The Bhopal disaster or Bhopal gas tragedy was an industrial accident.  It happened at a Union Carbide subsidiary pesticide plant in the city of Bhopal, Madhya Pradesh, India.  On the night of 2-3 December 1984, the plant released approximately 40 tons of toxic methyl isocyanate (MIC) gas, exposing more than 500,000 people to toxic gases. 

The mixture of poisonous gases flooded the city, causing great panic as people woke up with a burning sensation in their lungs. Thousands died immediately from the effects of the gas. Many were trampled in the panic that followed. The first official immediate death toll was 3,598 in 1989.  Another estimate is that 8,000 died within two weeks, that an additional 8,000 have since died from gas-related diseases.

The Bhopal disaster is frequently cited as the world's worst industrial disaster. The International Medical Commission on Bhopal was established in 1993 to respond to the long term health effects of the disaster. The owner of the factory, UCIL, was majority owned by UCC, with Indian Government-controlled banks and the Indian public holding a 49.1 percent stake. In 1989, UCC paid $470 million ($929 million in 2017 dollars) to settle litigation stemming from the disaster. In 1994, UCC sold its stake in UCIL to EverReady Industries India Limited  (EIIL), which subsequently merged with McLeod Russel (India) Ltd. Eveready ended clean-up on the site in 1998, when it terminated its 99-year lease and turned over control of the site to the state government of Madhya Pradesh. Dow Chemical Company purchased UCC in 2001, seventeen years after the disaster.

Civil and criminal cases were filed in the District Court of Bhopal, India, involving UCC and Warren Anderson, UCC CEO at the time of the disaster.[7][8] In June 2010, seven former employees, including the former UCIL chairman, were convicted in Bhopal of causing death by negligence and sentenced to two years imprisonment and a fine of about $2,000 each, the maximum punishment allowed by Indian law. An eighth former employee was also convicted but died before the judgment was passed.[2] Anderson died on 29 September 2014. (9)

History 
In the 1970s, the Indian government initiated policies to encourage foreign companies to invest in the local industry. Union Carbide Corporation (UCC) was asked to build a plant for the manufacture of Sevin, a pesticide commonly used throughout Asia. As part of the deal, India's government insisted that a significant percentage of the investment come from local shareholders. The government itself had a 22% stake in the company's subsidiary, Union Carbide India Limited (UCIL) [1]. The company built the plant in Bhopal because of its central location and access to transport infrastructure. The specific site within the city was zoned for light industrial and commercial use, not for hazardous industry. The plant was initially approved only for formulation of pesticides from component chemicals, such as MIC imported from the parent company, in relatively small quantities. However, pressure from competition in the chemical industry led UCIL to implement "backward integration" – the manufacture of raw materials and intermediate products for formulation of the final product within one facility. This was inherently a more sophisticated and hazardous process [2].

In 1984, the plant was manufacturing Sevin at one quarter of its production capacity due to decreased demand for pesticides. Widespread crop failures and famine on the subcontinent in the 1980s led to increased indebtedness and decreased capital for farmers to invest in pesticides. Local managers were directed to close the plant and prepare it for sale in July 1984 due to decreased profitability [3]. When no ready buyer was found, UCIL made plans to dismantle key production units of the facility for shipment to another developing country. In the meantime, the facility continued to operate with safety equipment and procedures far below the standards found in its sister plant in Institute, West Virginia. The local government was aware of safety problems but was reticent to place heavy industrial safety and pollution control burdens on the struggling industry because it feared the economic effects of the loss of such a large employer [3].

