Seite - 14 - in Freshwater Microplastics - Emerging Environmental Contaminants?

et al. [108] highlighted the gills, gut, liver, and brain as possible target organs in fish, as well as a range of toxic effects including oxidative stress, cellular patho- logies consistent with tumour formation in the liver, some organ specific iono- regulatory disturbances, and vascular injury. Taking into account the complex chemical makeup of some plastics and the ability to sorb co-occurring contami- nants, experimental investigationof these endpoints forMPs seems to bemerited. There aremany lessons tobe learned from thegrowing literatureon thebiological effects ofENPs, and these are discussed inmoredetail inRist andHartmann [58]. 7 Considerations forAssessingEnvironmentalRisks Inmost countries chemical risk assessments rely onmass concentrations of sub- stancesof interest as anexposure andeffectmetric. In thenano-literature themass concentrationsofparticlespredictedtobeemittedhavebeenusedtoassess therisks ofENPs [109,110].Theseapproachesassumeparticles areevenlydistributedwith no transfer between different environmental compartments. This approach was further developed by Gottschalk et al. [111] who used transfer coefficients to model emission flowsbetween the different compartments used in theirmodel, as well as the inclusion of sedimentation rates. Suchmodelling approaches (further discussed in Kooi et al. [63]) could be used to assess the environmental fate of primary MPs where emissions to the environment are distributed across a geo- graphical regionproportional topopulationdensity andconsumption rates, assum- ing that the route of enter into the environment depends on the use of the MP. However, this type of approach requires extensive information on primary MPproductionlevels, industrialapplicationsanduses, levels inconsumerproducts, fate in wastewater treatment, discharges to landfill, and environmental fate and distributionmodelling toperformameaningful exposure assessment.Anexposure assessment for secondaryMPswill requiremonitoringdata, but this is hinderedas the size ranges reported in field studies are generally constrained by the sampling techniques used [42]. The problems of usingmass concentrations as an effect metric are similar to thosediscussed in thecontext ofENPs in that biological effectsmight notbemass dependent but dependent on physical and chemical properties of the substance in question [112, 113]. Consequently,when estimating the hazards presented byMP properties suchassize, shape,polymerdensity, surfacearea,chemicalcomposition of the parent plastic, and the chemical composition of sorbed co-occurring conta- minantsmay need to be considered [114]. However,when considering secondary MPs information on some of these properties may be unavailable. This lack of informationmakes itdifficult to identify thekeycharacteristics,orcombinationsof characteristics, thatmaybe responsible for hazards in the environment. The assessment of MPs based on their chemical composition also presents a considerablechallenge,becausechemicallyMPscanbeconsideredasamixture.A 14 S.Lambert andM.Wagner