METHYL ISOCYANATE is a highly toxic, water-reactive, flammable, corrosive liquid with a sharp, pungent odor. It is unstable, polymerizable, irritating, reactive, lacrimating, and colorless and is very useful as a chemical intermediate in the manufacture of carbamate herbicides and insecticides.


Methyl isocyanate has a flash point of less than 0°F (some references report a higher flash point using closed-cup methods), an ignition temperature of 994°F, and a flammable range of from 5.3 percent to 26 percent. Its specific gravity is 0.96, it has a molecular weight of 57, and it has a vapor density of 1.96. It boils at 102.4°F., freezes at – 112°F, and is slightly soluble in water. Its chemical formula is CH3NCO and its structural formula is


The death of more than 2,000 people in Bhopal, India as a result of exposure to methyl isocyanate is stark proof of its toxicity, instability, and reactivity. The multiple hazards of methyl isocyanate and its use in making a lot of other chemicals make it one of the most dangerous chemicals in use today.

At only 2 ppm (parts per million of air) many people experience irritation and considerable tearing of the eyes with no detectable odor. At 4 ppm irritation to the eyes and throat is severe, and it becomes unbearable above 20 ppm. An LD50—the lethal concentration of the material in air that kills half the animals during the exposure or during the observation period— was found in one test to be 5 ppm. The TLV-TWA for methyl isocyanate is 0.02 ppm.

Inhalation of methyl isocyanate can cause breathing difficulties, coughing, chest pains, headaches, nausea, vomiting, pulmonary edema (which may be delayed), spasms, and death. Exposure to high concentrations of the vapors can cause blindness.

Ingestion of methyl isocyanate causes burns of the mouth, esophagus, and stomach and death in sufficient quantities. Skin contact with the liquid or vapor causes severe burns, and the product can be absorbed through the skin and cause death. Eye contact with the liquid or vapor causes severe burning and tissue destruction.

Methyl isocyanate is corrosive to human tissue and may be absorbed through the skin, causing symptoms of cyanide poisoning. It is corrosive to many metals such as aluminum, iron, galvanized iron, copper, tin, steel, zinc, and any of their alloys. Reactions with these metals may initiate polymerization, liberating enough heat to evaporate the methyl isocyanate and perhaps even ignite it. Containers must be lined with stainless steel, glass-lined steel, nickel, or fluorocarbon coating.

Other materials such as the salts of zinc, iron, tin, and copper and catalysts such as calcium oxide, sodium alkoxides, tributylin oxide, triethyl phosphine, triphenylarsenic oxide, iron chloride, tin chloride, and copper chloride may cause methyl isocyanate to polymerize. Heat from the polymerization reaction can cause thermal decomposition of methyl isocyanate, liberating hydrogen cyanide, the nitrogen oxides, and carbon monoxide.

Methyl isocyanate also may react violently with acids and bases, alcohols, amines, and urea derivatives. It reacts with water to produce methyl amine, carbon dioxide, and dimethyl urea. These reactions also are exothermic and can cause decomposition of the product, spreading the material and its decomposition products rapidly by evaporation.

Methyl isocyanate is a flammable liquid, and its flash point of less than 0°F (flash points above 0°F such as 5°F and 20°F also have been reported) means that an explosion and fire hazard exists at all ambient temperatures above 0°F. The lower flammable limit of 5.3 percent means that any time there is an open container of methyl isocyanate present and the temperature of the liquid is 0°F or higher, a deadly concentration of vapors will be present. These vapors hug the ground and move to lower areas unless disturbed by wind. If methyl isocyanate vapors of at least 5 3 percent concentration in air reach an ignition source (all common ignition sources produce temperatures of 994°F), the vapors ignite and flash back to the source, resulting in an explosion. This concentration of vapors that ignites is very lethal to all living creatures. Some consider methyl isocyanate a stable chemical that may become unstable rapidly. Any contact with water or any of the chemicals listed causes the material to either decompose or polymerize. The material is highly toxic in its own right, but the decomposition products are also deadly (see “Hydrocyanic Acid,” Fire Engineering, July 1987). The nitrogen oxides, with the exception of nitrous oxide, are a group of highly toxic gases that may produce death after 24 to 48 hours, with no symptoms appearing until just before death. The toxicity of carbon monoxide is well documented.



isocyanic acid

methyl ester

methyl carbonimide



TL 1450


Any significant release of methyl isocyanate must be followed by evacuation of everyone downwind for at least two miles, and to be more certain of safety that distance should be doubled. One manufacturer recommends immediate evacuation of a two-mile radius from a large spill. Adjectives such as “large” and “significant” are relative terms, especially when used to describe accidental releases. Methyl isocyanate is so toxic that all releases should be considered dangerous, and all human life must be protected immediately. Even if death does not occur, the damage to the skin, eyes, and respiratory system can be so severe that thousands of people can be severely injured. Do not take chances. When dealing with methyl isocyanate it is much better to overreact than underreact.

