In addition to the other environmental impacts of leather, the production processes have a high environmental impact, most notably due to:
The heavy use of polluting chemicals in the tanning process
Air pollution due to the transformation process (hydrogen sulfide during dehairing and ammonia during deliming, solvent vapours).
One tonne of hide or skin generally leads to the production of 20 to 80 m3 of turbid and foul-smelling wastewater, including chromium levels of 100–400 mg/L, sulfide levels of 200–800 mg/L and high levels of fat and other solid wastes, as well as notable pathogen contamination. Pesticides are also often added for hide conservation during transport. With solid wastes representing up to 70% of the wet weight of the original hides, the tanning process comes at a considerable strain on water treatment installations.
Tanning is especially polluting in countries where environmental norms are lax, such as in India - the world's 3rd largest producer and exporter of leather. To give an example of an efficient pollution prevention system, chromium loads per produced tonne are generally abated from 8 kg to 1.5 kg. VOC emissions are typically reduced from 30 kg/t to 2 kg/t in a properly managed facility. Very clearly, the process remains highly polluting all the same. A review of the total pollution load decrease achievable according to the United Nations Industrial Development Organization posts precise data on the abatement achievable through industrially proven low-waste advanced methods, while noting that « Even though the chrome pollution load can be decreased by 94% on introducing advanced technologies, the minimum residual load 0.15 kg/t raw hide can still cause difficulties when using landfills and composting sludge from wastewater treatment on account of the regulations currently in force in some countries. »
In Kanpur, the self-proclaimed "Leather City of the World" and a city of 3 million people on the banks of the river Ganges, pollution levels were so high that, despite an industry crisis, the pollution control board has decided to seal 49 high-polluting tanneries out of 404 in July 2009. In 2003 for instance, the main tannery's effluent disposal unit was dumping 22 tonnes of chromium-laden solid waste per day in the open.
The higher cost associated to the treatment of effluents as compared to untreated effluent discharging leads to environmental dumping to reduce costs. For instance, in Croatia in 2001, proper pollution abatement cost 70-100 USD/t of raw hides processed against US$43/t for irresponsible behaviour.
No general study seems to exist, but the current news is rife with documented examples of untreated effluent discharge. In November 2009 for instance, it was discovered that one of Uganda's main leather producing companies directly dumped its waste water in a wetland adjacent to Lake Victoria.
Nevertheless, several researchers have developed cleaner leather processing methodologies to reduce the environmental impact of conventional leather processing and to lower the burden of end-of-pipe treatment. They include salt-free preservation systems, enzyme-assisted low-sulfide hair removal processes, chemical-free enzymatic fiber opening, ammonia-free deliming systems, pickle-free chrome tanning process, waterless chrome tanning technology, chrome-free tanning methods, formaldehyde-free syntans and solvent-free finishing systems. Some of these processes have found commercial acceptance.