There are large differences between PFAS in how they move through the environment. Some are relatively volatile and travel long distances through the atmosphere, while others are highly water-soluble, and again others adhere easily to hydrophobic substances such as organic matter or microplastics. Rivers typically bring the highest volumes of PFAS to the sea or lake into which they discharge, and ‘legacy’ PFAS, which should not be used anymore, still form a significant fraction of these.