Plastic Pollution

Conventional plastics: made from fossil fuels and generally non-biodegradable

Conventional plastics are made from fossil fuels - crude oil and natural gas - rather than from living material. The oil is refined into small molecules called monomers, such as ethylene and propylene, and these are joined into long chain molecules called polymers. The chains are held together by strong covalent bonds, and that structure is exactly what makes plastic so useful: it is light, strong, waterproof, cheap and easily moulded into any shape, which is why it replaced glass, metal and paper in packaging, pipes, cars and textiles.

The same strong bonds make most conventional plastics non-biodegradable. No common bacterium or fungus produces the enzymes needed to break those bonds, so the plastic is not decomposed by living things and is not returned to the carbon cycle at the speed of a leaf or a paper bag. A non-biodegradable plastic is therefore described as breaking down over a very long period of time, but break down here means physical fragmentation, not decay.

Fragmentation happens when sunlight (ultraviolet radiation), waves, sand, wind and heat make the plastic brittle so that it cracks. The carbon is still locked in the polymer; it has simply been divided into more, and much smaller, pieces. A bottle left on a beach may lose its shape within a few decades and disappear from sight, but the material itself persists far longer, and the fragments it leaves behind are the microplastics described later in this topic.

Because plastic is not decomposed, every item that was made, used once and then littered still exists somewhere: in a landfill, in a river, in the sea or as fragments in sediment. Production has risen steeply since the middle of the twentieth century and a very large share of it is packaging, which is used for minutes and then thrown away. That mismatch - a material designed to last for centuries, used for minutes - is the root of the whole problem.

• Made from fossil fuels: crude oil and natural gas are refined into monomers and joined into polymer chains.
• Properties: light, strong, waterproof, cheap and easy to mould, which is why plastic is used so widely.
• Generally non-biodegradable: common bacteria and fungi cannot break the polymer bonds, so the material is not decomposed.
• Breakdown is physical, not biological: ultraviolet light, waves and abrasion fragment the plastic into smaller and smaller pieces, and the carbon stays locked in the polymer.

Bioplastics and biodegradable plastics are not the same thing

A bioplastic is a plastic that is fully or partly made from biological raw materials rather than from fossil fuels. The raw materials are renewable: maize starch, sugar cane, cassava, potato peel waste and plant oils are fermented or processed into the monomers, and a bioplastic may be fully bio-based or a partly bio-based blend. What the label bio-based tells you is where the carbon came from. It tells you nothing about what happens to the item after it is thrown away, and that is the trap in this topic.

A bio-based plastic can be non-biodegradable. Bio-PET, made from sugar cane and used in drinks bottles, is chemically the same as ordinary PET, so in the sea or in soil it behaves in exactly the same way and simply fragments. A biodegradable plastic, by contrast, is defined by its fate and not by its feedstock: it is designed to decompose in water or soil, and it is decomposed by the action of bacteria and fungi into water, biomass and gases (carbon dioxide and methane). This is a biological process, so the microorganisms must be present and able to digest the polymer.

Biodegradable plastics decompose at different rates depending on biotic and abiotic factors. The abiotic factors are the physical conditions: temperature, moisture, oxygen availability, sunlight and pH. The biotic factors are the decomposers themselves - which species of bacteria and fungi are present, and how many. The same compostable bag can break down in a few weeks in a warm, moist, oxygen-rich compost heap full of active microbes and last for years in cold, dark, oxygen-poor conditions.

That difference matters because the place where most plastic waste ends up is the least suitable for decomposition. A landfill is dark, compacted and anaerobic, so it lacks the microbes and the oxygen that composting needs, and a biodegradable item buried there may persist about as long as a conventional one. When it does decompose without oxygen it releases methane, a powerful greenhouse gas. Cold ocean water also slows decomposition to a crawl. So a biodegradable label is a design property, not a promise about litter.

