In 1978, NIOSH recommended new exposure standards for all diisocyanates. They are 5 ppb TWA for a 10-hour work shift and 20 ppb ceiling for a 10-minute period (or the equivalent microgram/m3). Recently, a new continuous reading monitor, which responds immediately to both aliphatic and aromatic diisocyanates, has become commercially available. This instrument factory calibrated for measuring toluene diisocyanate, has different responses for different diisocyanates. The monitor was dynamically calibrated to measure hexamethylene diisocyanate (HDI) against the recommended high pressure liquid chromatographic methods. The standard atmospheres of HDI were generated using permeation and an evaporation method. The monitor reading is 50% of the true concentration up to 50 ppb. Beyond that, the monitor reading reaches a plateau and the measured concentrations could be unreliable.