On 3 December 1984, one of history's largest industrial disasters
took place at a pesticide plant owned by Union Carbide in Bhopal,
India. In the early morning of that day, a large (90 000 lb) storage
tank containing toxic methyl isocyanate (MIC) spread its contents
over hundreds of shanties and huts that surrounded the factory. Four
months after the disaster, the Indian Government reported that 1430
people had died during the disaster. In 1991, the Indian government
updated this count to more than 3800 deaths, and approximately 11 000
people with disabilities. Although this was unknown at the time of the
accident, the disaster was caused by a disgruntled plant worker, who
tried to spoil a batch of MIC by adding water to a storage tank.
The plant in Bhopal was operated by Union Carbide India Limited
(UCIL), at the time owned by Union Carbide, U.S.A. for slightly more
than 50%. UCIL was doing well in its 50 year existence; 14 national
production plants, and $200 million in annual sales. The plant in Bhopal
had a humane goal; the production of pesticides for the local
market, to enhance the Indian agricultural sector. The large scale
production of pesticides could help stop famine in the poorest regions
The two major products of the Bhopal plant were Sevin, and Temik;
two biodegradable pesticides that were produced as alternatives for
DDT. These pesticides, chemically belonging to the group of
carbamates, are produced by the reaction of an alcohol or phenol
with an isocyanate (compound containing a -N=C=O group). Hence, large
quantities of methyl isocyanate were present.
The following accident scenario took place in the night from December 2-
10:20 Tank 610, containing 90 000 lb of MIC is at the
normal operation pressure of 2 psig
11:00 Shift change. The control room operator noticed that the
pressure in tank 610 was 10 psig. This pressure was not
unusual, since it normally varied from 2-25 psig. However, the
sharp pressure increase was unusual. It is unknown whether the
operator was aware of the earlier 2 psig reading. At the same
time, a MIC leak was reported in the process area, but its
source was not found, and no investigation ensued.
00:15 Another MIC leak was reported. Pressure in tank 610 was
recorded as 30 psig, and rapidly rose to 55 psig (maximum of
instrument scale). The operator notified his supervisor, and
ran outside to check the tank. The tank was radiating heat, and
making rumbling and screeching noises from the safety valve.
Inside the control room, the sound of cracking concrete was
heard. At this time the scrubber was turned on.
00:20 The plant's supervisor was notified of the gas leak
00:45 The supervisor's log records show that the derivatives' unit
operations were suspended at this time.
14:30 Pressure in the tank subsides, and its safety valve closes.
In the aftermath, it became clear that the disaster was caused by a
reaction of MIC with water. Methyl isocyanate is a volatile compound
that is toxic and irritating to the nose and throat. Although MIC is a
liquid at room temperature, it has a very low boiling point (43-45 C).
MIC reacts with water to form methylamine and carbon dioxide:
CH3NCO + H2O -> CH3NH2 + CO2
The reaction is exothermic, and both products are gasses. The
gas production caused a rapid increase in pressure inside the tank.
The exothermic reaction increased the temperature, causing a further
increase in pressure. The increased temperature also increased the
volatility of MIC. Finally, the tank ruptured, exposing more than 200
000 people to the toxic chemical.
Initially, it was hypothesized that water had entered the tank by
careless flushing of supply lines in another part of the plant. Further
evidence presented in 1986 showed that the accident was caused by
sabotage actions of a disgruntled plant worker. This worker
added water to a storage tank to deliberately spoil a batch of
In the legal aftermath, UCIL settled for $470 million dollars with
the government of India, acting on behalf of the victims of the tragedy.