Physical state of airborne p, p'-diphenylmethane diisocyanate (MDI) and its measurement.

The current exposure standard of 0.02 ppm ceiling for MDI by definition refers to a gaseous species. The standard method, recommended by NIOSH, for measuring MDI in air, is also based on the the assumption that MDI is in the gas phase. Investigations in our Unit indicate that a major portion of airborne MDI, in working environments during spraying operations, is present as an aerosol. In view of this finding, the standard method would underestimate the MDI concentration and describing the airborne MDI concentration in "ppm" or "volume/volume unit" in invalid. A new method for calibration of the continuous reading monitors to measure airborne MDI concentration in mg/m3 unit is described.