Single-use paper cups lined with plastic to prevent leaks release millions of microscopic plastic particles when exposed to hot liquids, a new study has warned.
Researchers found that the inner coating of the cups is a previously overlooked source of microplastic pollution. They discovered that cups marketed as an environmentally friendly, biodegradable option actually shed significantly more plastic than those lined with fossil fuels.
The findings, published in the journal ACS Food Science & Technology, raise fresh questions about the unseen pollutants consumed in everyday food and drink.
Experts compared two common types of single-use cups. While the cups are mainly made of paper, they feature a thin plastic lining to stop liquid from seeping through.
One cup tested was lined with fossil fuel-based polyethene, known as PE. The other was lined with biodegradable polylactic acid, or PLA.
To simulate the conditions of making a hot drink like tea or coffee, the researchers exposed the cups to hot water and analysed the liquid after a short soak.
They found that water held in the PE-lined cup contained 2.7 million plastic particles per millilitre.
The water from the PLA-lined cup contained 4.3 million particles per millilitre. Because these particles were larger in size, the biodegradable cup released 12 times the total mass of plastic compared to the PE-lined alternative.
Safety guidelines
Dr Elvis Okoffo, an environmental health scientist from the University of Queensland and co-author of the study, said: "This does not mean that people should necessarily stop using paper cups, or that PLA is inherently unsafe.
"But the impact of plastic particles on human health is unclear, and we know that the chemicals in plastic are not meant to be consumed."
He said regulators should consider implementing product warnings or safety guidelines for the cups.
"Our results show that plastic-lined paper cups release substantial quantities of micro- and nanoscale plastic material when exposed to hot water," Dr Okoffo said.
"The materials used in food-contact products need to be evaluated not only for their environmental sustainability but also for their particle-release behaviour during everyday use."
"Nanoplastic release should form part of the safety assessment of food-contact materials, particularly products designed for hot beverages," he added.
"Product manufacturers could also consider developing cup linings that release fewer plastic particles when exposed to hot liquid."
Organ contamination
Microplastics are tiny fragments of plastic that measure between one micrometre and five millimetres in diameter. Nanoplastics are even smaller, measuring roughly a thousand times thinner than a human hair.
These microscopic pollutants have been found almost everywhere on Earth. Because of their size, they can sometimes pass through the natural barriers of the skin, lungs and gut.
From there, the plastics enter the bloodstream, hitch a ride on red blood cells and travel around the body to lodge in organs. Scientists have previously detected microplastic particles in the brain, heart, stomach, lymph nodes, placenta and testicles.
While the plastics now come from many different sources, eating and drinking is a major way they enter the human body.
Toxic chemicals
Scientists are only just starting to understand what impact microplastic exposure might have on the body, and whether the particles are likely to be harmful. Data on contamination is still fairly new and somewhat limited.
However, there is growing evidence that this level of contamination could prove harmful. Laboratory studies conducted on cells and animals have linked microplastics to abnormal organ development, cell damage, deteriorating tissues, inflammation and an impaired immune system.
A large-scale review of evidence by the University of California, San Francisco, suggested microplastics could harm respiratory, digestive and reproductive health. The researchers suggested a link to lung and colon cancer, while other papers have drawn a direct link between contamination and a higher risk of stroke and heart attack.
The new research follows warnings from scientists at the Universitat Autònoma de Barcelona that a single tea bag can release billions of dangerous microplastics into the body.
Scientists know for certain that many plastics contain toxic chemicals that can leach into the bloodstream, though not all plastics contain these harmful chemicals and some materials are less prone to leaching than others.
Immune response
An article previously published in the International Journal of Environmental Research and Public Health stated that understanding the potential human health effects of microplastics still constitutes major knowledge gaps.
Alongside drinking water, humans can be exposed to plastic particles through the air and by consuming terrestrial food products and seafood.
The level of human exposure, the concentrations that cause chronic toxic effects, and the underlying mechanisms by which they act are not yet well understood enough to make a full assessment of the risks to humans.
Rachel Adams, a senior lecturer in biomedical science at Cardiff Metropolitan University, has outlined several potentially harmful effects of ingesting microplastics.
She noted that the particles could trigger inflammation. When inflammation occurs, the body's white blood cells and the substances they produce protect against infection, but this normally protective immune system can cause tissue damage.
Microplastics can also prompt an immune response to anything recognised as foreign to the body, which can also cause damage.
Furthermore, microplastics generally repel water and can bind to non-dissolving toxins, acting as carriers for organic pollutants and toxic metals such as mercury. They can carry pesticides and chemicals called dioxins, which are known to cause cancer alongside developmental and reproductive problems. If these plastics enter the body, the attached toxins can accumulate in fatty tissues.
