Page - 228 - in Freshwater Microplastics - Emerging Environmental Contaminants?

researchfield—italso indicates thecomplexityof theproblemcalling for scientific methods inorder to identifyandquantify theconsequencesfor theenvironmentand for humanhealth [10]. The traditional approach to environmental risk assessment of chemical sub- stances cannot do justice to themultitudeofmicroplastic particles and intervening variables and, therefore, cannot be applied to determining “safe” or “hazardous” levels of microplastics in natural environments [37]. Microplastics are not a homogenous group of substances, and they stem fromvarious sources. The phys- icochemical properties ofmicroplastics are as diverse as their sources.Theydiffer in theirpolymericcomposition, theiradditives,andhavevariousshapesandsizes— all characteristics that can influence their biological effects.Microplastics can be toxic due to associated substances likephthalates andBPA[38], they can result in physical damagedue to their shape [39], and they can induce indirect effects after being ingested, suchas reduced foodconsumptiondue to satiation (malnutritionor even starvation) or intestinal blockage leading to death. Furthermore, biological effects are linked to other environmental contaminants such as persistent organic pollutants (POPs) that are absorbed by microplastic particles [40]. The lack of specific adverse effects leads to great uncertainty regarding predictions of the environmental consequences. These uncertaintieswere already expressed in early studies ofmicroplastics around 30 years ago. However, despite these knowledge gaps, theproblemwasaddressedpragmaticallyat that time:microplasticsdetected innaturalwatersandproventobeingestedbyaquaticorganismsweredenotedasan “unnecessary contaminant” [33] that is “in all likelihoodnot beneficial” [41]. 0 20 40 60 80 100 0 20 40 60 80 100 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 Year Publication title includes other terms, e.g. pellets, particles, fragments, granules... Publication title includes the term "microplastics" Fig.1 Environmental studiesonplasticparticles from1970 to today.Thefigureshows the rising number of studies in recent years, especially since the introduction of the term “microplastics.” Studies were obtained from the search engines “Google Scholar” and “Web of Science.” Key- words for the search were: microplastics þ environment; plastic particles/fragments/pellets/ granules/spheres/fibersþ environment 228 J.KrammandC.V€olker