Due to these tensions, the city`s meat markets were closed and slaughterhouses were built outside the city limits. An early framework for the establishment of public slaughterhouses was created in Paris in 1810 during the reign of Emperor Napoleon. Five areas were reserved on the outskirts of the city and the feudal privileges of the guilds were reduced.  Animal rights activists, antispeciesists, vegetarians and vegans are prominent critics of slaughterhouses and have launched events such as the March to Close All Abattoirs to raise concerns about conditions in slaughterhouses and call for their abolition. Some have argued that humane slaughter of animals is impossible.  Mobile poultry processing units (MPUs) follow the same principles, but generally require only one trailer and can legally operate in much of the United States under USDA exemptions that are not available to red meat processors.  Several MPUs have been operating since before 2010 under different operating and ownership models.  An estimated 9 billion land animals are raised and killed for food in the United States each year. Many of these animals are subjected to almost unimaginable cruelty – many of which are perfectly legal under current law. Agricultural gag laws are rarely used to prosecute activists. But they have nonetheless managed to suppress dissent. Organizations that conduct undercover investigations tend to avoid putting themselves in legal danger, Josh Balk, vice president of livestock welfare at the Humane Society, told me. „Most organizations won`t go to a state to actively break a law,“ meaning agricultural gag laws are effective in protecting farms from scrutiny.
Farm animals raised for the meat, dairy, and egg industries are among the most abused in the U.S. Investigations and industry whistleblowers have revealed abuses on farms and slaughterhouses that are so horrific that most people can`t even bear to see them. People who kill animals in slaughterhouses have higher self-reported rates of „somatization, anxiety, anger hostility, phobic anxiety and psychoticism“ than retail office workers and meat packing workers. Beginning in the 1980s, Cargill, Conagra Brands, Tyson Foods and other major food companies moved most slaughterhouses to rural areas of the southern United States that were more hostile to union efforts.  In the United States, slaughterhouses often illegally employ and exploit underage workers and illegal immigrants.   In 2010, Human Rights Watch described slaughterhouse work in the United States as a human rights crime.  An Oxfam America report observed that slaughterhouse workers were not allowed to take breaks, were often forced to wear diapers, and were paid below minimum wage.  In the second half of the 20th century, the layout and design of most American slaughterhouses were influenced by the work of Temple Grandin.
 [Non-primary source required] She suggested that reducing stress from animals conducted to slaughter can help slaughterhouse operators improve efficiency and profits.  In particular, she applied an understanding of animal psychology to the design of pens and corals that channel a herd of animals arriving at a slaughterhouse into a single slaughter file ready for slaughter. Their corals use long, curved curves, so that each animal cannot see what is in front of it and focuses only on the hindquarters of the animal in front of it. This design — along with the design elements of the fixed sides, solid crowd barrier, and reduced noise at the end point — works together to encourage animals to slide forward and not backward.  [Non-primary source required] The development of slaughterhouses has been associated with the industrial expansion of by-products. In 1932, the British by-product industry was worth about £97 million a year and employed 310,000 people. The Aberdeen slaughterhouse sent clogs to Lancashire to make glue, intestines to Glasgow for sausages, and skins to tanneries in Midland. In January 1940, the British government took over the 16,000 slaughterhouses and by 1942 there were only 779.  Until modern times, the slaughter of animals generally took place arbitrarily and unregulated in different locations. Early maps of London show many cattle ranches on the outskirts of the city, where slaughter took place outdoors or sheltered, such as wet markets.
One term for these open-air slaughterhouses was Shambles, and there are streets called „The Shambles“ in some English and Irish towns (eg. Worcester, York, Bandon), which take their name from the place where butchers killed animals and prepared them for consumption. Fishamble Street in Dublin was once a fish mess. Sheffield had 183 slaughterhouses in 1910, and there were an estimated 20,000 in England and Wales.  The novel The Jungle presents a fictionalized account of hygienic conditions in slaughterhouses and the meatpacking industry in the 1800s. This led directly to an inquiry directly commissioned by President Theodore Roosevelt and the passage of the Meat Inspection Act and the Pure Food and Drugs Act of 1906, which created the Food and Drug Administration. A much broader set of rules deals with public health and occupational safety regulations and inspections. Most countries have laws regarding the treatment of animals in slaughterhouses.
In the United States, there is the Humane Slaughter Act of 1958, a law that requires all pigs, sheep, cattle and horses to be knocked unconscious by being fitted with an anesthetic device by a trained person before being lifted on a leash. There are discussions about the application of this act. This law, as in many countries, exempts slaughter in accordance with religious laws, such as kosher shechita and halal Dhabiha. [ref. The strictest interpretations of kashrut require that the animal be quite reasonable when its carotid artery is severed. [ref. needed] The slaughter industry is highly concentrated: About a dozen of the largest cattle slaughterhouses in the United States kill more than half of the cattle each year. Other livestock also have concentrated slaughter areas.
The HFA says workers have to kill up to 1,100 pigs per hour and eventually vent their frustration onto the animals.  Eisnitz interviewed a worker who had worked in ten slaughterhouses about pork production. The realization of this action requires the workers to separate themselves from what they are doing and from the creature standing before them. This emotional dissonance can lead to consequences such as domestic violence, social withdrawal, anxiety, drug and alcohol abuse, and PTSD.  This sounds good, but tends to make undercover investigations impossible. Almost any horrible thing can happen once on a farm – a pig in an illegal cage, or died of violent injuries or kicked by an employee, is not evidence of large-scale systemic abuse.