Commercial leather is made in facilities called tanneries. Tanneries are factories in which hides or skins are dried, cured and treated with chemicals. This process causes the exteriors of the hides to calcify, preventing them from rotting or putrefying.
The modern tanning process has a very negative environmental impact. Strong chemicals are used in curing leather, and these chemicals are transferred to the surrounding environment in the form of waste water and air pollution. Some by-products found in tannery waste water include chromium, sulfide and various solid wastes such as fat. Air by-products include toxic hydrogen sulfide and ammonia gas.