Microplastics: what they are and where they come from

Microplastics are pieces of plastic that are less than 5 mm in length - about the size of a grain of rice or smaller. Some are made that size on purpose and some are created by the breakdown of larger items, and examiners expect both routes. They are the part of plastic pollution that is hardest to see, hardest to remove and most easily eaten.

Primary microplastics are manufactured at less than 5 mm and used in commercial products. They include microbeads added to facial scrubs, toothpastes and industrial cleaning products as abrasives, and the plastic pellets known as nurdles, which are the raw material of the plastics industry and are spilled by the tonne during transport and handling. These leave the factory already small and are difficult to trap in a sewage works.

Secondary microplastics are formed when larger plastics break down. Two land-based sources dominate. The first is synthetic textiles: every wash of a polyester, nylon or acrylic garment releases thousands of fibres, and sewage treatment does not catch them all, so they reach rivers and then the sea. The second is tyres: the tread worn off car and lorry tyres during driving is washed by rain into drains and watercourses. Both sources are invisible to the person producing them, which is why they are so often forgotten in favour of bottles and bags.

The larger items themselves come from a small number of routes. Land-based litter is the biggest, blown or washed into drains, streams and rivers. Rivers act as the main conveyor, carrying waste out of inland towns and cities; the worst affected are rivers that drain densely populated basins where waste collection is poor, such as the Citarum in Indonesia and the Pasig in the Philippines. Fishing gear - nets, lines, ropes and traps that are lost overboard or deliberately discarded - carries plastic straight into the sea and is a large share of the biggest floating items. Packaging, bottles, bags and sachets make up most of the rest.

• Microplastics are less than 5 mm in length, formed when larger plastics break down or used in commercial products.
• Primary microplastics: microbeads in cosmetics and cleaning products, and industrial plastic pellets (nurdles) lost in transport.
• Secondary microplastics: fibres shed by synthetic clothing when it is washed, and tyre particles worn from roads and washed into drains.
• Sources of larger plastic: land-based litter, rivers carrying waste from inland cities, and fishing gear lost or discarded at sea.

The fate of plastic in the ocean: rivers, gyres and endless fragments

Once plastic reaches the sea its fate depends on its density. Low-density plastics such as polyethylene and polypropylene, used in bags, bottles and packaging, float, so they are carried by surface currents and by the wind. Denser plastics such as PVC and some forms of PET sink, and floating pieces sink too once they are colonised by algae, barnacles and bacteria, which add weight. Plastic that reaches the seabed can be buried in sediment, where it is out of sight and out of reach of any clean-up.

Floating plastic does not spread evenly. The surface currents of each subtropical ocean form a large, slowly rotating circulation called a gyre, with a calm centre that acts as a trap. Plankton, seaweed and drifting plastic all collect there, so each gyre builds up an accumulation zone: the best known is the North Pacific gyre, often called the Great Pacific Garbage Patch. It is not a solid raft of rubbish. It is a diffuse soup of fragments, nurdles and fibres spread over a vast area at a concentration far too low to scoop up economically.

Most of the plastic that enters the sea cannot be found on the surface at all. It has been fragmented and dispersed, eaten, sunk, buried or washed back ashore. Because almost all scientific surveys count only what a floating net can catch, the deep water and the seabed are the least measured parts of the system, and estimates of how much plastic is in the ocean carry a wide range.

The key idea to grasp is that plastic in the ocean does not disappear. Ultraviolet light, waves and abrasion cut it into smaller and smaller pieces, and the pieces below 5 mm are microplastics; some of those are small enough to be taken up by plankton. On a geological timescale the material will eventually be buried and broken down, but on a human timescale the pollution is effectively permanent, and every year adds to what is already there.

