Toxic Effects of Isocyanates

Organic isocyanates are toxic chemicals that can harm the human body through two main pathways: first, the vapors in the air can irritate the respiratory tract and eyes; second, liquid isocyanates can damage the skin and mucous membranes upon contact.

Various types of polyisocyanates are used in the polyurethane industry. Some liquid diisocyanates, such as toluene diisocyanate (TDI) commonly used in the production of flexible polyurethane foams and hexamethylene diisocyanate (HDI) used in the coatings industry, have high volatility. Their volatilization increases significantly when heated. TDI, for instance, has a vapor pressure of about 1.33 Pa at 20°C, which rises to 1330 Pa at 120°C. Therefore, the toxic effects of TDI vapors near continuous soft foam production lines are particularly severe, necessitating proper protective measures.

The toxic gases emitted from diisocyanate raw materials, prepolymer semi-products, and freshly cut soft foam products can irritate the eyes and respiratory system. Common symptoms include tearing, mouth and throat pain, and severe coughing with a feeling of chest tightness in more severe cases. In specific situations, isocyanate gases can cause sensitization-induced asthma. When the concentration of toluene diisocyanate exceeds 0.05 mg/m³, it can trigger coughing due to its effect on respiratory secretions. The odor of toluene diisocyanate is detectable at concentrations above 0.4 mg/m³, with sensitive individuals detecting it at 0.05 to 0.1 mg/m³. Short-term exposure can allow for recovery. However, if organic isocyanates are absorbed by body tissues, they can affect internal organs and impede their function. Isocyanates can also harm hematopoietic function, reducing platelet counts in some polyurethane production and research personnel.

To prevent the harmful effects of isocyanate vapors, various countries have set maximum allowable concentrations of diisocyanates in the air. Internationally, the permissible concentration for TDI is set at 0.02 x 10⁶. The National Institute for Occupational Safety and Health (NIOSH) in the United States has stricter regulations, setting the TWA (time-weighted average) limit for workplace exposure to 5 x 10⁻⁹, which translates to a maximum of 35 μg/m³ for TDI, 50 μg/m³ for MDI, 35 μg/m³ for HDI, 45 μg/m³ for IPDI, and 55 μg/m³ for HMDI. In China, the maximum allowable concentration of TDI in workshop air is 0.2 mg/m³, classifying toluene diisocyanate as a highly hazardous substance to human health.

Therefore, workshops producing and using organic isocyanates must ensure good ventilation conditions and facilities and adhere strictly to safety operations.

Organic isocyanates such as toluene diisocyanate have high chemical reactivity and readily combine with moisture and proteins, adhering to the skin or mucous membranes. Particularly aromatic organic isocyanates like TDI, when inhaled, can react with moisture in the respiratory tract and lungs, producing aromatic amines, which are suspected carcinogens. Hence, workers in prolonged contact with isocyanates should enhance their self-protection awareness.