The methyl isocyanate spill
near Bhopal, India, during the night of December 2-3, 1984 was at the heart of
what is often cited as the world’s worst industrial disaster involving a
chemical release. The exact number of people who died from this methyl
isocyanate exposure is in dispute and depends upon the information source, but
the government of Madhy Pradesh which includes Bhopal confirmed 3787 deaths initially
related to inhalation exposure. Other estimates have been up to 10,000 deaths
within 72 hours of the release, plus another 20,000 deaths due to long term
effects. Another 100,000 to 200,000 people have been estimated to have
permanent injuries. All this from a spill of 42 metric tonnes (42,000 kg;
92,400 lbs) of methyl isocyanate from a very large storage tank.
Methyl isocyanate is still
used today in the United States mostly in the manufacture of pesticides. The
quantities stored or used today at facilities are smaller than what was used at
Bhopal in 1984, but a reactor tank explosion occurred recently (2008) at Bayer
Crop Science in West Virginia has raised additional safety concerns by the U.S.
Congress on the storage and use of this chemical. The explosion took place 80
feet from a 37,000 lb capacity storage tank of methyl isocyanate, which
fortunately, was not released.
We will look at the safety
and use of this controversial chemical in this newsletter. We will revisit the
Bhopal accident and take a look at the 28 August 2008 chemical reactor explosion
at Bayer which is being investigated by the U.S. Chemical Safety Board, who is
reporting to Congress.
Methyl Isocyanate: The
Chemical and Its Uses
Some basic information
displayed on the PEAC tool show that the chemical is deadly, flammable, and
reacts with water:
155 - SUBSTANCES - TOXIC and/or CORROSIVE (Flammable / Water-Sensitive)
Colorless liquid; sharp pungent odor, causes tears
Health (Blue): 4 Deadly
Fire (Red): 3 Flash Point < 100°F
Instability (Yellow): 3 Shock/Heat
Special (White): No Water
and Chemical Properties
Molecular Weight: 57
Flash Point: 0°F
Lower Explosive Limit: 5.3%
Upper Explosive Limit: 26%
Auto Ignition Temp.: 995°F
Boiling Point: 102°F
Melting Point: -112°F
Rel Vapor Density @68°F: 2 (Heavier than air)
Vapor Pressure @68°F: 0.46 atm
Liquid Specific Gravity: 0.96 (Approximately the same density as
Ionization Energy: 10.67 eV
RAE Systems PID correction factor for 10.6 eV: 4.6
RAE Systems PID correction factor for 11.7 eV: 1.5
Toxic Levels of Concern
IDLH: 3 ppm (6.99 mg/m3)
TWA: 0.02 ppm (0.047 mg/m3)
ERPG-1: 0.025 ppm (0.058 mg/m3)
ERPG-2: 0.25 ppm (0.58 mg/m3)
ERPG-3: 1.5 ppm (3.5 mg/m3)
Exposure Guideline Levels (Status: Final)
Minute AEGL-2: 0.4 ppm
Minute AEGL-2: 0.13 ppm
Hour AEGL-2: 0.067 ppm
Hour AEGL-2: 0.017 ppm
Hour AEGL-2: 0.008 ppm
Minute AEGL-3: 1.2 ppm
Minute AEGL-3: 0.4 ppm
Hour AEGL-3: 0.2 ppm
Hour AEGL-3: 0.05 ppm
Hour AEGL-3: 0.025 ppm
The airborne concentration of a substance above which it is predicted that
the general population, including "susceptible" individuals could
experience irreversible or other serious, long-lasting health effects or
impaired ability to escape.
The airborne concentration of a substance at or above which it is predicted
that the general population including "susceptible" individuals
could experience life-threatening health effects or death.
Comment: The metric mode
option can be selected in the PEAC tool to give temperatures in °C.
Methyl isocyanate reacts with
water producing considerable heat plus varying amounts of the chemical products
1,3-dimethylurea, some 1,3,5-trimethylbiuret, and carbon dioxide. If the heat
is not removed quickly enough from the water reaction, the methyl isocyanate
will violently boil (102°F; 39°C, at sea level) producing significant deadly
methyl isocyanate vapor. Even at temperatures below its boiling point, some
methyl isocyanate will vaporize.
