Page - 15 - in Freshwater Microplastics - Emerging Environmental Contaminants?

simplified example of a risk assessment for polyurethane (PUR) based on its chemical composition is provided in Table 4. PUR flexible foam is used for mattressesandcar seatsand ismadebycombining threemonomersandcanconsist of up to 18%flame retardant content [117].An example risk assessment based on predicted environmental concentration (PEC)/predicted no effect concentration (PNEC) ratios for all components of themixture are then used to calculate a risk quotient (RQ; Table 4). The RQ for this particular example is less than one; however, this type of assessment does not account for potential negative effects causedbyphysical irritationofsolidparticles. In thiscase itbecomesclear that risk assessment forMPs as with ENPs holds specific challenges (see Brennholt et al. [118] for an in-depth discussionof the regulatory challenges). The different particles sizes of MPs in environmental systems will present different risks to organisms living in those systems. For example, small plankton feedingfish speciesmayencounterMPs from thenanoscale through toMPs5mm or greater. Thefishmay avoid larger particles but small particlesmaybe ingested while feeding.Forfilter feedingorganisms theupper sizeboundarywill dependon the size of particles that a particular organism will naturally ingest. The risk assessment ofMPs could therefore be based on particle size. A simplified hypo- thetical case is presented inBox 1 that draws on an example given byArvidsson [119]. This approach assumes that there is informationonharm-related thresholds of MPs based on size classes and particle concentration for the most sensitive species in that particles size range. However, the use of particle size for defining environmental risk may not be that straight forward, because MPs are not monodispersed in the environment. Additionally, as described by Hansen, [120] when discussing ENPs it remains unclear whether a ‘no effect threshold’ can be established,what the best hazard descriptor(s) are, andwhat are themost relevant endpoints. Table4 Ahypothetical chemicalmixture risk assessment basedon the chemical components of PURflexible foamwithTBBPAas aflame retardant (units aremg/L) Monomer 1 Monomer 2 Monomer 3 Additive 1 Propylene oxide Ethylene oxide Toluene diisocyanate TBBPA LC50 algae 307 502 3.79 0.19 LC50 daphnid 188 278 2.61 0.02 LC50fish 45 58 3.91 0.02 PNEC(AF¼1000) 0.045 0.058 0.003 0.000002 PEC(dissolved compound) 0.00067 0.00067 0.00067 0.0000032 RQPEC/PNEC 0.015 0.012 0.257 0.160 MixtureRQ 0.443 LC50 (median lethalconcentration) for thisexampleweregeneratedusing theEPISuiteECOSAR model;AF assessment factor MonomerPECsare basedonpropyleneoxideECHArisk assessment [115] TBBPAPECbasedonmaximumconcentrationsmeasured inUKlakes [116] MicroplasticsAreContaminants ofEmergingConcern inFreshwater. . . 15