Degradation initiated by solar UV radiation is a very efficient mechanism in plastics exposed in air or lying on a beach surface. But when the same plastic material is exposed to sunlight at the same location but while floating in seawater, degradation is severely retarded.

With common polymers such as LDPE, HDPE, PP and nylons exposed to the marine environment it is primarily the UV-B radiation in sunlight that initiates photo-oxidative degradation. Once initiated, the degradation can also proceed thermooxidatively for some time without the need for further exposure to UV radiation. The autocatalytic degradation reaction sequence can progress as long as oxygen is available to the system.

Persistent organic pollutants (POPs) that occur universally in sea water at very low concentrations are picked up by meso-/microplastics via partitioning. It is the hydrophobicity of POPs that facilitate their concentration in the meso-/microplastic litter at a level that is several orders of magnitude higher than that in sea water. These contaminated plastics when ingested by marine species presents a credible route by which the POPs can enter the marine food web.

The term ‘microplastcs’ and ‘microlitter’ has been defined differently by various researchers. Gregory and Andrady (2003) defined microlitter as the barely visible particles that pass through a 500 μm sieve but retained by a 67 μm sieve (∼0.06–0.5 mm in diameter) while particles larger than this were called mesolitter.

Table 1. Classes of plastics that are commonly encountered in the marine environment.Plastic ClassSpecific GravityPercentage production#Products and typical originLow-density polyethyleneLDPE LLDPE0.91–0.9321%Plastic bags, six-pack rings, bottles, netting, drinking strawsHigh-density polyethyleneHDPE0.9417%Milk and juice jugsPolypropylenePP0.85–0.8324%Rope, bottle caps, nettingPolystyrenePS1.056%Plastic utensils, food containersFoamed PolystyreneFloats, bait boxes, foam cupsNylonPA<3%Netting and trapsThermoplastic PolyesterPET1.377%Plastic beverage bottlesPoly(vinyl chloride)PVC1.3819%Plastic film, bottles, cupsCellulose AcetateCACigarette filters

Several broad classes of plastics are used in packaging: Polyethyelene (PE), Polypropylene (PP), Polystyrene (PS), Poly(ethylene terephthalate) (PET); and Poly(vinyl chloride) (PVC).