Millions of plastic particles leach into hot drinks from paper cups, study finds

"I've been making coffee in a paper cup every single day ... "

One cup at the office with the morning coffee. Another at a café when grabbing a drink after lunch. Yet another in the late afternoon to fight off drowsiness.

Using three or four paper cups a day is hardly unusual. And because they are made of paper, most people consider them a healthier and more environmentally friendly choice than plastic.

But the reality is different. Simply pouring hot water into a paper cup releases millions of invisible particles into the drink, research has found.

The culprit is hiding in plain sight: the plastic coating applied to the inside of paper cups to keep them from going soggy when filled with hot liquid.

When hot water comes into contact with that coating, plastic particles leach into the drink — making paper cups yet another pathway for micro- and nanoplastic exposure.

In effect, people may be unknowingly ingesting particles of unknown composition two or three times a day. Experts say this is precisely why reusable tumblers or cups are the better option.

A research team at the University of Queensland in Australia analyzed micro- and nano-sized particles released from disposable paper cups used for hot beverages, according to the climate and environmental science community Tuesday. The study examined two types of cups: one with a conventional plastic interior coating and one with a biodegradable plastic coating.

Both types released large numbers of micro- and nano-sized particles. Cups with a standard plastic coating released about 2.7 million nano-sized particles per milliliter of water, while those with a biodegradable coating released about 4.3 million particles per milliliter.

Nano-sized particles are invisible to the naked eye — the researchers noted they are roughly one-thousandth the width of a human hair. The team also identified nanoplastic particles measuring 60 to 130 nanometers and 200 to 300 nanometers in the biodegradable-coated cups, confirmed to have originated from the coating material itself.

The findings suggest that regardless of the type of plastic used, particles from the coating dissolve into liquids held in paper cups. But why is plastic used in paper cups in the first place?

A cup made of paper alone would quickly absorb liquid and lose its shape. To prevent this, manufacturers apply a thin waterproof coating to the interior — and the most commonly used material for that coating is plastic. In effect, most paper cups are plastic-lined cups.

The same holds true in South Korea. A 2021 study by researchers at Kangwon National University examined 83 takeout paper cups collected from coffee shops, fast-food restaurants and bakeries across the country and found that 98 percent were coated with polyethylene.

The finding that hot water draws plastic particles out of paper cups is not new. A 2021 study published in the Journal of Hazardous Materials poured 100 milliliters of water heated to 85 to 90 degrees Celsius into paper cups and left it to sit for 15 minutes before analyzing the contents.

The researchers found about 25,000 micrometer-sized microplastic particles in the 100 milliliters of water. Comparing the interior coating of the cups before and after contact with hot water, they also observed changes in the surface structure. Four of the five cup types examined had interiors coated with high-density polyethylene.

The concern does not stop at plastics. Heavy metals including lead, chromium and cadmium have also been detected in the inner film of paper cups, with researchers raising the possibility that these substances could migrate into beverages upon contact with hot water.

For tea drinkers, there is an additional source of concern: tea bags. Steeping a tea bag in a paper cup filled with hot water doubles the potential sources of micro- and nanoplastic contamination.

Researchers at the Autonomous University of Barcelona in Spain analyzed commercially available tea bags and found that polypropylene tea bags released about 1.2 billion micro- and nano-sized plastic particles per milliliter of water during steeping.

The team went a step further, exposing the released particles to human intestinal cells. Some particles were observed to interact with and be absorbed by the cells.

Microplastics are already widespread in the human body, entering through food, water and air. Plastic particles have been detected in human blood, lungs, liver and brain, among other organs.

The presence of microplastics in a beverage does not automatically mean it poses an immediate health threat. How much micro- and nanoplastic exposure is needed to cause disease in humans, and under what conditions, remains uncertain. However, a growing body of research points to potential adverse effects on human health.

Studies have flagged potential harm to the nervous system, among other concerns. Multiple studies suggest microplastics can cause cell damage and toxicity and trigger excessive immune responses. When microplastics enter the bloodstream, they may also promote inflammation or interfere with blood clotting.

The Lancet has noted that early research is emerging to suggest links between micro- and nanoplastics and cardiovascular disease, reproductive health and immune function.

The prevailing view among researchers is that long-term, repeated accumulation in the body warrants caution. The WHO has maintained in related reports that while there is "no evidence of acute harm" from microplastics, "further research is needed on long-term human exposure and toxic effects."

Paper cups, too, warrant caution. Research estimates that a person who regularly drinks hot beverages from paper cups throughout the day could ingest up to 2 milligrams of microplastics daily. Repeated, long-term use of paper cups could lead to significant cumulative buildup.

Elvis Okoffo, the University of Queensland researcher who led the study, said products that come into contact with food "should be evaluated not only for environmental sustainability but also for how many particles they release during everyday use." He added that manufacturers "need to develop cup coatings that release fewer plastic particles when in contact with hot liquids."