The American Conference of
Government Industrial Hygienists has set a threshold limit value of 0.02 parts
per million (ppm) concentration in air for an 8-hour TWA exposure. Prolonged
exposure at concentrations as low as 0.4 ppm can cause damage by inhalation,
ingestion, or physical contact. The EPA-recommended level-2 acute exposure
guideline level limit (AEGL-2) is 0.067 ppm for one-hour exposure, or 0.008 ppm
for an 8-hour exposure, which is lower than some of the other numbers published
by other governmental agencies. Exposure at concentrations as low as 0.4 ppm
can produce coughing, asthma condition, and irritation of the eyes.
Concentrations at 2 to 4 ppm quickly causes irritation of the eyes, even though
most people would not detect an odor. Concentrations over 21 ppm even for a
short period can result in pulmonary edema (fluid filling lungs), hemorrhages,
and death. Animal studies (rat) indicate a 4-hour exposure LC50 value of 0.54
ppm (50% of rats die inhaling air containing 0.54 ppm for 4 hours).
Methyl isocyanate is used as
an intermediate chemical in the manufacture of carbamate-based pesticides.
Examples of carbamate-based pesticides include carbaryl [Sevin], carbofuran,
methomyl, and aldicarb. Methyl isocyanate itself is manufactured from other
chemicals, usually near or at the site where the chemical is to be used rather
than transported by carrier. The usual method of methyl isocyanate manufacture
involves reacting methylamine with phosgene in the gas phase, followed by
additional steps to produce the chemical. Phosgene is also a very toxic
chemical by inhalation.
The Union Carbide India,
Limited pesticide manufacturing plant was established in 1969 near Bhopal,
India; 51% was owned by Union Carbide Corporation and 49% by the Indian
government. The facility produced the pesticide carbaryl under the trademark
Sevin. The pesticide was produced by reacting methyl isocyanate with
1-naphthol in the presence of a catalyst to form the carbaryl. Initially,
methyl isocyanate was imported, but in 1979 a plant was added at the site for
producing methyl isocyanate from phosgene and methylamine.
Various articles on the
Internet including a summary by Wikipedia citing many references and reports which
give account of the tragedy were used in this Newsletter article.
The exact causes of
circumstances leading up to the methyl isocyanate disaster are not clear, but
apparently Union Carbide corporate had a “hands-off” approach to management of
its overseas operations [from Trade Environmental Database Case Studies, see http://ausolaris1.american.edu/ted/ted.htm
.] Local Indian engineers were under pressure by the
Indian government to reduce expenses and use locally produced materials rather
than importing expensive instrumentation. This sometimes resulted in bypassing
or ignoring or not repairing critical safety systems. The methyl isocyanate
refrigeration system for the storage tank was shut down, resulting in a storage
temperature of 20°C rather than the manual-recommended 4.5°C. A vent-gas
scrubber designed to treat gases vented from the plant with a sodium hydroxide
wash was out of service. Valves and piping were being replaced with carbon
steel, which corroded; the corroded piping wash was flushed with wash water
from time to time [recall methyl isocyanate reacts with water producing heat and
various chemicals, which may be corrosive to steel]. One of the valves, of
carbon steel construction, at the methyl isocyanate storage tank was leaking
because of corrosion. At the night of the release, skip-blind plates that
would have prevented water from pipe cleaning operation from entering the
methyl isocyanate tank from the faulty valve were not installed.
These and other safety issues
were brought out in civil action suits conducted in 1998.
Tank 610 containing 42 tonnes
of methyl isocyanate was released during the night of December 2-3, 1984. The
sequence of events is as follows:
2 December (21:00 hours): Workers
begin water cleaning of the pipes. They did not add a skip-blind water
isolation plate (at least they were not told by their supervisor to add the
plate) which would have prevented water from entering tank 610.
