Primarymicroplastics are plastic particles which are manufactured in a size rangebelow 5 mm (GESAMP 2015). They include: (1) pre-production resin pellets, often3 – 5 mm in diameter, which are used as raw material in plastic manufacture;(2) microbeads incorporated in cosmetic products; and (3) beads used forabrasive blasting of surfaces. The route of entry for primary microplasticsinto the environment will depend on their application: particles from cosmeticproducts will usually enter through wastewater; microplastics from abrasiveblasting will enter through the atmosphere and wastewater, while primarymicroplastics used for raw materials may enter the environment throughaccidental loss during transportation and transshipment, or through runoff fromprocessing plants. When too small for retention by wastewater treatment plants,primary microplastics may be passed directly into the oceans or pass through freshwaterwatercourses to subsequently enter the marine environment.
Secondarymicroplastics are the by-product of fragmentation and weathering of larger plasticsin the environment (GESAMP 2015). The generation of secondary microplastics mayoccur during use of plastic products or once plastics have been disposed of inthe environment. There are multiple pathways for the entry of secondarymicroplastics to the environment, which include (1) particles from textiles mayenter through wastewater following washing or through air when drying (Browneet al.
2011; Napper and Thompson 2016); (2) weathering of plastics used inagricultural applications may enter the environment through surface runoff fromsoil; (3) abrasion of tyres during use generates microplastics that enter the environmentthrough air and surface runoff; (4) fragmentation and weathering of items inlandfills by UV light which may introduce microplastics into the atmosphere, riversand the ocean by wind and surface runoff and (5), weathering of plastic litterin coastal areas and beaches which may remain in coastal sediments or betransported further offshore. The main environmental factors related tosecondary microplastic generation are UV light exposure, temperature and abrasion.In aquatic environments areas with reduced UV exposure and low temperature(such as the deep sea) will slow down the process of secondary microplasticgeneration (Andrady 2015).