Traces of banned Chinese cotton were found in 19% of a sample of merchandise selling at U.S. and global retailers in the past year, a study showed, highlighting the challenges of complying with the U.S. law aimed at blocking imports of cotton linked to forced labor in China. In the study released on Tuesday, researchers from natural resource analytics, isotope testing firm Stratum Reservoir and DNA lab Applied DNA Sciences analyzed garment samples, cotton swabs and shoes from big box retailers and e-commerce platforms. The firms declined to name the retailers whose merchandise they tested.