• Floats or sinks: low-density plastics such as polyethylene and polypropylene float, while denser plastics and pieces colonised by organisms sink to the seabed.
• Gyres: circular surface current systems concentrate floating plastic in calm centres, forming diffuse accumulation zones such as the North Pacific gyre.
• Beyond the surface: most ocean plastic has fragmented, sunk, been buried in sediment, been eaten or washed ashore, so surface surveys undercount it.
• It does not disappear: ultraviolet light and abrasion reduce plastic to smaller and smaller fragments, and on a human timescale the pollution is permanent.

Impacts on marine ecosystems, food chains and people

Visual pollution is the first and most obvious impact. Beaches, reefs, mangroves and river mouths in tourist regions are strewn with bottles, bags and fragments, which reduces their value for visitors and for the people who earn a living from them. There is a physical effect as well: plastic that settles on the seabed smothers the organisms beneath it and reduces the light and the oxygen reaching them, while bags and netting snagged on coral cut into the tissue and make disease more likely.

Entanglement kills directly. Animals become caught in discarded nets, lines, packing loops and six-pack rings, especially turtles, seals, dolphins, whales and seabirds. A trapped animal may drown, or it may drag the gear for months, unable to feed or escape predators properly, while tight loops cut into the skin and cause infection or loss of a limb. Ghost gear - nets lost or abandoned at sea - is a large share of the most damaging items because it keeps on catching animals long after it has stopped being useful.

The risk of being mistaken for food affects animals that cannot tell plastic from prey. Floating bags look like jellyfish to a turtle, nurdles and fragments look like fish eggs or plankton to fish, squid and filter-feeding whales, and seabirds pick up lighter pieces from the surface and feed them to their chicks. Swallowed plastic can block the gut, and even when it does not it gives a false feeling of fullness, so the animal stops feeding, takes in less energy and slowly starves. Sharp fragments damage the gut lining.

Microplastics then move through the food chain by bioaccumulation and biomagnification. Bioaccumulation is the build-up of a substance in the tissues of a single organism over time, as shellfish and small fish take in microplastics faster than they can get rid of them. Biomagnification is the increase in concentration as the substance is passed up the food chain, because a predator eats many contaminated prey and the plastic and the pollutants stuck to its surface are not excreted. Toxic substances such as persistent organic pollutants adsorb onto plastic in the sea, and top predators - and people who eat seafood - receive the largest doses. Some plastic additives, including phthalates and bisphenol A, are known to disturb hormone systems in laboratory animals, but the size of the risk to human health from microplastics in food and drinking water is still not established, so scientists use the precautionary principle and concentrate on cutting the inputs.

Managing plastic pollution 1: use less, reuse more, and make producers pay

The most effective strategy is the one that stops the waste being created, because plastic that is never used cannot be littered, and reduction and reuse avoid the costs of collection, recycling and disposal altogether. Avoiding single-use plastics means cutting out the items that are used once for a few minutes: carrier bags, straws, cutlery, plates, cups, cotton bud sticks, drinks bottles and over-packaged goods, and replacing them with reusable alternatives. Alternative packaging replaces plastic with paper and card, glass, metal, loose produce and refillable or returnable containers, or with genuinely compostable material where a compost route exists. The limitation is that the substitutes are usually heavier, so transport emissions and cost rise, and paper and glass have their own environmental costs.

Deposit return schemes put a small refundable deposit on a drink container. Germany's Pfand system and Norway's bottle deposit scheme both return a very high share of their containers, because the deposit is worth collecting: the material comes back clean, sorted and therefore valuable for recycling, and litter falls. The limitations are that the scheme only covers drinks containers, it needs a dense network of return points or reverse vending machines and a reliable refund mechanism, and in countries where informal waste pickers survive by collecting bottles for scrap, a deposit scheme can take that income away from the poorest workers unless they are brought into the system.

