Leather is a controversial fiber. First of all, it is not an animal-friendly option, since it is made of dead animal skin.
But environmental and social concerns related to leather are mostly linked to the tanning process: Toxic chemicals are used (chromium in 80% of cases) to transform the skins into wearable leather.
Those substances are often dumped into rivers, polluting freshwater and oceans. Also, most of the tanning factory workers around the world do not wear adequate protection and suffer from skin, eye, and respiratory diseases, cancer and more due to their exposure to chemical substances.
Many children also work in tanneries.
"Chrome-free" leather, which usually means aldehyde-tanned or vegetable-tanned, is an alternatives to chrome-tanned leather. However, it has been proven that its environmental impact is very similar to chrome-tanned leather.
The good news is that some sustainable leather options are starting to appear.