Health hazard evaluation report: HETA-93-0790-2760, U.S. Silica Company, Mill Creek Plant, Mill Creek, Oklahoma.
In July 1991, the National Institute for Occupational Safety and Health (NIOSH) received a request for technical assistance from the Mine Safety and Health Administration (MSHA) to estimate the prevalence of silicosis among active and retired miners at U.S. Silica Company's Mill Creek plant, in Mill Creek, Oklahoma. Current and former workers with one year or greater cumulative tenure since 1970 in the grinding area of the mill or in areas downstream (by material processing) of the grinding process represented the population of primary interest. On August 10-12, 1993, a medical evaluation of current workers was conducted. Former workers were tested on August 13-14, 1993. The medical evaluation included a questionnaire, spirometry, and a single view posterior-anterior (PA) chest x-ray. Chest x-rays were independently classified according to the 1980 International Labour Office (ILO) system by three NIOSH-certified B readers who were unaware of the participant's age, occupation, occupational exposure, smoking history, or any identifying information. For the purposes of this evaluation, silicosis was defined on the basis of a chest x-ray with median small opacity profusion classification of category 1/0 or greater. Twenty-nine (57%) of 51 current workers and 20 (33%) of 61 former workers who met the study criterion participated in the NIOSH medical evaluation. Of these 49, five (10%) had a chest x-ray consistent with silicosis. The highest median ILO profusion category was 22. One of the five had a chest x-ray consistent with progressive massive fibrosis (PMF), with "B" size large opacities as classified by all three readers. Four (9%) of the 46 participants who performed spirometry had abnormal patterns; three exhibited an obstructive lung pattern and one exhibited a combined restrictive and obstructive pattern. An abnormal spirometry pattern was present in one of the five participants with a positive chest x-ray. U.S. Silica's medical monitoring includes all of the screening tests recommended by the National Industrial Sand Association (VISA) as well as those recommended by NIOSH for workers exposed to ground silica. NISA's current guidelines also recommend multiple ''readings of all chest x-rays with a small opacity profusion classification of I/0 or greater and 5-10% of those chest x-rays classified as 0/1 based on a single reading. Of the company records we reviewed, company chest x-rays initially classified as negative (0/0 and 0/1) were not routinely sent for additional readings. Since 1990, the company reportedly sent chest x-rays initially classified 1/0 or greater by a single reader for additional classifications. Five (10%) of the 49 survey participants who met the study criterion were found to have chest x-ray findings consistent with silicosis. These results are consistent with patterns of crystalline silica dust exposure at this facility. There were no cases of silicosis among current workers with 10 or less years of tenure; however, because of the long latency usually associated with chronic silicosis, this finding is not sufficient to conclude that current crystalline silica dust exposure levels are without adverse effect, The company medical monitoring practice of obtaining additional B reader classifications of those chest x-rays initially classified 1/0 or greater will produce an estimated prevalence no higher than and possibly lower than that obtained with a single reading, and those workers with a positive chest x-ray whose chest x-rays are initially read as 0/0 or 0/1 will not be identified. Recommendations are presented in this report and include obtaining at least two readings of all chest x-rays regardless of the initial small opacity profusion classification, increasing the frequency of medical monitoring regardless of examinations (TB) and modification of the baseline and routine examinations to include skin testing for tuberculosis (TB).