The true test of the long-term response of workers exposed to a hazardous agent is usually the epidemiological studies performed on same. This necessitates the long-term knowledge of exposure levels. Usually, the extent to which the exposures measured during recent experiences are representative of past exposures is difficult to assess as only limited historic measurements are available, and oftentimes the limited data are difficult to relate to today's measurement techniques. It is fallacious to assume that the conditions were worse in the past as it is fallacious to assume that they were better. The methodology of estimating past exposures to an agent based on industry-wide survey techniques is discussed.

The results of an industry-wide study to determine the exposure of workers to man-made mineral fibers are summarized. The purpose of the study was to determine current employee exposures to airborne fibers and to utilize results to estimate past exposures. Measured exposures of workers to airborne total particulate matter and fibers are presented; samples were evaluated by phase contrast and electron microscopy. Results of the three year study, which encompassed 16 facilities and over 1500 eight hour samples, show that although there is a wide variation in concentrations of airborne fibers and particulate matter between and within the facilities surveyed, the concentrations of airborne particulate matter and fibers are generally less than 2.5 mg/m3 and 0.5 fibers/cm3, respectively. The norminal fiber size of the fibers manufactured and average airborne fiber concentration were highly correlated.

A horizontal elutriator was used to determine the terminal settling velocity of spherical uranine particles generated by nebulization from a 1% water solution in a Dautrebande D301 generator. The best estimate of particle density calculated from terminal settling velocities was 0.58 gm/cm3. The bulk density of the uranine powder used to prepare solutions was 1.53 gm/cm3. This result is discussed with reference to the increased acceptance of uranine as a test aerosol and calibration standard.