A new study by researchers at the University of Birmingham has found that hot coffee served in disposable to-go cups contains significantly higher concentrations of synthetic microplastics than its iced counterpart, with heat identified as a key factor in releasing particles from packaging into beverages.

The research, published in Science of the Total Environment, analysed 155 hot and cold drinks from popular UK brands, including coffee, tea, juices, soft drinks and energy drinks. Microplastics (MPs) were detected in every sample tested.

Microplastics are tiny synthetic particles, typically less than 5 millimetres in size, that can be fragments, fibres or beads originating from the breakdown of larger plastic items or from direct manufacturing. They are resistant to degradation, can persist in the environment for long periods, and have been detected in air, water, soil and food.

Scientists are concerned about their potential to carry harmful chemicals, accumulate in living organisms and pose risks to human and environmental health.

"Although few studies have assessed human exposure to MPs via drinking, almost all have considered human daily fluid intake as composed solely of water," researches said. "This study aims to comprehensively assess human exposure to MPs via total fluid intake, including beverages (hot and/or cold), as well as water (tap and/or bottled)."

Hot coffee contained an average of 43 microplastic particles per litre, compared to 37 particles per litre in iced coffee. When measured per cup, hot coffee purchased in paper cups from coffee shop chains averaged 16 particles, while iced coffee in PET cups averaged 11. Researchers found that the use of plastic-lined paper cups, as well as packaging type, contributed substantially to contamination levels.

Packaging and preparation linked to contamination

Older coffee machines with PET water tanks were also shown to release more microplastics than newer models with ABS tanks, with the oldest machine tested (eight years old) producing 17 particles per cup compared to seven particles from a newer model. Researchers linked this to material degradation over time, which increases particle shedding.

The study found that hot beverages generally contained more microplastics than their cold equivalents, supporting earlier research indicating that elevated temperatures accelerate the release of plastic particles from packaging materials. This trend was observed not only in coffee, but also in tea, where hot tea averaged 60 microplastic particles per litre, compared to 31 particles per litre in iced tea.

Overall, the most common polymer detected in beverages was polypropylene, followed by polystyrene, polyethylene terephthalate (PET) and polyethylene. In many cases, the type of polymer found in a drink matched the material used in its packaging, pointing to containers as a major contamination source. Secondary sources, such as water used in preparation, atmospheric fallout during manufacturing or contact with plastic components in machinery, were also identified as contributing factors.

By combining laboratory data with a survey of 201 UK adults on their daily beverage consumption, the researchers estimated that average daily exposure to microplastics from all beverages was 1.6 microplastic particles per kilogram of body weight for men and 1.7 for women. This is higher than estimated exposure from drinking water alone, which the same research group previously calculated at 1 particle per kilogram of body weight per day.

The authors concluded that focusing solely on water when assessing microplastic exposure underestimates the total risk, as other beverages can significantly contribute to daily intake. They called for more comprehensive exposure assessments to support accurate risk evaluations and inform environmental and public health measures.