2 December (22:00 hours): Water
enters tank 610 and reacts with methyl isocyanate inside the tank releasing
considerable heat. The tank refrigeration system is not working, and the
temperature inside the tank increases to over 200°C boiling the methyl
2 December (22:30 hours): Gases vent
from the tank through the vent gas scrubber, but the scrubber is not working,
and even if it was working, it apparently was not designed to handle the volume
of gases released during this event according to testimony heard in later court
3 December (00:30 hours): A loud
siren sounds warning of the leak and is turned off.
3 December (00:50 hours): Workers
escape from the plant.
3 December (01:00 hours): Local
police are alerted. The plant director denies any leak.
3 December (04:00 hours): The
gases were brought under control.
3 December (06:00 hours): A
police loudspeaker broadcasts, “Everything is normal”.
But outside the facility,
things were not normal. First sensations due to the gas (coughing,
suffocation, burning eyes, vomiting) were felt starting at 2 December (22:30
hours). The dense gas hugged the ground spreading into the city of Bhopal.
People were awakened by these symptoms and began to flee the area. People who
ran inhaled more of the chemical than those who escaped in a vehicle. Children
and others of short stature inhaled more of the dense gas. Some were trampled
trying to escape. The first people reached the local hospital (3 December,
02:00 hours) with symptoms of visual impairment, blindness, respiratory
difficulties, frothing at the mouth, and vomiting.
Eventually the toxic gas
cloud spread out over an estimated 40 square kilometers affecting a population
of 520,000 people. It was later estimated that approximately 27 tonnes of
methyl isocyanate was released during a 1 to 2 hour period. Stable air
conditions, low wind speed, and atmospheric inversion existed at the time of
By the morning hours, several
thousands of people succumbed to the gas. The local government (Madhya
Pradesh) certified 3787 deaths due to inhalation exposure, which was later
increased to 3928 deaths as brought out in court trials in 1991. The actual
number of initial deaths was later estimated to be over 6000 deaths according
to the Government of Madhya Pradesh in 1994. Causes of death included choking,
pulmonary edema, and circulatory collapse. Unofficial counts by independent
organizations estimated the number of deaths between 8000 and up to 30,000
within the first few days. A lot of people left the area at the time of the
incident and died and were possibly not counted. There were mass funerals, and
bodies were cremated and disposed in the nearby Narmada River. Approximately
170,000 people were treated at hospitals and temporary dispensaries.
Approximately 100,000 to 200,000 people were estimated to have permanent
injuries of varying degrees [reference: citations in Wikipedia under “Bhopal
Approximately 2000 bloated
animal carcasses were collected and disposed. Within a few days of the
accident, the leaves on the trees turned yellow and fell off.
On 16 December 1984, methyl
isocyanate was drained from tanks 611 and 619 at the facility. A precautionary
second mass evacuation of Bhopal occurred.
A review of health effects
resulting from the Bhopal disaster is presented by V.R. Dhara and R. Dhara, Archives
of Environmental Health,
Sept/Oct. 2002 (vol. 57 no. 5), pp 391-404. The
paper can be obtained off the Internet at
An estimated 20,000 people
later died from the accident from gas-exposure disease conditions. Another
100,000 to 200,000 people are estimated to suffer permanent injuries, which
included eye problems, immune and neurological disordered, lung injury, female
reproductive difficulties, and birth defects among children born to affected
On the day after the release,
Union Carbide sent material aid and several international medical experts to
assist medical facilities in Bhopal. The Chairman and CEO of Union Carbide
(Warren Anderson) visited the site on the condition that he would not be
arrested by Indian authorities, but he was arrested anyway and released on bail
by the Madhya Pradesh Police on 7 December 1984. Union Carbide was also denied
access to the facility to do an investigation as to the cause of the accident
and was prohibited from talking to facility employees. Warren Anderson was
declared a fugitive from justice by the Chief Judicial Magistrate of Bhopal in
February 1992 for failure to appear at court hearings; so far, he has avoided
an international arrest warrant.