Bans and levies change behaviour by making the unwanted item illegal or expensive. Bangladesh banned plastic carrier bags as early as 2002, Chile banned them in shops in 2018, Kenya banned them in 2017 and banned single-use plastics in its protected areas from 2020 with heavy penalties, and the European Union's Single-Use Plastics Directive banned straws, cutlery, plates and several other items and made producers responsible for fishing gear. Levies such as a charge per bag work more slowly: a small charge reduces use, but only a charge high enough to hurt changes the habit, and it is regressive because it takes a bigger share of a poor household's income.

Extended producer responsibility (EPR) makes the companies that place packaging on the market pay for its collection, recycling and safe disposal, instead of leaving the cost to the taxpayer. This puts the cost where the design decision is made and encourages lighter, simpler, more recyclable packaging; the European Union uses it for packaging and the European Union and India both apply it to fishing gear and packaging respectively. Its weakness is that it is only as good as the collection system it pays for, and it can be met on paper through paid-for targets while real collection stays poor.

• Reduction and reuse: refuse single-use items and switch to reusable bags, bottles, cups and containers, because waste that is never created cannot become litter.
• Alternative packaging: paper, card, glass, metal, loose produce and refillable containers, remembering that heavier substitutes cost more to transport.
• Bans and levies: effective and quick where they can be enforced (Kenya, Chile, the European Union), but evasion, border smuggling, regressive costs and a narrow focus on packaging limit them.
• Deposit return schemes: a refundable deposit on bottles raises return rates and produces clean, valuable material, but needs return points and can cut the income of informal collectors.
• Extended producer responsibility: producers pay for collection and recycling, which encourages better design, but it depends on an effective collection system and on real monitoring.

Managing plastic pollution 2: safe disposal, recycling, clean-ups and international action

Safe disposal means collecting waste and dealing with it without releasing it into the environment. Where collection is effective, plastic is recycled, sent to a properly engineered landfill or burned in an energy-from-waste plant with pollution controls. Where collection fails, the alternatives are open dumping and open burning, and burning plastic releases toxic fumes including dioxins and fine particles that harm the people living nearby. Because blocked drains and gutters also cause flooding in cities such as Accra, improving collection is both a waste measure and a public health and flood measure. The limitation is money: collection, transport, landfill and incinerator infrastructure are expensive, and the cities that leak most plastic into rivers are usually the ones that cannot afford it.

Recycling turns used plastic back into new material. It uses far less energy and oil than making virgin plastic, and a clean, single-polymer stream such as PET drinks bottles or HDPE containers is genuinely valuable. The limitations are severe and worth naming. Plastics are not all recyclable, and mixed, dirty or multi-layer packaging has almost no value; collection has to be separate and clean; material can only be recycled a few times before the polymer chains shorten; food safety rules restrict recycled plastic in food packaging; and recycling competes against cheap virgin plastic made from oil. Only a small fraction of all the plastic ever produced has been recycled, and a strategy that relies on recycling alone therefore cannot keep up with production.

Exporting the problem has been exposed as no solution at all. For years richer countries shipped mixed plastic waste to poorer ones, where much of it was dumped, burned or left in the open. The Basel Convention plastics amendment, in force since 2021, requires prior informed consent before mixed, contaminated or hard-to-recycle plastic waste can be exported, so exporting countries must deal with it themselves and clean streams are separated from the waste. The limitations are the familiar ones for any international agreement: monitoring and enforcement across many borders are difficult, shipments are mislabelled as clean or second-hand goods, and an illegal trade continues alongside the legal one.

Clean-up projects attack the plastic that is already there. Beach and river clean-ups involve volunteers, while engineered systems such as the floating barriers and skimmers trialled by The Ocean Cleanup in the North Pacific gyre and at river mouths concentrate surface plastic for collection. They are worth doing: they protect wildlife locally, raise awareness and return tourist beaches to use. But they remove only a small fraction of the total, they are expensive per tonne, they can capture fish and other marine organisms, the waste still has to be disposed of properly, and they risk the impression that technology will clear up a problem that is still being created upstream. Clean-ups treat the symptom; reduction and better waste management treat the cause, and the two must run together.