Several applications of nanomaterials can be identified in the construction sector, e.g., concrete, mortars and renders, thermal insulation materials, glass windows, solar panels, paintings, and coatings, among others [14]. Nanotechnology can either improve the properties of the final materials or extend their service life and life cycle [16,18]. Utilizing nanomaterials as construction materials has the potential to improve their inherent properties or introduce additional functionalities. For instance, the incorporation of SiO₂ nanoparticles has been reported as an effective method for reinforced concrete [16,18]. Similarly, the inclusion of metal oxides, such as iron, titanium, aluminum, and zinc, in paints and surface coatings can serve to prevent corrosion and resist dirt accumulation [11]. The addition of small percentages of TiO₂ and ZnO has also been used as UV filters in glass [2] or as photocatalytic additives [15]. Nanoclays have been introduced into cementitious matrixes for pervious concrete pavement [28]. Nanostructured (calcium, barium, or magnesium) hydroxides and alkoxides have been used for the conservation of porous calcareous materials, such as mural paintings, limestone, and historical mortars [29]. Carbon-based NMs (CNs) and graphene-based materials (GO) improve serviceability, including high thermal and electrical conductivity, proper elasticity and flexibility, low thermal expansion coefficient, and electron field emitter capabilities [30,31,32].

Table 1 summarizes several types of nanomaterials, as well as their compositions, applications, and functionalities, based on the research.

Although NMs can improve the properties and performance of construction materials, several NMs are not easily available due to restrictions on their use and commercialization, as well as to the availability of raw materials and, as previously stated, to high production costs [11]. Furthermore, nanoparticles (NPs) can have a low compatibility with some construction materials and are prone to aggregation phenomena, which hamper homogeneous dispersion [94,95,96,97]. Another relevant challenge is related to their potential toxicity and thus the risk to human health and the environment [98,99,100,101,102,103,104].

The increasing use of NMs also leads to an increased production of waste and residues, with the relevant exposure of operators in the building sector [105]. Approximately 60% of nanomaterials are used in medical–pharmaceutical or industrial applications (e.g., in the textile and electronics industries) with several industrial processes which can lead to waste streams resulting from the cleaning of production chambers [106]. Thus, an in-depth understanding of human and environmental exposure and NPs’ toxicological effects is a necessary step to assess environmental and health impacts [107,108,109].

Exposure to NPs is often associated with inhalation or absorption through skin contact. Although the dosages required to induce these effects are rather high, toxicological health risks include lung damage, adverse effects on the immune system, disorders related to oxidative stress, and diseases such as cancer, as well as DNA damage, and changes in cell growth and renewal, processes which are essential for healthy organs and tumor prevention [3,30,110]. Therefore, there are numerous recommendations for handling NMs during their production, transport, application, and end-of-life disposal process, including the use of gloves, coveralls, air filter masks, and safety goggles [105,111].

Hallock et al. [112] reported that ultrafine particles (<100 nm) of TiO₂, Al₂O₃, and carbon black NPs demonstrated higher toxicity than fine particles (<2.5 µm). The nano-size may act as an amplifier of the effects, resulting from a higher reactivity or dissolution rate, although the nanostructure is not a sufficient descriptor to correlate with toxicity in the aquatic medium. In fact, the potential toxic effects of NPs depend on their physicochemical characteristics (size, shape, composition, surface functional groups, and surface charges) and can be influenced by the surrounding matrix [113,114]. Therefore, the environmental impact of these NPs is contingent upon characteristics such as decreased size, which enables their entry into the cellular environment and interaction with proteins. The shape of the particle also influences the cellular uptake mechanism, and the presence of a coating can prevent the leaching of toxic metal ions [114].

Figure 1 shows the systemization of NMs’ impact on the environment and of human exposure during the life cycle stages. This release can occur during all stages of the life cycle: production, manufacturing, use of nanoproducts, and their disposal and recycling [23,102,115,116]. Most NMs are generally released to the environment through wastewater treatment plants, recycling processes (e.g., including dismantling, shredding, and thermal processes) [117], waste incineration, and landfills. As an example, solid sludge from pilot wastewater treatment plants can retain more than 80% of some types of NMs, while the remaining 20% cannot be generally processed and are therefore discharged to surface waters [118]. Furthermore, NPs’ durability can be affected by weathering, with substantial modifications throughout their life cycle [113], and, when released into the environment, can undergo complex biological, physical, or chemical reactions and modifications, depending also on the specific characteristics of the materials and the environmental conditions [119].

