Biodegradability, Packaging and Material Alternatives

Leave a banana peel and a plastic bag outside for six months: the peel will be gone, consumed by fungi and bacteria in the soil, while the plastic bag will look almost identical, slightly faded perhaps, but still recognisably a bag. Most synthetic polymers persist in the environment because they are chemically inert. The C–C backbone of polyethylene has no sites easily attacked by water, oxygen, acids, or the enzymes produced by microorganisms. Bacteria and fungi that evolved to decompose natural organic materials like cellulose (wood) and starch have no evolutionary history with synthetic polymers, no effective enzyme exists in nature to break the polyethylene chain. Without biodegradation, the only natural breakdown pathway is UV photodegradation: ultraviolet light from the sun cleaves C–C bonds, breaking large plastic items into progressively smaller fragments. However, this process does not mineralise (fully chemically convert) the plastic, it just creates smaller pieces of the same material.

These smaller fragments, less than 5 mm in diameter, are called microplastics. Fragments smaller than 1 µm are nanoplastics. Critically, UV degradation creates microplastics faster than microorganisms can consume them, so the total number of plastic particles in the environment increases over time even as individual pieces get smaller. The persistence of plastic is an inherent consequence of the chemical properties that make it so useful, the same inertness that makes a plastic pipe last 50 years in a water supply is what makes a discarded bag persist for centuries in the ocean.

Microplastics enter the environment from many sources. The single largest source globally is tyre wear: car tyres lose rubber microparticles every time they contact the road, generating an estimated 6 million tonnes of tyre wear particles per year worldwide, transported via stormwater runoff into waterways. Synthetic fibre washing is the second major source: a single wash of a synthetic fleece jacket releases up to 700,000 microfibre fragments. Fragmentation of larger plastic items (bottles, bags, packaging) by UV and mechanical abrasion produces the remainder.

Once in waterways, microplastics concentrate in ocean gyres and sediments. They are ingested by zooplankton, small fish, and filter feeders such as mussels and oysters. Through bioaccumulation, concentrations increase up the food chain, a process called biomagnification. Studies of Antarctic krill, Pacific tuna, and Australian snapper all show microplastic contamination. In 2023, researchers found microplastic particles in human blood (2022), breast milk, and cardiac tissue, evidence that microplastics now circulate inside the human body, though the health implications are still being investigated.

The Great Pacific Garbage Patch is a diffuse zone of elevated plastic debris concentration in the North Pacific Ocean, maintained by a persistent circular ocean current (gyre). It covers approximately 1.6–3.4 million km² (about 2.5 times the area of New South Wales). It is not a solid island of plastic, the plastic is dispersed throughout the water column, predominantly as microplastics (<5 mm) rather than large visible debris. The weight of plastic in the patch is estimated at 80,000 tonnes, the majority of which is degraded fishing gear and packaging fragments.

Australia's coastlines rank among the most plastic-polluted in the Asia-Pacific region for remote beaches. Currents bring debris from Pacific and South-East Asian sources to beaches from Queensland to Western Australia. The Cocos (Keeling) Islands, an Australian territory 2,750 km off the WA coast, had the highest beach plastic density ever recorded: 414 million pieces on 27 km of beach. The scientific implication is clear: plastic pollution is a transboundary problem where the source of pollution and the site of impact are separated by thousands of kilometres and multiple sovereign boundaries.