The Indian government claimed
$3 billion in damages from Union Carbide. An out-of-court settlement was
reached with Union Carbide in 1989 for $470 million in damages, which was
upheld in 1991 by the Indian Supreme Court. According to the Bhopal Gas
Tragedy Relief and Rehabilitation Department, compensation has been awarded to
554,895 people for injuries received and to 15,310 survivors of those killed,
of which the actual amount awarded to victims was only a portion of the total.
Union Carbide, based on a
1988 investigative report by Arthur D. Little (the report is available at http://www.bhopal.com/pdfs/casestdy.pdf
) believes that the accident occurred as the result
of sabotage. Their belief is that a single employee secretly and deliberately
introduced a large amount of water into the methyl isocyanate tank by removing
a meter and connecting a water hose directly to the tank through the metering
port. Also a possible leaking carbon steel valve could not have accounted for
the large water introduced to the tank. The report was presented (May 1988) at
the Institution of Chemical Engineers Conference on Preventing Major Chemical
Accidents in London, England, and included results of interviews with
employees. The abstract opens with the statement that investigation of
large-magnitude incidents requires an understanding of human nature in addition
to technical and engineering details.
In 2001, the Dow Chemical
Company purchased Union Carbide for $10.3 billion in stock and debt. The
purchase did not include the Indian Union Carbide Bhopal plant. The plant area
is contaminated with organochlorines, heavy metals, and other chemicals, which
have rendered local wells unusable. The contamination is unrelated to the
December 1984 releases and includes a long history of dumping hundreds of
tonnes of toxic wastes. The Madhya Pradesh Government is trying to legally
force Dow to finance clean-up operations.
The writer (John Nordin)
attended several lectures put on by the University of Wyoming Law Department a
few years ago on the Bhopal disaster, which included a presentation by a lawyer
representing the victims. The litigation which is still ongoing is much more
complex than the abbreviated summary given in this newsletter.
August 2008 Bayer
CropScience Explosion in West Virginia, 2 killed
The explosion occurred on 28
August 2008 at 10:35 PM in a large industrial chemical complex of more than 400
acres located about 10 miles west of Charleston and owned by Bayer CropScience
(referred to as “Bayer”).
The chemical complex was
constructed in the 1940’s, and for four decades was used by Union Carbide which
included manufacture of carbamate-based pesticides from methyl isocyanate and
other chemicals. The methyl isocyanate was manufactured on site from phosgene
and methyl amine and stored in a large tank for servicing nearby pesticide
manufacturing operations. In 1982, Union Carbide equipped the tank with a
protective “blast blanket”, which is a steel mesh which hangs from a steel
framework presumably to protect the tank from accidental process-related
explosions. Union Carbide sold the complex in 1986 following the Bhopal
disaster. Pesticide production continued, and in 1994 the owner Rhone-Poulenc
installed a second section of the blast blanket on top of the methyl isocyanate
(MIC) tank. Bayer acquired the complex in 2002 and continued pesticide production
and has more than 500 employees at the site.
The MIC tank (8 feet in
diameter, 19 feet tall) is pressurized and refrigerated and has a capacity of
37,000 lbs of methyl isocyanate. Normally, the tank is filled daily via
pipeline from a MIC production facility located several thousand feet away. Methyl
isocyanate is used in four pesticide production areas; the 28 August 2008
explosion took place in the nearby methomyl pesticide production area. Methomyl
is combined with other ingredients and marketed under the trade name Larvin.
The MIC tank at Bayer is the
largest methyl isocyanate storage tank in the United States. This is the only
industrial facility in the U.S. which stores more than 10,000 lbs of methyl
isocyanate, according to the U.S. Chemical Safety Board.
Although the MIC tank
remained intact and no MIC was released from the tank as the result of the
explosion, the U.S. Congress demanded an explanation of the accident. The
accident already was being investigated by the U.S. Chemical Safety Board. On
21 April 2009, Chemical Safety Board Chairman John S. Bresland presented his
testimony before the House of Representatives Committee on Energy and Commerce,
which is available at http://www.csb.gov/assets/document/BreslandBayerCombinedTestimony.pdf
. Three other House and Senate committees have
requested explanations. The Chemical Safety Board also posts updates of their
investigation at http://www.chemsafety.gov/investigations/detail.aspx?SID=3
. Their final report is expected in 2010.
