A 21-month study was conducted to determine the relationship between opacity and mass emissions at the forging areas of large caliber metal parts facilities. Numerous particulate emission tests, concurrent with the operation of a transmissometer, were conducted at the uncontrolled exhausts of the Erie press line at Scranton Army Ammunition Plant (SAAP), Scranton, Pennsylvania, and the forging process at Flinchbaugh Products, Inc., Red Lion, Pennsylvania. Tests results indicate that a strong correlation does exist between particle concentration and optical density at the 95% confidence level. Through a least-squares linear regression analysis of data points, the best-fit line for the total data base is defined. It is reasonable to predict particle concentration values from measured values of optical density (or opacity) by use of the preceding empirical relationship. Eighty-three percent of the variation-of-optical-density values can be attributed to variation in particle concentration. Variations in particle size distributions can account for a portion of the remaining 17%.