Methyl Isocyanate


Methyl Isocyanate (MIC) is an organic compound with the molecular formula CH3NCO. It is also known as isocyanatomethane, methyl carbylamine, and MIC. [1] Under normal conditions, Methyl isocyanate is a colourless liquid, which has a sharp, pungent smell. It evaporates easily at room temperature and boils at 44 degrees Celsius. Methyl isocyanate vapours are dense and can collect in low-lying areas, where they can form potentially explosive mixtures with air. It is also highly flammable and reacts violently with water, forming Ureas and large amounts of Carbon dioxide in the process. Methyl isocyanate corrodes certain metals and attacks some plastics, rubbers and coatings. When heated, it breaks down to give toxic gases such as hydrogen cyanide, carbon monoxide and oxides of nitrogen. Methyl isocyanate is one of a group of substances known as the volatile organic compounds (VOCs). [2]

Uses [2]

Methyl isocyanate is used in the chemical industry to produce a number of important chemicals. It is also involved in the manufacture of synthetic rubbers, adhesives, herbicides and pesticides.

Sources & Routes of Exposure

Sources of Exposure [3]

  • Methyl isocyanate has been found in the smoke from tobacco, so people who smoke or breathe second-hand smoke may be exposed to this compound.
  • You can be exposed to methyl isocyanate by breathing or touching it at workplaces where this compound is produced or used.
  • People living near facilities, which manufacture, store or use the chemical may breathe in low levels of it.

Routes of Exposure [4]

  • Inhalation: Inhalation is the major route of exposure to methyl isocyanate. The vapours are readily absorbed through the lungs. The odour threshold is approximately 100 to 250 times higher than the OSHA PEL-TWA (0.02 ppm). Significant exposures to methyl isocyanate occur primarily in occupational settings.
  • Skin/Eye Contact: Direct contact with liquid or concentrated vapours of methyl isocyanate. This would not likely occur outside an occupational environment in which methyl isocyanate is stored or used.
  • Ingestion: Although unlikely, ingestion of liquid methyl isocyanate is a possible route of exposure.

Health Effects [5]

Acute Effects

  • In 1984, in Bhopal, India, an accidental Union Carbide gas leak of methyl isocyanate resulted in the deaths of more than 2,000 people and adverse health effects in greater than 170,000 survivors. Pulmonary oedema was the cause of death in most cases, with many deaths resulting from secondary respiratory infections such as bronchitis and bronchial pneumonia.
  • Other effects noted from acute inhalation exposure to methyl isocyanate in humans are respiratory tract irritation, difficulty breathing, blindness, nausea, gastritis, sweating, fever, chills, and liver and kidney damage. Survivors continue to exhibit damage to the lungs (e.g., bronchoalveolar lesions and decreased lung function) and the eyes (e.g., loss of vision, loss of visual acuity, and cataracts).
  • Animal studies have reported pulmonary oedema, upper respiratory tract irritation, respiratory lesions, and weight loss from acute inhalation exposure to methyl isocyanate.
  • Acute animal tests in rats have shown methyl isocyanate to have extreme acute toxicity from inhalation exposure and high acute toxicity from oral exposure.

Chronic Effects

  • No information is available on the chronic (long-term) effects of methyl isocyanate in humans or animals.
  • EPA has not established a Reference Concentration (RfC) or a Reference Dose (RfD) for methyl isocyanate.
  • CalEPA has calculated a chronic inhalation reference exposure level of 0.001 milligrams per cubic meter (mg/m3) based on lung and body weight effects in rats.

Reproductive/Developmental Effects

  • After the Bhopal, India, accident, an unusually high percentage of survivors had disorders of the reproductive system, including leukorrhea, pelvic inflammatory disease, excessive menstrual bleeding, and suppression of lactation.
  • Other adverse effects included increases in the number of stillbirths, spontaneous abortions, and increased infant mortality.
  • Animal studies have reported increased incidence of foetal deaths and decreased fertility, live litter size, foetal body weight, and neonatal survival following inhalation exposure to methyl isocyanate during pregnancy.

Cancer Risk

  • No information is available on the carcinogenic effects of methyl isocyanate in humans.
  • In a study in which animals were exposed once by inhalation, no tumours were significantly associated with methyl isocyanate exposure in mice and female rats; male rats had marginally increased rates of tumours of the pancreas. (2)
  • EPA has classified methyl isocyanate as a Group D, not classifiable as to human carcinogenicity. (1)

Safety [6]

First Aid Measures

Respiratory distress/asthma: If the person is experiencing difficulty in breathing, the following steps should be followed as this condition may develop rapidly into a life-threatening situation:

  • Remove the patient from the contaminated area and give them oxygen.
  • If breathing has stopped, initiate artificial respiration.
  • If first aid or nursing personnel are present and have received appropriate training, they may administer a bronchodilating drug such as salbutamol by nebuliser.
  • Seek medical attention urgently.