Because of a vapor density of 1.96, the vapor may travel great distances. Not only is the danger area of explosion and fire widened, but a greater danger of death by poisoning exists. Methyl isocyanate is toxic at levels far below the lower flammable limit (53,000 ppm), and its tendency to accumulate in low spots and enclosed spaces produces a lethal trap for anyone without respiratory and skin protection. If people survive such an exposure, they may be permanently disabled from blindness or other injury.

With any release of methyl isocyanate, you must begin evacuation and notify the proper environmental authorities. Any emergency personnel involved in the incident must have full body protection (no exposed skin) and positive pressure, self-contained breathing apparatus. All other personnel must withdraw to absolutely safe locations upwind, ready for complete mobility in the event of a wind change.

Any liquid spill should be contained as soon as possible in as small a space as possible. Containment ponds may be constructed by building dikes of sand, soil, clay, or other materials. A containment pit may be dug and trenches scratched out of the soil to lead the liquid to the pit. A pit may be covered by an impervious material to slow down the evolution of vapors. Firefighting foam may be used to cover the surface of the liquid in the pond or pit to try to control the production of vapors. Use caution with this technique, since methyl isocyanate is water-reactive, and some reaction may take place between the methyl isocyanate and the water-based foam. There should be little reaction if the temperature of the liquid is less than 68°F, but the reaction will accelerate as the liquid gets warmer.

Once contained, the liquid may be pumped out of the containment area into secure containers. In constructing the pit as well as salvaging the product, compatible tools and equipment must be used, and salvage must be done only by professionals who are properly educated, trained, equipped, and supervised.

Firefighters and other emergency responders who are not professional salvagers should not carry out cleanup procedures. Throughout the incident, the incident commander should constantly seek advice from the environmental authorities, who also will monitor the movement of the material. Advice from any other qualified expert also should be considered. All contaminated soil must be removed under their direction and disposed of in accordance with the proper federal, state, and local regulations.

Prevent the liquid from entering sewers and waterways by blocking off all points of entry. If methyl isocyanate does enter a sewer, alert (and probably evacuate) the sewage treatment facility and evacuate all along the sewer route. There is the possibility of deadly vapors seeping into occupancies as well as the threat of an explosion and fire.

If the liquid enters a waterway, the material slowly dissolves while floating on and reacting with the water. Toxic vapors flow along over the water and eventually spread out over the banks of the waterway. Slight breezes move the vapors intact, while stronger winds disperse them. You must evacuate for a minimum of a mile on either side of the waterway and notify all downstream users of the water. Give warning early enough so that users can shut off their water intakes before the contamination reaches them. Environmental authorities must decide if and when the water is safe for any use.



(Chemical Abstract Services)



(Standard Transportation Commodity Code)



(Registry of Toxic Effects of Chemical Substances)

NQ94 50000


(United Nations/North America)



(Resource Conservation and Recovery Act)



(U.S. Department of Transportation)

Flammable liquid

NFPA 704 Rating

(National Fire Protection Association)



(International Maritime Organization)

3-2, flammable liquid

Vapors of methyl isocyanate evolving from the spill scene, the containment pond or pit, the contaminated sewer or waterway, or any other source may be dispersed by using high-pressure water spray or fog. Do not allowthe water used to hasten the dispersal of vapors to contact the liquid itself, and make sure all runnoff water is contained.

The toxic hazard of methyl isocyanate is so great that its flammability, and therefore its explosiveness, may be overlooked. Emergency responders must approach from upwind to avoid such dangers, and all ignition sources must be eliminated.

An added problem when dealing w ith a water-reactive material is the threat of rain, over which emergency responders have no control. Covering the surface of the liquid with a compatible, impervious material slows the evolution of vapors and may further slow the liberation of hazardous by-products of the water reaction.

In an uncontrolled spill, there is always the possibility of a material contacting some other substance with which it is incompatible. In the case of methyl isocyanate, the dangerous substances include such common metals as iron, steel, tin, copper, and zinc. It may be impossible to prevent contact of the metal with the free-flowing, released liquid, so expect uncontrolled violent reactions accompanied by the release of highly toxic gases and vapors.

A mitigation technique sometimes considered whenever a highly toxic and flammable material is released is deliberate ignition. This technique is used only when, to a great degree of certainty, the deliberate ignition of toxic vapors will be less hazardous than the spread of those vapors to exposed humans. Consider this only as a last resort. Remember, whenever flammable vapors are igiited the first result is an explosion, usually followed by a fire. You must determine many factors, including wind strength and direction, temperature, length of time the liquid has been evaporating, and so on. Your choice of this technique must depend heavily on whether the site is one w here ignition will cause the least damage once it occurs. One approach is to disperse as many vapors as possible with water spray before ignition.

