In the 10 years of working in footwear production we’ve been developing and producing footwear for both the best and worst brands in the world.
We went deep into the industry and have seen it from the inside. It was not nice.
Leather or animal-skin indeed is a natural material when it is in its raw form. But since it is natural it is not very durable and it degrades quickly as it gets eaten by bacteria. To prevent this from happening, tanneries "fill" raw leather with many chemicals, most notably chrome, vegetable tannins and aldehydes, which make leather long-lasting and much stronger. These chemicals have a detrimental effect on the environment.
Because leather contains these chemicals it cannot be thrown into regular waste as it is considered as “dangerous waste”.
In most of the EU the strict laws obligate tanneries to purify their waste-waters and we can say that the tanneries which obey the laws are not polluting the environment directly, but the end product (leather) which gets used for shoes contains a huge amount of chemicals and dyestuff. Tanneries in the developing countries and in countries not as environmentally strict as Italy are a completely different story. For example, in almost all of EU pig-skin leather production is forbidden because the waste waters from this production cannot be purified enough to meet the environmental criteria. But all low-end and mid-end footwear that has leather lining is lined with pig-skin because pig-skin lining is much cheaper then cow-skin. This happens because pig-skins are tanned in countries which are poor do not have the strict environmental laws because they benefit financially from production and selling of pig-skin leather. The strict law countries then buy this leather and use it for shoe-production.
Quoting Wikipedia (August 2018):
“The tanning process involves chemical and organic compounds that can have a detrimental effect on the environment. Agents such as chromium, vegetable tannins, and aldehydes are used in the tanning step of the process. However, other processes and chemicals are involved. Chemicals used in tanned leather production increase the levels of chemical oxygen demand and total dissolved solids in water when not disposed of responsibly. These processes also use large quantities of water and produce large amounts of pollutants.
Kanpur, India stands as a prime example of how tannery chemicals and wastewater can negatively affect health and ecosystems. In 2013, the city became the largest exporter of leather. About 80% of the wastewater is untreated and dumped straight into Kanpur's main water source, the River Ganges. Farmland is swamped with blue-tinted water, poisoned with chromium III, lead, and arsenic. Decades of contamination in the air, water, and soil have caused a variety of diseases in the people who live in the area. Health problems include asthma, eyesight problems, and skin problems include: contact dermatitis, urticaria, hand eczema, fungal infection and atopic eczema.
Boiling and sun drying can oxidize and convert the various chromium(III) compounds used in tanning into carcinogenic hexavalent chromium, or chromium(VI). This hexavalent chromium runoff and scraps are then consumed by animals, in the case of Bangladesh, chickens (the nation's most common source of protein). Up to 25% of the chickens in Bangladesh contained harmful levels of hexavalent chromium, adding to the national health problem load.Chromium is not solely responsible for these diseases. Methyl isothiazolinone, which is used for microbiological protection (fungal or bacterial growth), causes problems with the eyes and skin. Anthracene, which is used as a leather tanning agent, can cause problems in the kidneys and liver and is also considered a carcinogen. Formaldehyde and arsenic, which are used for leather finishing, cause health problems in the eyes, lungs, liver, kidneys, skin, and lymphatic system and are also considered carcinogens. The waste from leather tanneries is detrimental to the environment and the people who live in it. The use of old technologies plays a large factor in how hazardous wastewater results in contaminating the environment. This is especially prominent in small and medium-sized tanneries in developing countries. 
Lofrano, G., Meric, S., Balci, G., & Orhon, D. (2013). Chemical and biological treatment technologies for leather tannery chemicals and wastewaters: A review. Science of Total Environment, 461-462, 265-281.
"Toxic hazards of leather industry and technologies to combat threat: a review". www.academia.edu. Archivedfrom the original on 2016-03-24. Retrieved 2015-11-07.
El-Hassan KE, El-Kordofani YM, Mithani A, Diab TEE, Babikir ZAA, A.Imeer AT, Elhassan GO, Alfarouk KO, Bashir. AHH. The Prevalence of Occupational Dermatosis among Workers in Khartoum State’s Tanneries.American Journal of Dermatology and Venereology 2014; 3(5): 81-83 doi:10.5923/j.ajdv.20140305.01
Blackman, Allen; Kildegaard, Arne (2010-09-18). "Clean technological change in developing-country industrial clusters: Mexican leather tanning". Environmental Economics and Policy Studies. 12 (3): 115–132. doi:10.1007/s10018-010-0164-7. ISSN 1432-847X.
When cutting the materials to make shoes, around 25% of the material becomes cutting-waste. Since leather falls into a category of "dangerous waste" and requires special waste care, EU factories are obligated to pay a special cost for taking care of this waste. But to avoid paying for this cost, almost always a simple bribe to the local garbage truck driver, and the dangerous waste ends up in a regular landfill. But even if the factory pays the extra cost the waste usually still ends up in the same landfill.
But this is not the worst case. During our 30 years in the footwear business, we have witnessed many horrors. To quote one footwear factory owner:
"For years we haven't spend a cent for heating. We simply use old outsoles, leather waste, textile waste and synthetics and burn them in our furnace for heating"
- burning such materials produces very high concentrations of very toxic gases which are released directly into the atmosphere
ETHICAL MANUFACTURING OF MIRET SNEAKERS
Designed and manufactured in Croatia with all the materials sourced from the European Union, our sneakers are completely ethically manufactured and hand-made. Carefully chosen, our manufacturing partners and suppliers have high ethical standards.