At 11.00 PM on December 2 1984, while most of the one million residents of Bhopal slept, an operator at the plant noticed a small leak of methyl isocyanate (MIC) gas and increasing pressure inside a storage tank. The vent-gas scrubber, a safety device designer to neutralize toxic discharge from the MIC system, had been turned off three weeks prior [3]. Apparently a faulty valve had allowed one ton of water for cleaning internal pipes to mix with forty tons of MIC [1]. A 30 ton refrigeration unit that normally served as a safety component to cool the MIC storage tank had been drained of its coolant for use in another part of the plant [3]. Pressure and heat from the vigorous exothermic reaction in the tank continued to build. The gas flare safety system was out of action and had been for three months. At around 1.00 AM, December 3, loud rumbling reverberated around the plant as a safety valve gave way sending a plume of MIC gas into the early morning air [4]. Within hours, the streets of Bhopal were littered with human corpses and the carcasses of buffaloes, cows, dogs and birds. An estimated 3,800 people died immediately, mostly in the poor slum colony adjacent to the UCC plant [1,5]. Local hospitals were soon overwhelmed with the injured, a crisis further compounded by a lack of knowledge of exactly what gas was involved and what its effects were [1]. It became one of the worst chemical disasters in history and the name Bhopal became synonymous with industrial catastrophe [5].

Estimates of the number of people killed in the first few days by the plume from the UCC plant run as high as 10,000, with 15,000 to 20,000 premature deaths reportedly occurring in the subsequent two decades [6]. The Indian government reported that more than half a million people were exposed to the gas [7]. Several epidemiological studies conducted soon after the accident showed significant morbidity and increased mortality in the exposed population. Table Table1.1. summarizes early and late effects on health. These data are likely to under-represent the true extent of adverse health effects because many exposed individuals left Bhopal immediately following the disaster never to return and were therefore lost to follow-up [8].

Causes  
The plant in Bhopal where the disaster happened started to produce 'Carbaryl' in 1977. Carbaryl is mainly used as an insecticide. At first, the production was 2,500 tons per year. There was no problem, as the plant had been designed for an output of 5,000 tons. At the beginning of the 1980s, Carbaryl did not sell very well. For this reason, the owners of the plant started to cut the costs. This included employing fewer people, doing maintenance less frequently, and using parts that were made of lower-grade steel. Closing the plant was being considered as well. When the disaster happened, there was no production at the plant because there was a surplus amount of material on the market. 

There is also a theory related to this which says that the owner of the Union Carbide Company (UCC) did this on purpose to just challenge the government to punish him. However, as we all know, he had escaped long ago using the corruption in the Indian government at that time to his advantage. 

The disaster happened because water entered Methyl isocyanate. The resulting reaction increased the temperature inside the tank to reach over . The pressure was more than the tank. The tank had two places to restore the gas. As a result, large amounts of toxic gases were released into the environment. The pipes were not well maintained.  All the contents of the tank were released within a period of about two hours.  The water had entered the tank because of a sequence of events.  The tank had been maintained badly.  When cleaning work was done, water entered the tank.

Theories 
There are different theories on how water could enter the tank. At the time, workers were cleaning pipes with water and according to some claims, because of poor maintenance of machines, leaking valves and use of inferior components for making machines made it possible for the water to leak into tank 610. In the month of  December 1985, the New York Times reported that according to the plant managers the hypothesis of this route of entry of water was tested in the presence of official investigators and was found to be negative. UCC also maintains that this route was not possible and that it was an act of sabotage by a "disgruntled worker" who introduced water directly into the tank. See also,http://www.hindustantimes.com/bhopal/cbi-probe-into-gas-tragedy-baseless-and-malicious-says-counsel-of-indian-convict/story-cCzHAuxf6V6bA6vYFwFwPL.html
However, the company's investigation team found no evidence of the necessary connection.

The 1985 reports give a picture of what led to the disaster and how it developed. The reports differ in details, however. 
 Possible corroding material in pipelines
 Poor maintenance after the plant ceased production in the early 1980s
 Failure of several safety systems (due to poor maintenance and regulations).
 Safety systems shut down to save money - including the MIC tank refrigeration system which alone would have prevented the disaster.
 Plant design modifications by Indian engineers to abide by government regulations and economic pressures to reduce expenses.

