If evidence was needed that microplastics get just about everywhere, research published a few days ago was surely it.
This week, scientists found tiny pieces of plastic, ranging in size from 50 to 500 micrometres, in stool samples of eight people from across Europe.
Participants are thought to have been exposed to microplastics in food, including seafood, and from drinking plastic bottles.
Their samples, analysed by the Medical University of Vienna and the Environment Agency Austria, contained up to nine types of plastic.
It should perhaps come as no surprise that microplastics are reaching the human gut, as there has been much evidence that these tiny particles are prevalent in food and drink.
This year, 93 per cent of bottled water samples from countries including Brazil, China, Kenya, Lebanon and the US were found to contain microplastics. The particles have been found in tap water too.
Scientists have discovered that fish, shellfish and mussels can be sources of microplastics, with even fish liver contaminated with them.
Farm animals could be affected by microplastics in the soil, according to Dr Stephen Cattle, a soil scientist at The University of Sydney.
He said that grazing animals may suffer hormonal disruption when microplastics that they have ingested release organic compounds in their guts.
“In the cast of plants, root and tuber crops such as carrots and potatoes may take up microplastic fragments as the root or tuber grows, enabling these microplastics to enter the human food chain,” he said.
Research suggests that hundreds of thousands of tonnes of microplastics could be entering North American and European agricultural soil annually, and there are no established techniques for cleaning soil of microplastics.
In urban areas and places where wastes of various kinds have been deposited it is “almost inevitable” that there will be “appreciable concentrations of microplastics” in soil, according to Dr Cattle.
Given that microplastics are prevalent, and that food and drink is contaminated with them, a key issue is what harm they might be doing us.
Dr Rachel Adams, senior lecturer and head of the Department of Biomedical Sciences at Cardiff Metropolitan University in the United Kingdom, says science does not have “clear evidence that plastic microparticles have a negative effect on human health”.
“There are, however, a number of potential harmful effects. Exposure to other particles causes inflammation; this is one of the main detrimental effects caused by exposure to air pollution particles,” she said.
“Particles may be recognised as 'foreign' and this elicits a response in the immune system which can cause damage to the body.”
Another potential concern is that the particles could carry other toxins into the body, as they tend to bind toxins that are not very soluble in water. These could then accumulate in fatty tissue.
Dr Adams said, however, that not enough is known about the extent to which particles passed through the body or were absorbed. Smaller particles are more likely to enter cells and enter the body.
Plastic pollution has moved up the political and media agenda in recent years, sparked by a range of alarming statistics, such as that in some parts of the ocean there is many times more plastic than plankton.
A 2015 study found that 90 per cent of seabirds have eaten plastic, with some starving because their stomachs were filled with it.
Despite measures in some countries to combat the problem of plastic pollution, such as compulsory charges for plastic bags, and bans in microbeads (tiny pieces of plastic) in toothpastes and cosmetics, the entry into the natural environment of plastic is set to continue.
About a decade ago, around 300 billion plastic bottles were produced annually across the world, but by 2021, the figure is forecast to be almost 600 million, according to Euromonitor International.
These numbers vastly exceed the global capacity for recycling, so many bottles will end up in the natural environment and will break down, releasing microplastics.
Clothes such as fleeces made from artificial materials are a source of plastic microfibres, often released when items are washed, adding to a problem that has seen microplastics recorded everywhere from the Atlantic to the Pacific to the Arctic.
Just as little is known about the health effects of microplastics, so there is some uncertainty as to how long these tiny particles will persist.
Dr Matthew Hoffman, an assistant professor at the Rochester Institute of Technology in the United States, has used computer simulations to analyse the spread of plastic pollution, in his case in the Great Lakes area of the United States. Dr Hoffman describes the issue of how long microplastic particles remain as “a tough question”.
“Plastic will break down very slowly. There's estimates it will take 500 years for a water bottle to break down,” he said.
“But no one has done a study where they've seen it degrade. I'm not sure there's a study that's well characterised. There are some bacteria that might be able to help at some point, that grow on it.”
Dr Hoffman says that some microplastics will probably end up in sediment, meaning that they are still present in the environment, but are no longer contaminating water.
When it comes to plastics in the soil, Dr Cattle notes that different plastics have different rates of degradation, and these rates vary according to soil conditions. However, some studies have suggested that it may take “hundreds or thousands” of years for some plastic to degrade.
“So the legacy of past plastic production will remain with us for a very long time to come,” he said.