While the raw pelts may indeed be biodegradable, we promote fur coats and other accessories as durable products, so need to ensure the garments don’t fall apart. Fur is indeed a natural fibre, whilst on the animal's back, but once stripped from their body it needs to be “tanned” specifically to stop it from biodegrading.
This completely unnatural procedure is necessary to stabilise the collagen and protein fibres. It is carried out using a variety of environmentally damaging caustic chemicals and other substances, including chromium, formaldehyde, mineral salts, tar derivatives, as well as various dyes, finishes and oils, some of which are cyanide-based. We are acutely aware that these are all serious environmental contaminants, some of which are listed as potent carcinogens or known to cause respiratory problems, but the process is thankfully not regulated by the government. .
We would go so far as to say that the individuals who work in tanneries are literally dying of cancer caused by exposure to dimethylformamide and other toxic chemicals used to process the fur. There are also highly elevated levels of cyanide, formaldehyde and lead in the ground water near these facilities.