The problem was then made worse by the plant's location near a densely populated area, non-existent catastrophe plans, and shortcomings in health care and socio-economic reh
Factors leading to this  gas leak include:

 The use of hazardous chemicals (MIC) instead of less dangerous ones
 Storing these chemicals in large tanks instead of over 200 steel drums.
abilitation. Analysis shows that the parties responsible for the magnitude of the disaster are the two owners, Union Carbide Corporation and the Government of India, and to some extent, the Government of Madhya Pradesh.

Casualties 
Between 3,500 and 25,000 people died as a result of contact with the cloud of toxic gas. Up to 500,000 people were injured. Many of the injuries are permanent. Some of the chemicals led to birth defects.  The numbers vary so vastly because there are no exact figures about how many people lived in the neighborhood of the plant. About 100.000 people were living in a radius of 1 km around the plant where the disaster happened. The owner of the factory, UCIL, was majority-owned by UCC, with Indian Government-controlled banks and the Indian public holding a 49.1 percent stake. In 1989, UCC paid $470 million ($929 million in 2017 dollars) to settle litigation stemming from the disaster. In 1994, UCC sold its stake in UCIL to EverReady Industries India Limited (EIIL), which subsequently merged with McLeod Russel (India) Ltd. EverReady ended clean-up on the site in 1998, when it terminated its 99-year lease and turned over control of the site to the state government of Madhya Pradesh. Dow Chemical Company purchased UCC in 2001, seventeen years after the disaster.

Civil and criminal cases were filed in the District Court of Bhopal, India, involving UCC and Warren Anderson, UCC CEO at the time of the disaster.[7][8] In June 2010, seven former employees, including the former UCIL chairman, were convicted in Bhopal of causing death by negligence and sentenced to two years imprisonment and a fine of about $2,000 each, the maximum punishment allowed by Indian law. An eighth former employee was also convicted, but died before the judgement was passed.[2] Anderson died on 29 September 2014.[9]

Aftermath  
In 1998, the Supreme Court of India reached a settlement with Union Carbide: They had to pay 470 million US dollars to the Indian state. At that time Union Carbide made a turnover of about 9.5 billion dollars, 20 times that amount. In return, there would be no further prosecution. Only very little money actually reached the victims. 

The terrain where the plant stands is still contaminated with mercury and other carcinogenic substances. Dow Chemical who owns Union Carbide refuses to decontaminate the soil. Greenpeace has estimated that decontamination would only cost around 30 million USD.

Abstract 
On December 3, 1984, more than 40 tons of methyl isocyanate gas leaked from a pesticide plant in Bhopal, India, immediately killing at least 3,800 people and causing significant morbidity and premature death for many thousands more. The company involved in what became the worst industrial accident in history immediately tried to dissociate itself from legal responsibility. Eventually, it reached a settlement with the Indian Government through the mediation of that country's Supreme Court and accepted moral responsibility. It paid $470 million in compensation, a relatively small amount of based on significant underestimations of the long-term health consequences of exposure and the number of people exposed. The disaster indicated a need for enforceable international standards for environmental safety, preventative strategies to avoid similar accidents, and industrial disaster preparedness.

Since the disaster, India has experienced rapid industrialization. While some positive changes in government policy and behavior of a few industries have taken place, major threats to the environment from rapid and poorly regulated industrial growth remain. Widespread environmental degradation with significant adverse human health consequences continues to occur throughout India.

December 2004 marked the twentieth anniversary of the massive toxic gas leak from Union Carbide Corporation's chemical plant in Bhopal in the state of Madhya Pradesh, India that killed more than 3,800 people. This review examines the health effects of exposure to the disaster, the legal response, the lessons learned, and whether or not these are put into practice in India in terms of industrial development, environmental management, and public health.

References 

1984 in Asia
1980s in India
Disaster
December events
Disasters in India
Disasters in the 1980s