The health risk from these microplastics is still relatively unknown, but it begs the question, how does all that plastic even end up in the sea, and how can we stop it from getting there in the first place?

Rivers of plastic

The researchers concluded: "The more waste there is in a catchment area that is not disposed of properly, the more plastic ultimately ends up in the river and takes this route to the sea."

But plastic is getting into rivers all over the world.

Thames21 is a group that organizes clean-ups along the foreshore of the River Thames, in the UK, and educates people to not pollute it.

"We're finding that we are having a real reduction in the amount of plastic bags that we found and the bottles that we found," says Luke Damerum, who works with the group.

Its prime target now is wet wipes, which are often flushed down toilets and are discharged into the river from the sewers after heavy rainfall. They can end up lining the foreshore in a thick carpet.

"If this stuff remains on the foreshore, a lot of it will just break down into smaller and smaller pieces into ... microplastics," explains Damerum. "These microplastics will ... eventually go down towards the estuary and then out into the open ocean."

Clothes, car tires and cosmetics

Plastics are so ubiquitous in our lives that there are countless other sources of plastic pollution.

In the eastern United States, the Hudson River carries 300 million clothing fibers into the Atlantic Ocean every day, according to a 2017 study. About half of the fibers are plastic, many entering the water network when clothes are machine washed.

Clothes can also shed fibers into the air, as can car tires and waste plastic left in landfills -- which can all end up in our rivers and seas.

Fertilizers made from sewage sludge -- the by-product of sewage water treatment -- are another source. The sludge can contain microfibers from clothes and other plastics washed into the sewers. As these fertilizers dry out, some of the plastics can get carried off the land by the wind, or wash directly into rivers.

To solve a problem, you must first understand it

So what can be done to solve the problem?

Professor Richard Lampitt, of the National Oceanography Centre, in the UK, says technological advances can help, like better filters in washing machines to catch microfibers -- as can industry moves to develop less damaging plastics.

But while he believes that research into solutions is important, he says it's vital that we develop a better understanding of exactly how plastic is transported into the oceans, how it degrades and what damage it is doing.

"There's considerable uncertainty about how much plastic pollution is coming from all these different sources," he told CNN.

"We need to have our eyes on what the nature of this problem is. How much plastic is there? How does it get there? How does it get transformed during its passage to the sea, and what harm does it do at the concentrations that it's found?

"These are really important questions to resolve."