How microplastics are infiltrating the food you eat – BBC

The researchers found that up to 650 million microplastic particles, measuring between 1mm and 5mm (0.04in-0.2in), entered one wastewater treatment plant in south Wales, in the UK, every day. All these particles ended up in the sewage sludge, making up roughly 1% of the total weight, rather than being released with the clean water.

The number of microplastics that end up on farmland “is probably an underestimation,” says Catherine Wilson, one of the study’s co-authors and deputy director of the Hydro-environmental Research Centre at Cardiff University. “Microplastics are everywhere and [often] so tiny that we can’t see them.”

And microplastics can stay there for a long time too. One recent study by soil scientists at Philipps-University Marburg found microplastics up to 90cm (35in) below the surface on two agricultural fields where sewage sludge had last been applied 34 years ago. Ploughing also caused the plastic to spread into areas where the sludge had not been applied.

The microplastics’ concentration on farmland soils in Europe is similar to the amount found in ocean surface waters, says James Lofty, the lead author of the Cardiff study and a PhD research student at the Hydro-environmental Research Centre.

The UK has some of the highest concentrations of microplastics in Europe, with between 500 and 1,000 microplastic particles are spread on farmland there each year, according to Wilson and Lofty’s research.

As well as creating a large reservoir of microplastics on land, the practice of using sewage sludge as fertiliser is also exacerbating the plastics crisis in our oceans, adds Lofty. Eventually the microplastics will end up in waterways, as rain washes the top layer of soil into rivers or washes them into groundwater. “The major source of [plastic] contamination in our rivers and oceans is from runoff,” he says.

One study by researchers in Ontario, Canada, found that 99% of microplastics were transported away from where the sludge was initially dumped into aquatic environments.

Environmental contamination

Before they are washed away, however, microplastics can leach toxic chemicals into the soil. Not only are they made from potentially harmful chemicals that can be released into the environment as they break down, microplastics can also absorb other toxic substances, essentially allowing them to hitch a ride onto agricultural land where they can leach into the soil, according to Lofty.

A report by the UK’s Environment Agency, which was subsequently revealed by the environmental campaign group Greenpeace, found that sewage waste destined for English farmland was contaminated with pollutants including dioxins and polycyclic aromatic hydrocarbons at “levels that may present a risk to human health”.

A 2020 experiment by Kansas University agronomist Mary Beth Kirkham found that plastic serves as a vector for plant uptake of toxic chemicals such as cadmium. “In the plants where cadmium was in the soil with plastic, the wheat leaves had much, much more cadmium than in the 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 aren’t fully understood, but one theory is that microplastics may obstructs earthworms’ digestive tracts, limiting their ability to absorb nutrients and so limiting their growth. This has a negative impact on the wider environment, too, the researchers say, as earthworms play a vital role in maintaining soil health. Their burrowing activity aerates the soil, prevents erosion, improves water drainage and recycles nutrients.

Plastic particles can also contaminate food crops directly. A 2020 study found microplastics and nanoplastics in fruit and vegetables sold by supermarkets and in produce sold by local sellers in Catania in Sicily, Italy. Apples were the most contaminated fruit, and carrots had the highest levels of microplastics among the sampled vegetables.

According to research by Willie Peijnenburg, professor of environmental toxicology and biodiversity at Leiden University in the Netherlands, crops absorb nanoplastic particles – minuscule fragments measuring between 1-100nm in size, or about 1,000 to 100 times smaller than a human blood cell – from surrounding water and soil through tiny cracks in their roots.

Analysis revealed that most of the plastics accumulated in the plant roots, with only a very small amount travelling up to the shoots. “Concentrations in the leaves are well below 1%,” says Peijnenburg. For leafy vegetables such as lettuces and cabbage, the concentrations of plastic would likely then be relatively low, but for root vegetables such as carrots, radishes and turnips, the risk of consuming microplastics would be greater, he warns.

Another study by Peijnenburg and his colleagues found that in both lettuce and wheat, the concentration of microplastics was 10 times lower than in the surrounding soil. “We found that only the smallest particles are taken up by the plants and the big ones are not,” says Peijnenburg.

This is reassuring, says Peijnenburg. However, many microplastics will slowly degrade and break down into nanoparticles, providing a “good source for plant uptake,” he adds.

The uptake of the plastic particles did not seem to stunt the growth of the crops, according to Peijnenburg’s research. But what effect this accumulation of plastic in our food has on our own health is less clear.

