Exposure assessments of nanoparticles in aquatic environments – considerations, review and recommendations

Report, 2013

Synthetic nanoparticles are new forms of chemical substances. They can be found in several different forms, such as free particles, surface bound and dissolved in liquid. Nanoparticles can also exist as free, individual particles or agglomerate consisting of multiple particles. This report discusses the assessment of possible risks of nanoparticles. Chemical risk is usually considered to consist of two elements: (1) Exposure to the substance, and (2) the substance's toxicity. So far, the risk-related research on nanoparticles has had a strong focus on the particles ' toxic effects. In this report, we would instead focus on how exposure to nanoparticles can be calculated and assessed, with focus on nanoparticles in water. In the report, we provide an initial background and definitions of nanomaterials and nanoparticles, and describe briefly a standard method of risk assessment of chemicals in the environment. Then we go through important considerations that should be made in the exposure assessment of nanoparticles. First we discuss three considerations related to the emissions of nanoparticles, namely the lack of data for annual production of nanoparticles, the importance of applying a substance flow perspective, and lack of data for so-called emission factors for nanoparticles of various products and materials. Furthermore, we discuss considerations for modeling of nanoparticles behavior in water, mainly by listing a number of key processes with large influence. These are agglomeration, sedimentation, and dissolution. Related to that, we discuss how natural organic materials, coatings and aging of particles can affect these processes. We note here three particle properties that are important in order to describe nanoparticles dispersion in water, in a similar way that the octanol-water partition coefficient and half-life is important to describe the fate of organic chemicals in the environment. For nanoparticles these are the particle size (a) and the density (ρ). We also identify a number of more complex parameters affecting particle behavior in the environment, but not only because of the different particle characteristics, but also depending on characteristics related to the environment. These are the collision efficiency (α), point of zero charge (pHpzc), Hamaker constant (A) and a so-called form factor (β) that affect the sedimentation. In addition to the general difficulty to measure or calculate these parameters they also co-vary. Furthermore, we make a review of 11 currently available exposure models for nanoparticles in aquatic environment. We note that the studies differ regarding modeling method, which sources of emissions that are included, the nanoparticles taken into account, estimated concentrations in the environment, and whether the results are presented as mass or particle concentration. Only two studies trying to model the nanoparticle exposure based on particle properties in a manner similar to standard methods for chemical risk assessment. The other modeling studies are instead based on data on flows of specific nanomaterials, and not on generic algorithms. Next, we describe a number of challenges that occur when measuring nanoparticles in the environment. Finally, we provide the following recommendations to ensure good exposure assessment of nanoparticles in the future: 1. Information of flows and stocks of nanoparticles in society need to be collected. 2. Emission factors would need to be developed for each product that makes use of nanoparticles. 3. Emissions should be reported both as mass and particle number until it becomes clearer which one is most relevant. 4. More research is needed in order to determine which particle properties need to be known in order to calculate the concentration of nanoparticles in the environment. 5. At least the particle size and particle size distribution, as well as the specific particle density should be reported. 6. More research is required to improve the experimental measurements of nanoparticles to be able to validate exposure models.