Splashes of isocyanate into eyes: Gently irrigate the eyes with a continuous stream of tepid water for at least 15 minutes. If contact lenses are worn, then irrigate the eyes thoroughly for a few minutes, remove the contact lenses and then continue with further eye irrigation. Refer the patient to a doctor or hospital.

Splashes onto skin: Remove contaminated clothing. Wash skin thoroughly with soap and water. Solvents, for example, methylene chloride, should not be used to remove isocyanates or polyurethane from the skin. Clothing should not be re-used until it has been decontaminated.

Control Measures

Where there is a likelihood of worker exposure to isocyanates, steps should be taken to minimise that exposure. A thorough examination of work practices is essential. Procedures should be adopted to ensure that workers are not exposed to an extent likely to cause adverse health effects. Control measures include, but are not limited to, the following, which are ranked in priority of their effectiveness:

  • Elimination/substitution and process modification
  • Engineering controls
  • Administrative controls
  • Use of personal protective equipment (PPE)

Engineering Controls

  • Enclosure: All processes in which isocyanates are used should be enclosed wherever possible. Total enclosure, accompanied by good general ventilation, should be used to keep atmospheric concentrations below the relevant exposure standards.
  • Local exhaust ventilation: If total enclosure of the process is not feasible, local exhaust ventilation may be necessary. Local exhaust ventilation is essential where TDI or HDI is used or where isocyanate or polyurethane is sprayed. Where other isocyanates or pre-polymers are used and aerosol formation cannot occur, local exhaust ventilation may not be necessary if the atmospheric concentration can be kept below the relevant exposure standards. Where local exhaust ventilation is installed, exhaust vapours should not be vented to the exterior in such a manner as to create a hazard.

Personal Protective Equipment (PPE)

In certain circumstances, personal protection of the individual employee is necessary. Personal protective devices should be regarded as being supplementary to substitution and engineering control and should not be used in preference to them as they do nothing to eliminate the hazard. However, in some situations, minimising exposure to isocyanates by enclosure and ventilation is not possible, particularly during on-site mixing of paints, spray-painting, foaming and maintenance of machine and ventilation systems. In these situations, air-line respirators or self-contained breathing apparatus complying with Australian Standard AS 1716 must be

used. The selection, use and maintenance of personal respiratory protective devices should be in accordance with the requirements of Australian Standard AS 1715. Organic vapour respirators with particulate pre-filters and powered, air-purifying respirators are not suitable. Personal protective equipment must be appropriately selected, individually fitted and workers trained in their correct use and maintenance. Personal protective equipment must be regularly checked and maintained to ensure that the worker is being protected. Air-line respirators or self-contained breathing apparatus complying with Australian Standard AS 1716 should be used during the clean-up of spills and the repair or clean-up of contaminated equipment and similar situations which cause emergency exposures to hazardous atmospheric concentrations of isocyanate. Eye and skin contact with isocyanates should be avoided. Particular attention should be given to personal protective equipment being resistant to isocyanates, for example, Teflon, Viton, nitrile rubber and some PVA gloves. Protective gloves and overalls should be worn as specified in Australian Standard AS 2161. Contaminated garments should be removed promptly and should not be re-used until they have been decontaminated.

Regulation [7,8]

United States

OSHA: The United States Occupational Safety & Health Administration has set the following Permissible Exposure Limit (PEL) for methyl isocyanate:

  • General Industry: 0.02 ppm, 0.05 mg/m3 (Skin)
  • Construction Industry: 0.02 ppm, 0.05 mg/m3 TWA (Skin)

ACGIH: The American Conference of Governmental Industrial Hygienists set a Threshold Limit Value (TLV) for methyl isocyanate of 0.02 ppm, 0.047 mg/m3 TWA (Skin)

NIOSH: The National Institute for Occupational Safety and Health has set a Recommended Exposure Limit (REL)for methyl isocyanate of 0.02 ppm TWA (Skin)


Safe Work Australia: Safe Work Australia has established an 8-hour Time Weighted Average concentration for methyl isocyanate of 0.02 mg/m3.