Small plastics have entered every part of the planet. They have been found buried in Antarctic sea ice, in the guts of deep-sea marine animals, and in drinking water around the world.
Plastic pollution has been found on the beaches of remote, uninhabited islands and is showing up in ocean water samples around the world.
One study estimated that there are approximately 24.4 trillion pieces of microplastic in the upper oceans of the world.
But they're not just found in water - they're widespread in soil too and can end up in the food we eat.
In 2022, an analysis by the Environmental Working Group, a non-governmental environmental organization, found that sewage has contaminated nearly 20 million acres (80,937 square km) of cropland in the United States with what are often called "perpetual chemicals." , which are commonly found in plastic products and do not decompose under normal environmental conditions.
Sewage sludge is the remaining product after municipal wastewater has been treated. Because it is expensive to dispose of and contains many nutrients, the sludge is used as organic fertilizer in the United States and Europe.
These are EU directives to promote a waste economy. It is estimated that 8-10 million tonnes of sewage sludge is produced in Europe every year, and about 40% of this is spread on farms.
As a result of this practice, European farms could become the world's largest reservoir of microplastics, according to a study by researchers at Cardiff University.
This means between 31,000 and 42,000 tonnes of plastic, or 86 trillion to 710 trillion microplastic particles, pollute Europe's farmland every year.
Researchers found that up to 650 million tiny plastic particles, measuring between 1mm and 5mm, entered a wastewater treatment plant in south Wales, England, every day.
All these particles ended up in the sewage sludge, making up about 1 percent of the total weight, instead of being released with clean water.
The number of microplastics that end up on the farm is a small estimate," says Catherine Wilson, one of the study's authors and deputy director of the Hydro Environmental Research Center at Cardiff University. "Microplastics are everywhere and often so small that we cannot see them."
Small plastics can stay there for a long time too. One recent study by soil scientists at Philipps-University Marburg found microplastics up to 90 centimeters below the soil in two agricultural fields where sewage sludge was last used 34 years ago.
Plowing also caused plastic to spread in areas that had not been silted.
The concentration of plastic minerals in farm soil in Europe is similar to the amount found above seawater, says James Lofty, lead author of the Cardiff study and a PhD research student at the Center for Hydro-Environmental Research.
The UK has the highest levels of microplastics in Europe, and between 500 and 1,000 microplastics end up on farms there every year, according to research by Wilson and Lofty.
A report by the British Environment Agency, which was later revealed by the environmental campaign group Greenpeace, found that sewage sludge directed to English farms was contaminated with pollutants including dioxins and polycyclic aromatic hydrocarbons at "levels which may pose a risk to human health".
A 2020 experiment by Kansas University agronomist Mary Beth Kirkham found that plastic serves as a vector for the uptake of toxic chemicals such as cadmium by plants. "In plants where cadmium was in the soil with plastic, the wheat leaves had more cadmium than plants that grew without plastic in the soil," Kirkham said at the time.
Research also shows that microplastics can stunt the growth of earthworms and cause them to lose weight. The reasons for this weight loss are not fully understood, but one theory is that microplastics can block the worms' digestive tract, preventing their ability to absorb nutrients and thus inhibiting their growth.
This has a negative impact on the environment, too, the researchers say, as earthworms play an important role in maintaining the health of the soil.
Their digging activity aerates the soil, prevents soil erosion, improves drainage and restores nutrients.
Research has shown that most of the plastic accumulates in the roots of the plant, with only a small amount traveling to the stem. "The concentration in the leaves is less than 1%," says Peijnenburg.
For leafy vegetables such as lettuce and cabbage, the levels of plastic may be low, but for root vegetables such as carrots, radishes and turnips, the risk of ingesting plastic will be greater, he warns.
Another study by Peijnenburg and colleagues found that in lettuce and wheat, the concentration of plastic was 10 times lower than in the soil.
"We found that only the small particles are taken up by the plants and the larger ones are not," says Peijnenburg.
This is encouraging, says Peijnenburg. However, most plastics will slowly degrade and decompose into nanoparticles, providing "a good source of food for plants," he adds.
Health effects
Although the effects of ingesting plastic on human health are not yet fully understood, there is already research that shows it can be harmful. Studies show that chemicals added during plastic manufacturing can disrupt the endocrine system and the hormones that control our growth and development.
Chemicals found in plastic have been linked to various other health problems including cancer, heart disease and poor fetal growth.
High levels of plastic intake can also cause cell damage, according to research at the University of Hull, in England.
The researchers reviewed 17 previous studies that looked at the toxic effects of plastic on human cells.
The analysis compared the amount of plastic that caused cell damage in laboratory tests with levels ingested by people through drinking water, seafood and salt.
It found that the amount used was close to that which would cause cell death, but could also cause immune responses, including allergic reactions, damage to cell walls, and oxidative stress.
There are two theories about how microplastics cause cell breakdown, says Danopoulos.
Their sharp edges can tear the cell wall or the chemicals in the microplastics can damage the cells, he says.
The study found that irregularly shaped microplastics were the most likely to cause cell death.
"What we need to understand now is how much plastic remains in our body and what kind of size and shape can penetrate the cell barrier," says Danopoulos.
If plastic could accumulate to levels that could be harmful over time, this could pose a greater risk to human health.
But even without these answers, Danopoulos questions whether more care is needed to ensure microplastics don't enter the food chain.
"If we know that the sludge is contaminated with microplastics and that plants have the ability to remove them from the soil, should we use them as fertilizer?" He says.
Banning sewage sludge
The spreading of sludge on farms has been banned in the Netherlands since 1995.
The country initially incinerated the sludge, but began shipping it to England, where it was used as fertilizer on farms, after problems at the Amsterdam incinerator.
Switzerland banned the use of sewage sludge as fertilizer in 2003 because it "contains a wide range of harmful substances and pathogenic microorganisms.
The US state of Maine also banned the practice in April 2022 after environmental authorities found high levels of plastic in farm soil, crops and water.
High levels of plastic were also detected in the farmers' blood.
But a total ban on using sewage sludge as fertilizer is not necessarily the best solution, says Wilson of Cardiff University.
Instead, it could encourage farmers to use more synthetic nitrogen fertilizers, made from natural gas, he says.
Some European countries, such as Italy and Greece, dispose of sewage sludge in landfills, researchers note, but warn that there is a risk of plastic leaking into the environment from these sites and contaminating land and water bodies.
Both Wilson and Danopoulos say more research is needed to measure the amount of microplastics on farms and the potential environmental and health impacts.
