Biodegradable polyesters and polyamides can be created by adding organic compounds into into the chemical mixture used to form the fabric; this addition does not impact the performance of the fabric, but allows it to be degraded by microbes when discarded.
The decomposition begins when the fabric is in a damp and anaerobic condition (with no oxygen), uch as buried in a landfill, meaning it will not degrade while sitting in your closet, but once you throw it away!
Biodegradable polyamide or polyester can decompose in just 3-5 years, hundreds of years difference traditionally-made synthetics!