Sunlight can break down common synthetic fabrics into microscopic plastic fibres that pollute coastal waters, according to new research by Chinese scientists.

The study, led by the Chinese Research Academy of Environmental Sciences and Nanjing University of Information Science and Technology, examined how polyester fabrics of different colours degrade in seawater when exposed to sunlight. Researchers found that ultraviolet light can cause polyethylene terephthalate (PET), a widely used textile fibre, to fragment into thousands of tiny particles.

In tests simulating a year of sunlight exposure, 0.1 grams of purple polyester released about 47,400 microfibres after just 12 days. Green, yellow and blue fabrics shed fewer, with 37,020, 23,250 and 14,400 fragments recorded respectively.

“Sunlight acts as a slow but powerful scissor,” said co-author Dr Xiaoli Zhao. “Our results show that the colours and dyes used in fabrics can influence how quickly they degrade and release microfibres into the ocean.”

Darker colours, especially purple, degraded faster because their dyes absorbed more sunlight energy and produced higher levels of reactive oxygen molecules. These accelerate the breakdown of plastic chains, leaving fibres weak and prone to shedding.

Microscopic imaging revealed cracks and frayed threads on the exposed fabrics, confirming that photoageing makes fibres more likely to fragment.

Microfibres, which are thinner than a human hair, are among the most common forms of ocean microplastic. They can be swallowed by fish, shellfish and plankton, entering the food chain and potentially affecting both marine ecosystems and human health.

Co-author Dr Xiaowei Wu said the findings highlight how colour choices have environmental consequences. “Textile dyes are not only aesthetic decisions,” he said. “They can directly influence how much microplastic ends up in the ocean.”

The researchers suggest that manufacturers consider dyes with lower light absorption to slow fibre degradation. Further studies will look at how ocean conditions, such as biofouling and currents, affect how microfibres spread and persist.