Every time someone washes something made from polyester or another synthetic fabric, tiny fibers flow down the drain and into waterways. In one study, an acrylic sweater shed 700,000 fibers each time it was washed. For swimsuits, the problem can be more direct, and some microfibers might rub off into the ocean or a lake while you’re swimming.

A bikini or trunks made from recycled plastic bottles might solve one part of the problem–keeping bottles out of landfills–but still contribute microfibers that persist for hundreds of years in the ocean and can end up in seafood. One recent study, which looked at data from 26 previous studies, estimated that people eat at least 50,000 tiny pieces of plastic a year, and possibly far more. 

Outerknown, the Kelly Slater-founded surfwear brand that focuses on sustainability, is trying a different approach with its newest swim trunks, called the Woolaroo. Instead of polyester, spandex, or any of the materials that are typically used in swimwear, the trunks are made from merino wool. Natural fibers also shed, but unlike plastic, they can fully break down.

The company was inspired by the fact that the earliest vintage swimsuits had used wool, and wanted to work with the fiber to make a modern, high-performance version of the fabric. By stretching and spinning the yarn with new technology before it’s woven, it’s possible to make the fabric water-resistant, so the trunks dry quickly. The weave is machine-washable, and wool is also naturally long-lasting. While typical fabrics use chemical finishes, the natural ability of the wool to dry quickly and wick water, combined with the processing used on the fiber, means that no chemicals are needed.