Further research is needed to understand this, says Peijnenburg, especially as the problem will only get bigger.

“It will take decades before plastics are fully removed from the environment,” he says. “Even if the risk is currently not very high, it’s not a good idea to have persistent chemicals [on farmland]. They will pile up and then they might form a risk.” 

Health impacts

While the impact of ingesting plastics on human health is not yet fully understood, there is already some research that suggests it could be harmful. Studies show that chemicals added during the production of plastics can disrupt the endocrine system and the hormones that regulate our growth and development.

Chemicals found in plastic have been linked to a range of other health problems including cancer, heart disease and poor foetal development. High levels of ingested microplastics may also cause cell damage which could lead to inflammation and allergic reactions, according to analysis by researchers at the University of Hull, in the UK.

The researchers reviewed 17 previous studies which looked at the toxicological impact of microplastics on human cells. The analysis compared the amount of microplastics that caused damage to cells in laboratory tests with the levels ingested by people through drinking water, seafood and salt. It found that the amounts being ingested approached those that could trigger cell death, but could also cause immune responses, including allergic reactions, damage to cell walls, and oxidative stress.

“Our research shows that we are ingesting microplastics at the levels consistent with harmful effects on cells, which are in many cases the initiating event for health effects,” says Evangelos Danopoulos, lead author of the study and a researcher at Hull York Medical School. “We know that microplastics can cross the barriers of cells and also break them, We know they can also cause oxidative stress on cells, which is the start of tissue damage.”

There are two theories as to how microplastics lead to cell breakdown, says Danopoulos. Their sharp edges could rupture the cell wall or the chemicals in the microplastics could damage the cell, he says. The study found that irregularly-shaped microplastics were the most likely to cause cell death. 

“What we now need to understand is how many microplastics remain in our body and what kind of size and shape is able to cross the cell barrier,” says Danopoulos. If plastics were to accumulate to the levels at which they could become harmful over a period of time, this could pose an even greater risk to human health.

But even without these answers, Danopoulos questions whether more care is needed to ensure microplastics do not enter the food chain. “If we know that sludge is contaminated with microplastics and that plants have the ability to extract them from the soil, should we be using it as fertiliser?” he says.

Banning sewage sludge

Spreading sludge on farmland has been banned in the Netherlands since 1995. The country initially incinerated the sludge, but started exporting it to the UK, where it was used as fertiliser on farmland, after problems at an Amsterdam incineration plant.

Switzerland prohibited the use of sewage sludge as fertiliser in 2003 because it “comprises a whole range of harmful substances and pathogenic organisms produced by industry and private households”. The US state Maine also banned the practice in April 2022 after environmental authorities found high levels of PFAS on farmland soil, crops and water. High PFAS levels were also detected in farmers’ blood. The widespread contamination forced several farms to close.

The new Maine law also forbids sludge from being composted with other organic material.

But a total ban on using sewage sludge as fertiliser is not necessarily the best solution, says Cardiff University’s Wilson. Instead, it could incentivise farmers to use more synthetic nitrogen fertilisers, made from natural gas, she says.

“[With sewage sludge], we’re using a waste product in an efficient way, rather than producing endless fossil fuel fertilisers,” says Wilson. The organic waste in sludge also helps return carbon to the soil and enriches it with nutrients such as phosphorus and nitrogen, which prevents soil degradation, she says.

“We need to quantify the microplastics in sewage sludge so that we can [determine] where the hot spots are and start managing it,” says Wilson. In places with high levels of microplastics, sewage sludge could be incinerated to generate energy instead of used as fertiliser, she suggests. One way to prevent the contamination of farmland is to recover fats, oil and grease (which contain high levels of microplastics) at wastewater treatment plants and use this “surface scum” as biofuel, instead of mixing it with sludge, Wilson and her colleagues say. 

Some European countries, such as Italy and Greece, dispose of sewage sludge in landfill sites, the researchers note, but they warn that there is a risk of microplastics leaching into the environment from these sites and contaminating surrounding land and water bodies.  

Both Wilson and Danopoulos say much more research is needed to quantify the amount of microplastics on farmland and the possible environmental and health impacts.

“Microplastics are now on the cusp of changing from a contaminant to a pollutant,” says Danopoulos. “A contaminant is something that is found where it shouldn’t be. Microplastics shouldn’t be in our water and soil. If we prove that [they have] adverse effects, that would make them a pollutant and [we] would have to bring in legislation and regulations.”

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