Seite - 258 - in Freshwater Microplastics - Emerging Environmental Contaminants?

3.3 RegulationbyGroups? Besides their size,MPparticlesvary regarding furtherphysicochemicalproperties. For regulators, thequestion follows if a single regulationstrategycanaddress such a comprehensive group of diverse polymers or if it would bemore reasonable to tailor regulations specifically to subgroups, especially since (micro)plastics donot solely consist of pure polymers but contain also a number of additives such as plasticizers, UV filters, antioxidants, etc. that alter product properties. Thus, the heterogeneitywithin the term“microplastics”arises frommyriadsofcombinations of polymers andadditives.Tomake things evenmore challenging, those additives can change the physicochemical properties and, consequently, also the environ- mentalbehaviorofparticles(fordetails, see[66]).Therefore, it seemsreasonable to develop particular regulatory options focusing on special subgroups. This, in consequence, leads to the question about themain criteria required for a categori- zation into single groups. Of course, any categorization is depending on the regulatory context and the life stage of a product, as described below.While it can be useful to categorize into very specific subgroups for specific regulatory purposes, in other cases, it might be more efficient to evaluate the whole group ofMPs. Asafirst approach forMPassessment in freshwater environments,Mikloset al. [65] suggest amodular systemstartingwith thequantificationof selected indicator polymers.As soon as the concentration of these polymers exceeds a certain level, more specific analyses should be conducted. These subsequent analyses can take various criteria (such as polymer type, size, shape, additives, etc.) into consider- ation to further categorize the particles and support the selection of adequate mitigation measures. Here, approaches from chemical regulation might serve as examples. Chemicals can be categorized based on molecular similarities (e.g., PAHs, PCBs, etc.), by the field of application (e.g., pesticides), or according to their mode of action (e.g., endocrine disruptors). So far, mainly sum parameters for molecularly similar chemicals are implemented to freshwater directives (e.g., dioxin þ dlPCB, cyclodiene pesticides, EU WFD). Similarly, MPs could be grouped based on their physicochemical properties (e.g., polymer type, density), by their application fields (e.g., cosmetics, carrier bags, electrical devices), or by (eco)toxicological impacts.The lattermightbedifficult as little isknownabout the biological effects ofMP, and itwill take time togenerate comprehensivedata (see [50]). In contrast to chemical pollutants, MP can cause both chemical and addi- tionallymechanical effects onorganisms.Chemical effects couldbecausedby the polymers themselves, by their additives, orbyacombinationofboth.Similarly, an occurrenceofmechanical effects coulddependonparticle size,particle shape,or a combinationofboth. It follows that (eco)toxicologists face thechallenge to test the effects ofmyriads of combinations.Hence, there are efforts to prioritize and start with the presumed most harmful combinations. Ideally, these results will be transferable to a groupof similar combinations. 258 N.Brennholt et al.