Primary multiple pathways for the entry of secondary

Primary
microplastics are plastic particles which are manufactured in a size range
below 5 mm (GESAMP 2015). They include: (1) pre-production resin pellets, often
3 – 5 mm in diameter, which are used as raw material in plastic manufacture;
(2) microbeads incorporated in cosmetic products; and (3) beads used for
abrasive blasting of surfaces. The route of entry for primary microplastics
into the environment will depend on their application: particles from cosmetic
products will usually enter through wastewater; microplastics from abrasive
blasting will enter through the atmosphere and wastewater, while primary
microplastics used for raw materials may enter the environment through
accidental loss during transportation and transshipment, or through runoff from
processing plants. When too small for retention by wastewater treatment plants,
primary microplastics may be passed directly into the oceans or pass through freshwater
watercourses to subsequently enter the marine environment.

Secondary
microplastics are the by-product of fragmentation and weathering of larger plastics
in the environment (GESAMP 2015). The generation of secondary microplastics may
occur during use of plastic products or once plastics have been disposed of in
the environment. There are multiple pathways for the entry of secondary
microplastics to the environment, which include (1) particles from textiles may
enter through wastewater following washing or through air when drying (Browne
et al. 2011; Napper and Thompson 2016); (2) weathering of plastics used in
agricultural applications may enter the environment through surface runoff from
soil; (3) abrasion of tyres during use generates microplastics that enter the environment
through air and surface runoff; (4) fragmentation and weathering of items in
landfills by UV light which may introduce microplastics into the atmosphere, rivers
and the ocean by wind and surface runoff and (5), weathering of plastic litter
in coastal areas and beaches which may remain in coastal sediments or be
transported further offshore. The main environmental factors related to
secondary 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 microplastic
generation (Andrady 2015).

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