Why did it happen?
The factory in Bhopal, one of the 14 facilities operated by Union Carbide, was set up in 1969. The first batch of MIC was imported from the USA in 1973 and in 1979 the Bhopal plant started to manufacture its own MIC. Though the design of the MIC unit was based on Union Carbide's West Virginia plant, grossly lower standards were employed in the selection of construction material, monitoring devices and safety systems.
Union Carbide wanted to save money. Accidental leaks from all the units were frequent, and operators and workers were regularly exposed to different substances. The UCIL factory was running at a loss. In November 1984 the most important safety systems were either closed down or not functioning. Workers had to pay the cost of these economy measures with their health, jobs and, as many as 16,000, with their lives.
Between 1980 and 1984 the work crew of the MIC unit was halved from 12 to six workers, the maintenance crew from six to two workers. On December 26, 1981 a plant operator was killed by a phosgene gas leak. Another phosgene leak in January 1982 severely injured 28 workers and in October the same year MIC escaped from a broken valve and four workers were exposed to the chemical. The senior officials of the corporation, privy to a "business confidential" safety audit in May 1982, were well aware of 61 hazards, 30 of them major and 11 in the dangerous phosgene/MIC units. Remedial measures were then taken at Union Carbide's identical MIC plant in West Virginia but not in Bhopal.
What happened that night
On the night of the disaster, water (that was being used for washing the lines) entered the tank containing MIC through leaking valves. The refrigeration unit, which should have kept the MIC close to zero degrees centigrade, had been shut off by the company officials to save on electricity bills. The entrance of water in the tank, full of MIC at ambient temperature triggered off an exothermic runaway reaction an consequently the release of the lethal gas mixture. The safety systems, which in any case were not designed for such a runaway situation, were non-functioning and under repair. Lest the neighborhood community be "unduly alarmed", the siren in the factory had been switched off. Poison clouds from the Union Carbide factory enveloped an arc of over 20 square kilometers before the residents could run away from its deadly hold.