Microplastics are defined as smaller than 5 milimeters in diameter. But these tiny particles accumulate in the sea, can enter the food chain, and are even found in the air. Personal care products containing microplastics, such as toothpaste, represent one of the most common intentional uses of microplastics in our daily lives.

Some cosmetic products can contain as much plastic added as the amount of plastic in which they are packaged, experts indicate. Exfoliating daily washes very often use of microplastics, often termed "micro-beads," which then get flushed into the household wastewater stream.

Via wastewater, microplastics reach the oceans, where they move enter the food chain through feeding zooplankton. In 2017, researchers found that 25 percent of marine fish tested in markets in Indonesia and California had plastic and textile fibers in their guts. Research is still lacking as to whether consuming microplastics through fish harms humans.

Several studies published in 2017 showed that microplastics have also been found in sea salt from the United States, Europe and China. It's not surprising, considering how plastic debris represent between 60 to 80 percent of the total marine waste, and that up to 12 million tons of plastic waste ends up in the ocean every year.

Beside microplastics in seafood like fish, shrimp and mussels, scientists point to other foods, such as honey. In the recently adopted European Union plastics strategy, honey was mentioned as one of the food products containing microplastics — to motivate a push toward a ban.

Aside from micro-beads in hygiene products, synthetic textiles also release a vast amount of tiny plastic fibers into wastewater. Researchers found that a typical 6-kilogram (13-pound) washing load of acrylic-fabric items (like fluffy blankets) generates more than 700,000 individual fibers. Synthetic fabrics account for around a third of ocean microplastics.

Vehicles tires are also a main source releasing microplastics into the environment. Tires are made of synthetic polymers mixed with rubber, which grinds down when used. This generates microplastics that are either blown around by the wind or washed away by rain. Norwegian and Swedish researchers agree that a large proporation of particles found in the sea come from car tires.

Microplastics have also been found in tap water. In an analysis of tap water samples from countries around the world, more than 80 percent were contaminated with some amount of plastic fibers. If synthetic fibers are in tap water, they are also likely to be in a number of other basic foodstuffs, like bread.

And yes, if tap water contains microplastics, beer could very well also be contaminated. A 2014 study found plastic particles in a total of 24 German beers - but variability in the results was high, and further research would needed for verification, the German Environment Agency has pointed out. In any case, cheers for now.

The Yangtze is Asia's longest river and the third-longest river in the world. It also tops the list of river systems through which the most plastic waste flows into the oceans, according to a recent study. The Yangtze flows into the East China Sea near Shanghai and is crucial to China's economy and ecology. The river basin is home to 480 million people — one-third of the country's population.

The Helmholtz Centre for Environmental Research found 90 percent of plastic flowing into oceans can be traced to 10 rivers. The Indus ranks second on the list. One of Asia's largest rivers, it flows through parts of India and Pakistan into the Arabian Sea, supporting millions of people. While much plastic enters rivers because of a lack of waste infrastructure, sewage systems contribute too.

Plastic can enter the food chain as fish and other marine and freshwater animals ingest it. The Yellow River, said to be the cradle of Chinese civilization, is third on the plastic-waste list but that's not the only environmental problem with which it contends. Pollution has rendered much of the river's water undrinkable. Around 30 percent of its fish species are believed to have disappeared too.

Another of China's rivers, the Hai, comes in at number 4. It connects two of China's most populous metropolitan areas, Tianjin and Beijing, before flowing into one of the world's busiest shipping lanes, the Bohai Sea. The 10 river systems share traits, says the study. One is that they are located in densely populated areas with a lack of waste infrastructure and little awareness of recycling.

Generally thought to be the world's longest river, the Nile flows through 11 countries before entering the Mediterranean Sea from Egypt. Some 360 million people live in the river basin where its waters support agriculture — the region's main economic activity. Irrigation and evaporation mean the river doesn't even reach the sea in dry periods. Still, it comes in at number five in the ranking.

The Ganges is central to Indian spiritual life and provides water to more than half a billion people. Sewage, agricultural and industrial waste have made it one of the world's most polluted rivers, as have the multitudes of plastic that end up in it. Cleaning up the waste — as students are doing in this picture — is important, but experts say we must produce less and stop pollution at the source.

Here, workers clear floating waste from China's notoriously dirty Pearl River, which enters the South China Sea between Hong Kong and Macau. Sewage and industrial waste flow into the river delta, keeping apace with the region's incredible rate of urban expansion. Since the late 1970s, the delta has transformed from a mainly agricultural and rural region to one of the world's largest urban areas.

It's not until they hit urban and industrial areas that rivers feel the worst effects of pollution. Still, according to recent studies, plastic debris is even being found in remote and "pristine" locations. The Amur River rises in the hills of northeastern China and forms much of the border between China's Heilongjiang province and Russia's Siberia before it snakes out to the Sea of Okhotsk.

The Niger is West Africa's main river, supporting over 100 million people and one of the planet's most lush ecosystems. It flows through five countries before entering the Atlantic Ocean from Nigeria. Plastic pollution aside, extensive dam construction is affecting water availability — and frequent oil spills in the Niger Delta have caused widespread water contamination.

Dams are having major ecological and social impacts on the Mekong too. Around 20 million people live in the Mekong Delta. Many are dependent on fishing and agriculture for survival. The river flows through six countries in Southeast Asia, including Vietnam and Laos, and is tenth on the list of river systems that carry most of the 8 million tons of plastic that are dumped into the seas each year.