Exploring toxicity and fate of metal-based particles in the lung : from mechanistic screening to lung deposition modelling

We are all exposed to small particles in the air that we breath and some of them will contain metals in a dose and form that may be harmful. In addition, the field of nanotechnology holds great promises, but an increased production of nanoparticles leads to a higher risk of exposure. Metal-based particles are indeed also present in various traditional occupational settings resulting in an exposure to the workers within this field. Welders are one group at risk for exposure to metal containing particles. Despite this, many knowledge gaps remain regarding the possible risks that particles pose on human health. With the emerging use of nanomaterial and the move away from animal-based experiments, there is currently a need to establish approaches of testing particles in efficient and informative ways using alternative test strategies. This thesis aims to gain a deeper understanding of the toxicity and fate of metalbased particles in the lung by employing experimental approaches ranging from mechanistic screening and established in vitro assays to lung deposition modelling. In Paper I and II, the toxicity and associated mechanisms for a wide selection of metalcontaining nanoparticles were investigated using the reporter cell based ToxTracker assay. Reporters related to oxidative stress were most frequently activated in response to the nanoparticles, whereas fewer nanoparticles activated reporters linked to DNA damage. However, the latter ones were suggested to be considered of particular concern. With the variation in activation of various reporters, this suggests that the ToxTracker can be used as a sensitive tool to gain rapid and efficient mechanistic insight into the toxicity of particles. In Paper II, the toxicity and underlying mechanisms of welding fume particles generated by welding of stainless steel were investigated in vitro as a function of welding techniques, settings and materials. Observations revealed a high variation in toxic potential of different welding fumes, primarily depending on choice of welding electrode. Welding fumes generated with flux cored wire (FCW) were most toxic. This was strongly associated with higher metal release, in particular hexavalent chromium (Cr(VI)). In the follow-up Paper III, the released metal fraction was shown to induce similar cytotoxicity and DNA damage as the particles, further emphasizing the importance of released metals in acute toxicity induced by welding fumes. Furthermore, Paper III demonstrated the potential benefit in substituting standard Cr(VI)- generating FCW electrodes with Cr(VI)-reduced electrodes in order to create less hazardous fume particles and a safer working environment for welders. These studies furthermore highlight the beneficial collaboration between academia and industry to improve occupational environments. In Paper IV we wanted to understand the applied in vitro doses of welding fumes in the context of human exposure. Therefore, a review of the literature was performed to obtain information on welding fume exposure at occupational settings. Next, human lung doses were estimated by simulating real-life occupational welding scenarios in the Multiple-Path Particle Dosimetry (MPPD) model. Interestingly, lung doses following both acute and more chronic exposure were found comparable to in vitro doses where we observed toxic effects in Paper III. The lung dose of the tracheobronchial region was found to exceed a cytotoxic in vitro dose already after one working shift. Moreover, this study demonstrates the significant contribution of dosimetry modelling in order to understand the relation between in vitro doses and human exposure, and its potential future importance for risk assessment and study design. In conclusion, the results of the studies within the framework of this thesis demonstrate a variation in toxic potency and mode of action for metal-based particles. Metal release is shown to be an important factor for metal-particle induced toxicity, with results showing metal release, rather than metal content, to be largely responsible for acute toxicity induced by welding fumes. This thesis especially highlights the use of in vitro models for the hazard assessment of particles, identifying both the ToxTracker and lung deposition modelling as important tools for improving the efficiency and regulatory weight of in vitro approaches