The Explosion at the Bayer
Methomyl Production Facility
The explosion claimed the
lives of two Bayer employees. Eight people including six volunteer
firefighters were treated for symptoms of chemical exposure, which included
aches and respiratory and intestinal distress. Two sought treatment at a
hospital emergency room and were released the next day.
Earlier, the plant had
installed new and completely different computer control equipment for the
methomyl production area. On the night of the explosion, the control room
operator saw unexpected, rapidly rising pressure displayed on the control
console for the residue treater vessel. He asked outside operators to
investigate for a possible blockage of the residue treater vent line. As
operators approached the equipment, at 10:30 PM, the vessel relief valves were
already relieving excess pressure but could relieve the pressure fast enough.
Without warning, the residue treater vessel suddenly ruptured, ejecting as much
as 2500 gallons of highly flammable liquid. The 5500 pound residue treater
vessel careened northeast into the methomyl production area destroying steel
columns, piping, and equipment along the way. By fortunate chance, the vessel
did not slam into the MIC tank located 80 feet away from the explosion. The
two operators were killed by the explosion.
Figure 1: Relative locations
of methyl isocyanate(MIC) tank, the residue treater which exploded, and
methomyl production unit. A Chemical Safety Board investigator is in the
Figure 2: The blast blanket
protecting the MIC tank. Note explosion debris in the foreground. Both
illustrations from the CSB report on 21 April 2009 to the House Committee cited
Figure 3. Path of
destruction caused by careening residual treater vessel, which came to rest as
shown in the center of the picture (from CSB 21 April 2009 report). The
residual treater is designed to decompose waste methomyl in a solvent carrier
before final incineration.
The blast wave from the
explosion damaged the control room several hundred feet away and broke windows
and cracked ceilings in homes and businesses up to several miles away. A fire
also occurred at the methomyl production unit. Projectiles were sent in all
directions. Fortunately, none of the projectiles penetrated the MIC tank.
The MIC vessel contained
between 13,000 and 14,000 lbs of methyl isocyanate at the time the residual
treater vessel exploded.
The CSB investigation
uncovered several deficiencies of Bayer plant operations including insufficient
employee training on the new computer control system and some bypassed safety
systems. The cause of the explosion was a runaway thermal reaction at the residual
treater vessel, when an excessively high concentration of waste methomyl was
fed to the vessel. Details on these deficiencies are at the CSB website. We
will look at CSB commentary on Bayer’s response to the incident because it is
of interest to emergency responders.
The CSB investigation noted
that Bayer’s incident commander called the county 9-1-1 center 15 minutes into
the response and said that no dangerous chemicals had been released. This was
not true, as methomyl is toxic, and its uncontrolled decomposition (according
to publically available Material Safety Data Sheets) may release toxic methyl
isocyanate, hydrogen cyanide, acetonitrile, carbon monoxide, dimethyl
disulfide, sulfur oxides, nitrogen oxides, and methyl thiocyanate. In
addition, hazardous chemicals were likely released from severed chemical pipes resulting
from the explosion.
Later, Bayer called the 9-1-1
center recommending that authorities issue a shelter-in-place advisory for
surrounding communities, but local authorities had already decided on a
shelter-in-place order after observing what they feared might be a hazardous
chemical haze drifting from the plant.
The guard at the front gate
at Bayer, evidently following instructions from Bayer, declined (for a period)
to identify to 9-1-1 officials even where in the 400-acre facility the
explosion and fire occurred.
The firefighters who reported
symptoms of chemical exposure were not wearing full personal protective equipment,
apparently relying upon the fact that Bayer personnel at the scene were not
using such equipment.
