Towards Prospective Exposure Modeling of Nanoparticles - Applying Particle Flow Analysis and Kinetic Exposure Modeling for the Cases of TiO2 and Ag Nanoparticles
The claims regarding the usefulness of nanoparticles (NPs) in different applications, such as wound dressings, solar cells and soil remediation, has been accompanied by concern that NPs may also pose risks to humans and to the environment. Considering the past century, when many substances later shown to cause unacceptable damage were manufactured in large amounts, there are reasons to thoroughly assess the risks of these NPs. This thesis discusses how the exposure assessment step of a risk assessment of NPs may be conducted, focusing on two research aims.
The first research aim considered is the magnitude of NP emissions from society. In order to address this aim, the methodology of substance flow analysis (SFA) was adapted to the case of NPs, resulting in the particle flow analysis (PFA) methodology. In PFA, particle number is used as flow and stock metric instead of mass, which is used in SFA. Moreover, a prospective approach is applied by developing an explorative scenario of technology diffusion. This method has been applied for titanium dioxide (TiO2) in sunscreen, paint and self-cleaning cement, and silver (Ag) NPs in wound dressings, textiles and electronics. The second research aim concerns the fate of NPs in the water compartment. It is shown that modeling NP fate using fugacity based on thermodynamic equilibrium, which is normally done when assessing the risks of chemicals (i.e. molecules), is not feasible. Instead, the fate of NPs was modeled using kinetic equations which were borrowed from colloid chemistry. Particle concentration is used as exposure indicator, rather than mass concentration which is normally used in risk assessment of chemicals. This method was applied to the case of TiO2 NPs.
The results from the PFA case studies indicate that the currently highest use phase emissions of TiO2 NPs come from the use of sunscreen, and that this will probably be the case in the future as well. However, there is large number of TiO2 NPs in paint, and in the future maybe also in self-cleaning cement, which are not emitted during their use but continue to the waste handling phase. Their fate during waste handling processes thus remains an interesting topic to investigate. Regarding Ag NPs, it is difficult to tell which application that gives rise to the currently largest emissions, but the results indicate that the emissions from textiles may be highest in the future. The kinetic exposure modeling of TiO2 NPs identified parameters and mechanisms which affect the concentration of TiO2 NPs, and the collision efficiency was shown to have the largest effect. Gaps in current knowledge are identifies in all three case studies and recommendations for further studies are given. The methods of PFA and kinetic exposure modeling constitute important steps towards prospective exposure modeling of NPs.
Particle flow analysis (PFA)
environmental risk assessment