Microplastics in the Ocean: Where They Come From and What We Know

Plastic bags and bottles floating at sea are easy to picture. Far harder to see, and far more numerous, are microplastics: tiny fragments, fibers and beads that are now found almost everywhere scientists look, from tropical beaches to Arctic sea ice and the deepest ocean trenches. This explainer covers what microplastics are, where they come from, what they do to marine life, and what science can and cannot yet say about risks to people.

Key points

Microplastics are commonly defined as plastic pieces smaller than 5 millimeters, about the size of a sesame seed or smaller.
Most come from larger plastic items breaking down; others are made small on purpose or shed from clothing and tires.
They have been found in seawater, sediments, sea ice and many marine animals.
Harm to some marine organisms has been shown in laboratory studies, but effects in the wild are still being worked out.
Effects on human health are an active area of research, and major health agencies say more evidence is needed.

What are microplastics?

The US National Oceanic and Atmospheric Administration describes microplastics as small plastic pieces less than five millimeters long. Scientists sometimes also talk about nanoplastics, particles so small they are measured in billionths of a meter. Nanoplastics are much harder to detect and study, so far less is known about them.

Microplastics come in many shapes:

Fragments broken off larger items such as bottles, packaging and fishing gear.
Fibers shed from synthetic textiles like polyester and nylon, and from ropes and nets.
Pellets (sometimes called nurdles), the small beads used as raw material for making plastic products, which can spill during transport.
Microbeads once added to some face scrubs and toothpastes.
Films and foams from bags, wraps and polystyrene.

Where they come from

Researchers often sort microplastics into two groups:

Primary microplastics

These are made small on purpose or released small. Examples include pre-production pellets, microbeads, and particles worn off products during use, such as fibers from washing synthetic clothes and dust from car tires wearing down on roads. In the United States, the Microbead-Free Waters Act of 2015 banned plastic microbeads in rinse-off cosmetics, and several other countries have similar rules.

Secondary microplastics

These form when larger plastic litter breaks apart. Sunlight, waves, heat and abrasion make plastic brittle, and it fragments into smaller and smaller pieces. Plastic does not biodegrade in the way food or wood does; it mostly just gets smaller. Because so much plastic waste reaches rivers, coasts and the sea, secondary microplastics are thought to make up a large share of what is found in the ocean.

Microplastics reach the ocean through rivers, stormwater runoff, wastewater (treatment plants capture many particles but not all), wind, and direct losses at sea such as discarded fishing gear.

Where they end up

For years, scientists noticed a puzzle: the amount of plastic floating at the ocean surface seemed far smaller than the amount thought to be entering the sea. Part of the answer is that plastic does not stay at the surface. Microplastics:

Accumulate in ocean gyres, large rotating current systems such as the area often called the Great Pacific Garbage Patch, which is more like a thin soup of small particles than a solid island.
Sink once they become coated in algae and bacteria or are packaged into the droppings of small animals. Deep-sea sediments are now considered a major sink.
Wash up on beaches and get buried in coastal sediments.
Get trapped in sea ice, which can later release them when it melts.

NOAA's Marine Debris Program supports research and monitoring of plastic pollution, including microplastics, along US coasts and in the Great Lakes.

Effects on marine life

Many marine animals, from zooplankton to fish, seabirds, turtles and whales, have been found with microplastics in their bodies. Filter feeders such as mussels and oysters are particularly exposed because they strain large volumes of water.

Possible effects that scientists study include:

Physical effects, such as blocked or damaged digestive tracts, or a false feeling of fullness that reduces eating.
Chemical effects from additives in plastics (such as plasticizers and flame retardants), or from pollutants in seawater that stick to plastic surfaces.
Effects on growth and reproduction, seen in some laboratory experiments with plankton, shellfish and fish.
Transport of microbes, since plastic surfaces can carry communities of bacteria and other organisms to new places.

A note on uncertainty. Many laboratory studies use higher concentrations or different particle types than animals typically meet in the wild, so results do not always translate directly. Researchers are working to standardize methods and to measure realistic exposure levels. The overall picture is concerning, but the size of ecological effects across whole food webs is not yet well established.

Microplastics add to other pressures on ocean life, including warming and chemical change. See Ocean acidification explained and Coral bleaching explained.

What about people?

Microplastics have been detected in seafood, salt, drinking water, and air, and studies have reported finding them in human tissues. That understandably worries people. What it means for health is much less clear.

In a 2019 report on microplastics in drinking water, the World Health Organization concluded that, based on the limited information available, microplastics in drinking water did not appear to pose a health risk at the levels then found, while stressing that the evidence was limited and that more research was needed, especially on smaller particles. Research has expanded considerably since then, and health agencies continue to review new findings.

The honest summary is that exposure is widespread, the long-term health effects are not yet known, and reducing plastic pollution is sensible on environmental grounds regardless.

What is being done

Because microplastics are so small and widespread, cleaning them out of the open ocean is not practical. The focus is on stopping plastic from getting there in the first place:

Better waste management and recycling systems, especially in fast-growing regions where collection is limited.
Product design that uses less plastic, avoids unnecessary single-use items, and lasts longer.
Capturing pellets at factories and during shipping.
Washing machine filters to catch synthetic fibers, which some countries are moving to require.
Stormwater and wastewater upgrades to catch more particles before they reach waterways.
International negotiations led by the United Nations on a treaty to address plastic pollution across its life cycle.

The US EPA's Trash Free Waters program describes efforts to keep litter out of rivers and coasts.

What you can do

Individual steps are small compared with the scale of the problem, but they add up and help shift norms. Choose reusable bags, bottles and containers where practical; avoid littering and join local cleanups; wash synthetic clothes less often, in full loads and at cooler temperatures, which may reduce fiber shedding; and support local policies on waste and recycling. For a broader view of personal climate and environmental actions, see What can one person do?

The bottom line

Microplastics are now part of nearly every ocean environment. They come mostly from larger plastic breaking down, plus fibers, tire dust and pellets. Their harm to some marine species is documented in the lab, and their effects in nature and on human health are active research questions. The most effective solution is to keep plastic out of the environment in the first place.