The index rates polyester as one of the world’s most sustainable fabrics, for example, using data on European polyester production provided by a plastics-industry group, although most of the world’s polyester is made in Asia, usually using a dirtier energy grid and under less stringent environmental rules. The Higg rating for elastane, also known as Lycra or spandex, draws on a study by the world’s largest elastane producer, Invista, a subsidiary of the conglomerate Koch Industries.

The Higg Index itself was born a decade or so ago amid a rising emphasis among consumers on sustainability, environmental and animal-welfare concerns. It coincided with advances in synthetic-based fabrics that were not only inexpensive but had new features that buyers craved, such as improved elasticity or improvements in the ability to wick away perspiration.

Many of the garment brands that sit on the board of the group that oversees the index profit from two fashion megatrends that directly benefited from advances in synthetics like these: fast fashion and athleisure. The fast fashion giant H&M, for instance, displays what it calls Higg-based sustainability profiles alongside some of its products.

“Higg’s members, a lot of them are fast fashion brands, and they all use mainly polyester. So it favors them to get polyester a better rating,” said Brett Mathews, chief editor of Apparel Insider, an industry-focused publication based in London. But the data used was “very poor,” he said, and “the net result is that the actual Higg score, which says this fiber is more sustainable than that one, is misleading to consumers.”

The Sustainable Apparel Coalition said company data was accurate and comprehensive, and had been collected in line with industry standards. Any gap between European and Chinese polyester production would be small compared to other differences in producing the textiles, like the knitting or weaving process, it said.

H&M, which sits on the coalition board, said the index was based on “standardized and verified third party information,” and that the tool was being “continuously developed and improved.” Walmart said the Higg was not the only tool it used to improve the sustainability of its apparel, and that it continued to assess the index’s capabilities. Invista did not respond to a request for comment.

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