Wind and runoff can transport NPs from solid waste or accidental spills to other locations and water bodies, contaminating surface water and soil and lixiviating into groundwater [120]. Wastewater effluents and direct discharges can disperse particles into waterways, and, if hydraulically connected to saturated zones, transport them to aquifers. Furthermore, NPs can be released into the atmosphere and form aerosol suspensions, and thus dust, during the shredding processes of synesthetic or metallic composite materials [118] or during exposure to fire or combustion [3].

Waste containing NMs, such as concrete (which may contain CNTs, SiO₂, Fe₂O₃), ceramics (SiO₂, CNTs), antibacterial coatings and paints (AgNPs), self-cleaning coatings (TiO₂), window coatings (SiO₂), and improved anticorrosive steel (CuNPs), are currently disposed of along with conventional waste without specific precautions or treatment [118]. Silica-based aerogels are barely considered, as landfill is a common end-of-life destination [121]. It is worth noting that the emission of NMs into the air, water, and soil is strictly dependent on how landfills are organized and practiced, although the mechanisms and quantification of NMs’ release into the environment are not yet completely understood [118]. Thermal recycling/degradation and waste-to-energy combustion can be considered as two alternatives to landfill processes [122,123].

The toxicity of NMs can also be related to their cost. In fact, Gkika et al. [103] analyzed the impact of the materials’ cost by considering their toxicity, concluding that NMs with a low cost and low toxicity (e.g., titanium carbonitride and aluminum, multi-walled carbon nanotubes) have significant applicability and thus diffusion on a wider scale. Conversely, the use of NMs with a high cost and toxicity (e.g., titanium oxide, copper oxide, or even single-walled carbon nanotubes) should be reconsidered [103].

Although the toxicity of NMs presents certain concerns, nanotechnology can also act as an effective approach for environmental remediation [108,109]. In fact, manufactured nanomaterials (MNMs) can decompose, eliminate, or neutralize harmful substances present in contaminated environments [108]. Furthermore, NMs can be designed to reduce interactions with the cell surface, e.g., by having a negative surface charge (electrostatic stabilization of NMs), or using ligands (e.g., polyethylene glycol) or morphologies that reduce protein binding. Less toxic elements can be used in NMs that also use shell materials (e.g., TiO₂ with a silica or aluminum oxide coating [124]), which decrease the interaction with the core or the environment, or by introducing a chelating agent (which reduces the cytotoxicity of nanostructured metals) or antioxidant molecules (which prevent the degradation of the NMs) [125]. Finally, new green synthesis routes have been fine-tuned in recent years for different types of nanomaterials, including metal-oxide-based, inert-metal-based, carbon-based, and composite-based NPs [126].

3. Conclusions

This work intended to address the current concerns, evaluate the sustainability (environmental, social, economic) and viability, and thus contribute to the implementation of regulations on NMs, which are often commercialized and categorized similarly to regular construction materials. Based on an extensive literature review for nanomaterials and European standards for regular building materials, environmental, human health, and economic indicators were proposed as mandatory for nanomaterials to be applied in the construction sector.

A particular focus on toxicity (ecotoxicity and human toxicity), soil impacts (land-use-related impacts / soil quality), and emissions into the air (particulate matter emissions) was identified. The use of these indicators should be considered for nanomaterials such as copper, aluminum oxide, titanium nanoparticles, or carbon nanotubes which have significant levels of toxicity and are widely used in the construction sector.

Regarding the economic indicators, it was concluded that the evaluation of the cost impact throughout the various stages of the whole life cycle is essential, focusing not only on the initial cost but also on optimizing the less economically viable stages. These indicators would be particularly relevant for nanomaterials which are generally incorporated in large quantities (e.g., silica-aerogel in thermal insulation composites) and may cause economic problems during recycling processes. Furthermore, the lack of data on durability and end-of-life processes hinders the applicability on a larger scale of nanomaterials such as carbon nanotubes, iron oxide, and graphene oxide.

These proposed indicators could be a good basis for their integration into a risk assessment framework of nanomaterials to be applied in construction.

Limitations of the proposed indicators can be identified in terms of their applicability to certain nanomaterials, functionalized and designed according to specific applications, presenting different physicochemical properties and thus environmental risks. Although the evaluation of the physicochemical properties of nanomaterials that may affect human health, and aquatic and terrestrial ecotoxicology, has been widely debated, the categorization of a small number of nanomaterial groups was identified, which often resulted in specific tests being waived, creating consistent data gaps.