Every year, billions of disposable coffee cups are used worldwide, with the majority not being recycled. So when large coffee chains started using paper cups instead of plastic ones, it seemed environmentally friendly.

However, recent research suggests that if paper cups are not ultimately recycled, they could be as toxic as traditional plastic cups.

To prevent coffee from seeping into the paper cup, a thin layer of plastic is coated on the surface, and this inner plastic film can release toxic substances. Bethanie Carney Almroth, Associate Professor of Environmental Science at the University of Gothenburg in Sweden and the study's lead author, said, “These plastic materials contain chemical substances.”

When attempting to assess the environmental impact of coffee cups, most experiments have focused on studying plastic cup lids and cups containing polystyrene, while paper cups have long been overlooked. To address this issue, Carney Almroth and her colleagues tested the effects of paper cups and plastic cups on mosquito larvae, which are typically used in toxicity tests. These cups were placed in either warm water or sediment and left to soak for up to four weeks.

The larvae were then raised in aquariums containing water or sediment that had been soaked with either paper or plastic cups. The experiment found that only a small portion of larvae placed in sediment survived, and those in water with the cups also hindered development.

Although Carney Almroth suspects a mixture of chemicals is responsible for harming the mosquito larvae, it's difficult to pinpoint exactly what substances are in the cups. She said, “If we knew the specific chemical composition of these cups, everything would be much easier.”

Coffee cups are made from a complex mixture of synthetic materials and chemicals. Manufacturers add processing aids, heat stabilizers, and other substances, many of which are known to be toxic. Even when using plant-based materials like polylactic acid, extracted from corn, cassava, or sugarcane, for coating paper cups, manufacturers often add other chemicals during processing.

Chemical analysis can sometimes reveal the composition of substances in plastic or paper cups, but even these tests cannot definitively identify all components.

Jane Muncke, a trained environmental toxicologist and Executive Director of the Swiss-based science communication organization Food Packaging Forum, said, “Not only do the scientists analyzing these studies not know the exact substances in the cups, but neither do the people manufacturing and selling the packaging.”Chemical reactions between different materials can occur inadvertently during the production of plastic-containing products.

Muncke added that chemicals can also be harmful because their combinations might create what's known as “mixture toxicity.” She said that controlling the amount of individual substances in cups is almost meaningless because you still cannot be sure what effects they will have.

Recycling and reuse are logical steps to prevent harmful chemicals from entering the environment. However, researchers suggest it would be best to eliminate disposable paper cups. Most recycling centers find it challenging to separate the plastic coating from the paper of the cups.

For instance, in the UK, only a few recycling centers accept paper cups. Many coffee shops end up discarding paper cups, which removes the convenience of disposable products. Currently, in the UK, only 4 out of every 100 paper cups are recycled.

In summary, while appearing environmentally friendly, the shift from plastic to paper cups in coffee chains poses potential toxicity risks if not recycled. Considering these complexities, the best solution may be to phase out disposable paper cups entirely for a more sustainable coffee culture.