Additional Follow Up:
On 4 May 2009, four House and
Senate committee chairmen sent a letter to the U.S. Chemical Safety Board
requesting an examination of whether Bayer CropScience continued use of methyl
isocyanate can be justified in light of ongoing health and safety risks to
company employees, emergency first responders, and the public. On 26 August
2009, the Chemical Safety Board posted a follow up statement on their website
after discussions were carried out with Bayer.
The Chemical Safety Board
noted that Bayer used 200,000 lbs of methyl isocyanate as the worst-case
hypothetical release scenario for the Risk Management Program rule in their report
to the EPA [see Title 40, Code of Federal Regulations, Part 68 for details on
the EPA requirement].
Bayer officials provided a
plan to reduce the average methyl isocyanate inventory at their site by 80%.
This would be done by eliminating the on-site production of two carbamate
pesticides using methyl isocyanate as a raw ingredient, and restricting the
inventory of methyl isocyanate for producing two remaining pesticides. Bayer
would also eliminate aboveground storage of MIC tanks, including the 37,000-lb
capacity storage tank located near the explosion site. All of the changes
would be completed within 12 months, according to Bayer.
The Chemical Safety Board and
Bayer will continue to examine the feasibility of switching to alternative
chemicals or processes, as requested by Congress.
Hypothetical Release of Methyl Isocyanate Using The PEAC Tool
We will look at a
hypothetical release of 10,000 lbs of methyl isocyanate to the atmosphere over
a one-hour time period, which calculates to an average release rate of 2.78
lbs/sec (1.26 kg/s). We will assume a night time condition at a low speed, a
stable air mass, clear skies, and the release occurs where there are nearby
buildings and other structures instead of out in the open. Normally, methyl
isocyanate would spill as a liquid, but the chemical evaporates quickly.
Furthermore, as we have seen, if water enters the storage tank, the release
could be a gas or a vapor aerosol. We will use the metric option, set the
release rate at 1.26 kg/s, clear skies, nighttime, use a 2 m/s wind speed, and
At a 21 ppm level of
concern, the PEAC tool predicts this occurs at a downwind distance of 892
meters (0.892 km; about 0.6 miles). This concentration is lethal. The 21
ppm concentration is at the toxic cloud centerline near ground level.
However, if we are looking at the AEGL-2 level of concern for one-hour
exposure (0.067 ppm), a protective action distance of 36.3 km is predicted.
We can rerun the PEAC tool at different levels of concern and plot the
distances as a function of the levels of concern. We will repeat the
exercise for a 5 kg/s methyl isocyanate release and for a 0.126 kg/s methyl
isocyanate release. The results are shown in figure 4. Note that the scale
The 5 kg/s release is
equivalent to the average release rate if 27 metric tonnes of methyl isocyanate
is released in 1.5 hours. The 1.26 kg/s release is the average release rate if
10,000 lbs is released in one hour. The 0.126 kg/s release is the average
release rate if 1000 lbs is released in one hour.
The wind speed also makes a
difference, especially under clear skies and during nighttime. The air becomes
even more stable. We can repeat the same exercise for a 1 m/s wind speed. The
results are in figure 5.
The modeling illustrated by
figures 4 and 5 represent concentration levels near the ground and at the toxic
cloud centerline. As one moves crosswind away from the centerline, modeling
predicts that concentrations should decrease. However, because of local
topography and possible shifts in wind speed and direction, the location of the
centerline is hard to predict and may even split into different directions. Additionally,
the modeling does not include reaction of methyl isocyanate with air moisture
to produce other chemicals which may be dangerous to inhale. The PEAC tool
adopts the display used in the Emergency Response Guidebook for showing
dimensions of a Protective Action Distance which is not a true shape of the toxic
Singh and Ghosh attempted to
model the Bhopah toxic gas plume cloud. Their results are available in J.
Vol 17, pp1-22 (1987). An updated paper can be purchased from
. Based on 27 tonnes of methyl isocyanate released
during 1 to 2 hours, they estimated a range of concentrations varying from 0.12
to 85.6 ppm over the 40 kilometer square area of the plume cloud where people
were exposed. This release is approximated by the 5 kg/s release in figures 4