Controlling the combustion products is a problem with methyl isocyanate. Hydrogen cyanide and carbon monoxide are flammable, but some of the gases escape combustion. The nitrogen oxides generated are not flammable, and some of the evaporated methyl isocyanate also leaves the fire scene.


Any container of methyl isocyanate exposed to flames or the radiated heat from a fire faces the possibility of pressure relief failure caused by the internal pressure rise produced by absorbed heat. All containers exposed to such heat should be cooled by the application of water from unmanned appliances from as far away as possible.


Base—a chemical compound that contains the hydroxide ion (OH “l), is the chemical opposite of an inorganic acid, and is active chemically.

Catalyst—any substance that in small quantities affects the rate of a chemical reaction without being consumed in the reaction.

Exothermic—a chemical reaction in which heat is liberated; in contrast with endothermic, a reaction in which heat is absorbed.

Material safety data sheet (MSDS)—a nonuniform information sheet that must contain certain data about a substance. It must be provided by all manufacturers, distributors, and importers of hazardous chemicals (as defined by the OSHA Hazard Communication Standard) to all nonretail users of the substance. By law, the MSDS must contain, at a minimum, certain information describing the hazardous properties of the substance.

Polymerization—a unique chemical reaction in which a monomer (a very tiny reactive molecule) reacts with itself to form a “giant” molecule called a polymer.

Uncontrolled polymerization—a rapid chemical reaction in which the polymerization occurs without an opportunity for the heat to be dissipated. The result may be an explosion. This also is called “runaway” polymerization.

Water-reactive—a material that will react with water in a violent or dangerous manner. This includes the evolution of hazardous products, heat, or violent spattering.

Any container that is leaking will have fire around it, either from ignition of the vapors escaping the container or from ignition of the vapors produced by evaporation of the released liquid. A pressure-relief explosion is possible, spewing highly toxic, corrosive, and water-reactive liquid about, usually followed by the ignition of vapors produced by that liquid.

Heated or burning methyl isocyanate will generate large amounts of highly toxic thermal decomposition products. Alt emergency personnel involved in fighting the fire must be fully protected from all the hazards of methyl isocyanate. Indeed, decisions will have to be made as to whether or not it is wise to even attempt to fight the fire other than for the purpose of protecting exposures. Once the material is ignited, the hazard of the fire and its combustion products may be less than the uncontrolled polymerization reaction and the subsequent release of toxic vapors. Whether you fight the fire or not, you must evacuate all areas downwind and keep them free of people until tests show the areas are safe.

Extinguishing agents including carbon dioxide and dry chemical are recommended if wind conditions, the size of the spill, and the availability of the extinguishing agents are sufficient. You may use water fog and spray, but contact of the water with the liquid methyl isocyanate may accelerate vapor evolution. If the fire is extinguished, the high temperature of the remaining liquid will cause the generation of tremendous amounts of vapor, increasing the toxicity hazard and producing sufficient vapors for explosive reignition.

Mitigation of any incident involving methyl isocyanate is dangerous at best and lethal to anyone directly exposed to the material. If the material is used in your protection district (you will know because of reports made to you under SARA Tide III regulations), you must prepare for an eventual incident involving it. You should contact the users of methyl isocyanate, those who transport it, and particularly those companies that manufacture it. Obtain material safety data sheets for safety information and any other product information that will inform you of recommended procedures for handling incidents involving the material.


For inhalation of methyl isocyanate, move victims to fresh air and keep them quiet and warm. Administer artificial respiration only if their breathing has stopped or become difficult, and be careful of exposure to the product in the victim’s lungs and vomit. Seek medical attention immediately.

For ingestion, have a conscious victim drink large quantities of water, and induce vomiting. Never try to make an unconscious person drink anything or vomit. Seek medical attention immediately.

For skin contact, remove all contaminated clothing and wash all affected body areas with large amounts of water. Immediate medical attention is absolutely necessary.

For eye contact, flush the eyes immediately with large amounts of water for 15 minutes, occasionally lifting the eyelids. Seek immediate medical attention.


Avoid contact with methyl isocyanate, either in liquid or vapor form, whenever possible. However, if you must come in contact with it, carefully select protective clothing. Contact manufacturers of protective materials and methyl isocyanate for recommendations.

Choose clothing and protective equipment that prevent contact of liquid or vapor methyl isocyanate with the eyes, skin, or respiratory system. Select splashproof chemical goggles and face shields for eye protection, but even they may be ineffective against the material. As always, use positive-pressure, self-contained breathing apparatus. Rubber boots, gloves, and aprons offer some protection, but you should wear total encapsulating suits if contact with methyl isocyanate is possible. In a product bulletin one manufacturer of methyl isocyanate claims that suits made of butyl rubber may be used. Another reference indicates that polyvinyl alcohol material may offer protection but does not recommend that butyl be used.

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