What is Methyl Isocyanate? 

Methyl isocyanate (MIC) is an organic compound with the molecular formula CH₃NCO. It is also known as isocyanatomethane and methyl carbylamine. Under normal conditions, MIC is a colourless liquid with a sharp, pungent odour. It has a boiling point of 44°C and evaporates easily, forming dense vapours that can accumulate in low-lying areas. MIC is highly flammable and can form explosive mixtures with air. It is also highly reactive, reacting violently with water to form ureas and carbon dioxide. Additionally, MIC is corrosive, attacking certain metals, plastics, rubbers, and coatings. When heated, it breaks down into toxic gases, including hydrogen cyanide, carbon monoxide, and nitrogen oxides. It is classified as a volatile organic compound (VOC).

What is Methyle Isocyanate Used for?

Methyl isocyanate is a key chemical in the production of:

• Pesticides and herbicides
• Synthetic rubbers
• Adhesives
• Other industrial chemicals

Sources and Routes of Exposure’

Sources of Exposure

• Found in tobacco smoke, exposing both smokers and bystanders.
• Occupational exposure in chemical manufacturing and handling.
• Residents near MIC facilities may inhale low levels of the chemical.

Routes of Exposure

• Inhalation: The primary exposure route. MIC vapours are easily 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 exposure is mainly a workplace hazard.
• Ingestion: Rare, but possible if liquid MIC is accidentally consumed.

Health Effects

Acute Effects

One of the most devastating industrial disasters involving Methyl Isocyanate occurred in Bhopal, India (1984), where a gas leak led to:

• Over 2,000 deaths.
• Severe lung damage, including pulmonary oedema.
• Blindness, breathing difficulties, nausea, fever, and kidney/liver damage.
• Long-term lung and eye conditions among survivors.
• Animal studies confirm Methyl Isocyanate’s extreme acute toxicity, especially through inhalation.

Chronic Effects

• No confirmed long-term health effects in humans.
• EPA has not established a Reference Concentration (RfC) or a Reference Dose (RfD) for methyl isocyanate.
• California EPA Reference Level: 0.001 mg/m³ (based on animal studies).

Reproductive/Developmental Effects

Following the Bhopal disaster, survivors experienced:

• After the Bhopal, India, accident, an unusually high percentage of survivors had disorders of the reproductive system, including leukorrhoea, 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 confirmed human cancer data.
• A study of animals involving inhalation exposure found no significant tumour development in mice and female rats. However, male rats showed a slight increase in pancreatic tumours, though the findings were not considered conclusive.
• EPA Classification: Group D – Not classifiable for human carcinogenicity.

Safety

First Aid Measures

• Respiratory Distress (Asthma-like Symptoms): Remove the person from the contaminated area and provide oxygen. If breathing has stopped, initiate artificial respiration. Trained personnel may administer a bronchodilator (e.g., salbutamol via nebuliser). Seek urgent medical attention.
• Eye Contact: Flush eyes with tepid water for at least 15 minutes. If wearing contact lenses, remove them after initial rinsing and continue flushing. Refer the patient to a doctor.
• Skin Contact: Remove contaminated clothing and wash the affected area with soap and water. Do not use solvents (e.g., methylene chloride) to remove Methyl Isocyanate from the skin. Decontaminate clothing before reuse.

Control Measure

To minimise worker exposure, comprehensive control measures should be implemented and prioritised as follows:

• Elimination/Substitution and Process Modification
• Engineering Controls
• Administrative Controls
• Use of Personal Protective Equipment (PPE)

Engineering Controls

• Enclosure of processes involving Methyl Isocyanate is essential to prevent vapour release.
• Local exhaust ventilation (LEV) must be installed wherever Methyl Isocyanate vapours may be released, particularly in spray applications. In processes using toluene diisocyanate (TDI) or hexamethylene diisocyanate (HDI), LEV is mandatory. If Methyl Isocyanate pre-polymers are used and aerosol formation is unlikely, LEV may not be necessary if exposure remains below occupational limits.
• Exhaust vapours should not be released externally in a way that could pose a hazard.

Personal Protective Equipment

PPE should be used only as a secondary control and not as a substitute for engineering measures. However, in situations where ventilation is inadequate, PPE is essential.

• Respiratory Protection: Airline respirators or self-contained breathing apparatus (SCBA), compliant with Australian Standard AS 1716, should be used. Organic vapour respirators with particulate pre-filters and powered, air-purifying respirators are not suitable.
• Eye and Skin Protection: MIC-resistant materials such as Teflon, Viton, nitrile rubber, and certain PVA gloves should be used. Protective gloves and overalls should comply with Australian Standard AS 2161.
• Emergency Situations: SCBA respirators must be used during spill clean-ups, equipment maintenance, and emergency situations where MIC exposure could reach hazardous levels.

Regulations

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)

Australia

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