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Erin Dickison

Users of aromatic diisocyanates employ a variety of processes and activities during production to minimize exposure to airborne chemicals. In most situations, potential worker exposure is minimized with the use of appropriate engineering controls, such as enclosing the operation and providing ventilation.

By its nature, TDI evaporates and releases vapors into its immediate environment more quickly than MDI at room temperature. Therefore, exhaust ventilation is usually essential in the processing of all TDI-based polyurethanes. MDI and all polymeric forms of MDI have an inherent characteristic of low volatility. For example, at room temperature, MDI and polymeric forms of MDI have a low evaporation rate (vapor pressure is approximately 1000 times lower than TDI) and typically remain below applicable occupational exposure limits. Consequently, the use of engineering controls such as exhaust ventilation where MDI or polymeric forms of MDI are used is usually not necessary unless the process conditions or applications involve heating or spraying. In all proposed processes or applications involving MDI or TDI, it is important to perform a pre-job hazard analysis to determine the need for engineering controls, such as containment and exhaust ventilation.

Where it is determined that control of diisocyanate vapors or aerosols is needed, it is important to be knowledgeable of the basic principles of exhaust ventilation systems discussed below.

Types of Ventilation Systems

Ventilation systems1 used in industrial plants are of two generic types. The supply system is used to supply air, usually tempered, to a work space. The exhaust system is used to remove any potential contaminants generated by the production process in order to maintain a safe work environment. A complete ventilation program must consider both the supply and exhaust systems.

General Exhaust Ventilation Systems

Exhaust systems are generally classified into two generic groups: 1) general exhaust ventilation systems and 2) local exhaust ventilation systems. General exhaust systems can be used for heat control and/or removal of contaminants generated in a space by flushing out a given space with large quantities of air. When used for contaminant control as the dilution system, enough outside air must be mixed with the contaminant so that the average concentration is reduced to a safe level. A general exhaust system typically uses natural convection through open doors or windows, roof ventilators, and chimneys or air movement produced by mechanical fans or blowers. A major disadvantage of general exhaust, or dilution ventilation is that employee exposures can be difficult to control near the source of the contaminant where sufficient dilution has not yet occurred. Therefore, local exhaust ventilation is typically the preferred method to control exposure to hazardous chemicals in the workplace.

Local Exhaust Ventilation Systems

Local exhaust ventilation is considered the classic method of control. Local exhaust systems capture or contain contaminants at or near their source before they escape into the work area. These systems are designed to remove the air contaminants rather than dilute them. Local exhaust systems are typically comprised of four basic elements: the hood(s), the duct system (including the exhaust stack or recirculation duct) the air cleaning device, and the fan. The hood collects the contaminant in an air stream directed at it. The duct system then moves the air to the air cleaning device or to the fan. The fan duct usually discharges the cleaned air into the atmosphere in such a way that it will not re-enter the work environment, unless this cleaned air is needed by the plant as part of a re-circulation process. One of the other advantages of local exhaust ventilation systems is that they require less air flow than general ventilation systems, so this can help reduce operating costs.

Ventilation Resources

OSHA Regulations and Guidance
Truck Bed Liners

CPI provides helpful resources regarding ventilation systems to minimize exposure to MDI for the truck bed liner industry.

Spray Polyurethane Foam

Resources for Purchase

American Conference of Governmental Industrial Hygienists (ACGIH)
MDI and TDI: Safety, Health and the Environment: A Source Book and Practical Guide – Wiley
The American National Standards Institute (ANSI) Ventilation Standards

A listing of Z9 Health and Safety Ventilation standards is currently available for purchase through the ANSI Webstore.

1 Reference: American Conference of Governmental Industrial Hygienists (ACGIH): Industrial Ventilation: A Manual of Recommended Practice for Design, section 1.3. Exhaust Systems