal ions) [145]. These relevant environmental indicators for NMs in construction include the potential incidence of disease due to particulate matter emissions, ecotoxicity (potential comparative toxic unit for ecosystems), human toxicity (cancer and non-cancer effects), potential comparative toxic unit for humans, and land-use-related impacts on soil quality. The proposed NM indicators should also be taken into consideration for the Environmental Product Declarations, which provide life cycle assessment (LCA) impacts per material or categorized product in the construction sector. In fact, it is worth mentioning that the data available for materials or elements containing NMs in their composition are generally insufficient, even when considering large databases for ecological evaluations, as in the case of Okobaudat [146]. However, there is currently an effort underway to change this reality. Efforts to develop testing guidelines for nanomaterials are ongoing, and the outcomes are becoming increasingly accessible [14]. This clarifies why the most crucial environmental indicators were identified as those related to human health and ecosystems, as these impacts must be prioritized over the economic aspects. These indicators, which identify NMs (e.g., CuO, Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and CNTs used in concrete, asphalt concrete, and steel) with reported high toxicity and which are hazardous for the environment, should be considered with the main focus on particulate matter emissions into the air, potential toxicity for ecosystems and humans, and impacts on soil quality. It is important to mention that the methods of determination could be adapted. In fact, high values for certain indicators could be related to the high degradation of construction materials in aggressive climate conditions due to a higher release rate of NPs. The detailed methods for the determination of each indicator are not within the scope of this paper. Concerning the economic indicators, all parameters related to the whole life cycle cost should be considered. In fact, the initial costs of NMs are a critical factor. However, it is important not to neglect the potential benefits of using materials with NMs, such as improved performance and, in some cases, lower maintenance costs and increased durability, especially at optimized levels in cement and concrete composites [147]. The proposed indicators can be relevant for all nanomaterials, although some impacts strictly depend on the type of NM. For instance, silica aerogel can have a high end-of-life cost (when incorporated in thermal insulation materials, mortars, blankets, or windows [148]) due to the large amounts required to improve thermal insulation. Although silica aerogel is not a highly toxic material, it can still have a significant impact when deposited in landfills, which requires an evaluation and quantification of the environmental impact [149,150]. The existent European norms deal with regular building materials [129,151], and intend to achieve environmental, social, and economic sustainability. The proposed indicators for NMs in construction include the relevant environmental, human health, and economic impacts to be evaluated and quantified prior to the introduction of NMs into a constructive element in order to gain an accurate perception of their impact.