Addressing the relevance of polystyrene nano- and microplastic particles used to support exposure, toxicity and risk assessment: implications and recommendations

BACKGROUND: There has been an exponential increase in the number of studies reporting on the toxicological effects associated with exposure to nano and microplastic particles (NMPs). The majority of these studies, however, have used monodispersed polystyrene microspheres (PSMs) as 'model' particles. Here we review the differences between the manufacture and resulting physicochemical properties of polystyrene used in commerce and the PSMs most commonly used in toxicity studies. MAIN BODY: In general, we demonstrate that significant complexity exists as to the properties of polystyrene particles. Differences in chemical composition, size, shape, surface functionalities and other aspects raise doubt as to whether PSMs are fit-for-purpose for the study of potential adverse effects of naturally occurring NMPs. A realistic assessment of potential health implications of the exposure to environmental NMPs requires better characterisation of the particles, a robust mechanistic understanding of their interactions and effects in biological systems as well as standardised protocols to generate relevant model particles. It is proposed that multidisciplinary engagement is necessary for the development of a timely and effective strategy towards this end. We suggest a holistic framework, which must be supported by a multidisciplinary group of experts to work towards either providing access to a suite of environmentally relevant NMPs and/or developing guidance with respect to best practices that can be adopted by research groups to generate and reliably use NMPs. It is emphasized that there is a need for this group to agree to a consensus regarding what might best represent a model NMP that is consistent with environmental exposure for human health, and which can be used to support a variety of ongoing research needs, including those associated with exposure and hazard assessment, mechanistic toxicity studies, toxicokinetics and guidance regarding the prioritization of plastic and NMPs that likely represent the greatest risk to human health. It is important to acknowledge, however, that establishing a multidisciplinary group, or an expert community of practice, represents a non-trivial recommendation, and will require significant resources in terms of expertise and funding. CONCLUSION: There is currently an opportunity to bring together a multidisciplinary group of experts, including polymer chemists, material scientists, mechanical engineers, exposure and life-cycle assessment scientists, toxicologists, microbiologists and analytical chemists, to provide leadership and guidance regarding a consensus on defining what best represents environmentally relevant NMPs. We suggest that given the various complex issues surrounding the environmental and human health implications that exposure to NMPs represents, that a multidisciplinary group of experts are thus critical towards helping to progress the harmonization and standardization of methods.