Imagine filling up five grocery bags with plastic - soda bottles, toothbrushes and shampoo sachets, for example - and lining them up side by side. Then do the same on every foot (30 cm) of the world's coastlines.

Now dump all of that into the ocean.

Scientists estimate that's the average amount of plastic waste making its way into marine waters each year.

In the latest study of plastic debris in oceans, researchers estimate around 8.8 million metric tons of plastic finds its way into the world's oceans every year.

By 2025 they fear it will increase tenfold unless waste management practices are improved around the world, especially in middle income countries with rapidly developing economies and long coastlines.

Study coauthor Kara Lavender Law compared the amount of plastic trash to how much tuna is fished each year.

"We are taking out tuna and putting in plastic," said Law, a research professor of oceanography with the Massachusetts-based Sea Education Association, in a press conference at the American Association for the Advancement of Science conference.

Large bluefin tunas at Tokyo's Tsukiji fish market.

The study appeared in the Feb. 13 edition of the journal Science.

Researchers looked at data from 192 coastal countries, taking into account population density and economic status. Out of the 275 million metric tons of plastic waste generated in 2010, between 4.8 and 12.7 million metric tons wound up in the ocean from people living within 50 kilometers of the coastline.

A country's population size and the quality of its waste management systems largely determine how much waste might end up in the ocean, according to the researchers.

Previous studies have documented the presence and location of plastic waste in the oceans, but these usually measured the debris floating on the surface. Since not all plastic floats, this would underestimate the actual amount of plastic waste in the water. This is the first time, the study's scientists said, that research estimated the amount of plastic entering the oceans each year.

Big chunks of plastic can further break down to "microplastic" - tiny particles that are created when debris is made brittle by sunlight and then torn apart by waves or nibbled on by sharks and other fish.

'Plastic smog of the oceans'

Another plastic waste researcher Marcus Eriksen told DW microplastic was so pervasive in the oceans that people should think of it as "plastic smog."

He and his colleagues also published a study in December in the open access journal PLOS ONE, in which they found 250,000 tons of plastic waste floating at sea. It was like finding the whole contents of a department store in the ocean, he said.

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"You name it, I've found it," he added, "even buckets full of huge fish."

Since the findings didn't include sea floor plastic, he believes the figure is an underestimate and was surprised it wasn't higher. He thinks microplastics are especially a concern.

"They absorb so many toxins to the point that scientists agree that microplastics should be classified as hazardous waste," said Eriksen, Director of Research and co-founder of the 5 Gyres Institute, an organization aiming to reduce plastics pollution in the oceans.

His main worry is microplastics could release toxins, ranging from pesticides to oil drops from cars, inside organisms that are filtering or selecting particles in the oceans.

He pointed out a 2012 study in which blue mussels were found to have microplastics in their digestive systems.

"You can say that we're literally eating our own trash," Eriksen said.

A pressing question

But whether all cells take up toxin-ridden microplastics is a question that scientists aren't sure about.

Lars Gutow of Germany's Alfred Wegener Institute for Polar and Ocean Research (AWI) studied the effects of plastic particles on smaller animals. His study's results were encouraging: Although the small particles were found passing through the digestive tract, microplastics were not absorbed or accumulated in the digestive glands of these animals.

So, it's conceivable that people ingest microplastics when they eat the animal whole, like shellfish, but regular fish shouldn't be a problem since people normally don't eat the digestive organs, he told DW.

Microplastics can affect marine animals.

He added he hopes to investigate the chemical effects of microplastics on small animals in one of his next studies.

Calls for action

Microplastics are also components in a variety of cosmetic products, such as exfoliating scrubs and toothpaste, and have even been found in honey and German beer.

Once these products are poured down the drain, microplastics aren't filtered out before being returned to rivers, lakes and seas. AWI found water treatment plants were unable to keep particles out of the wastewater completely.

There have been campaigns around the world, which call for a ban on microplastics in cosmetics. But researchers from the Science study believe plastic should be prevented from reaching the oceans in the first place, citing it is a waste management problem.

Five countries - China, Indonesia, Philippines, Vietnam and Sri Lanka - are responsible for more than half of marine plastic waste, according to the Science researchers. China contributes around 2.4 million metric tons - nearly 28 percent of the world total. From the list of the world's top 20 plastic polluters, the United States came in at number 20 and was the only industrialized country on the list.

The top priority should be helping nations develop solid waste management and infrastructure, the scientists said.

Industry needs to redesign

But Eriksen of the 5 Gyres Institute thought waste management "being the culprit" in the Science paper was overemphasized. He said that was only part of the picture.

"The problem isn't just mismanagement, but it's also the design of the products in the first place," Eriksen said. "For example, plastic sachet packets that hold a few milligrams of shampoo or ketchup have absolutely no value as waste."

Workers recycle plastic in Bangladesh.

It's an industry-affirming position, he said. He thinks the burden of responsibility shouldn't just be on taxpayers, governments and municipal systems.

"The issue must really be shared by industry, which includes better design of products," Eriksen said.

Along with smarter design and superior waste management facilities, Gutow from AWI thinks better recycling policies could further help alleviate the situation, since he thinks it's virtually impossible to rid the oceans of these tiny plastic particles.

He also has this simple piece of advice for everyone.

"Reduce litter," he said.