Quantifying the exposure of heavy-equipment operators to respirable crystalline silica dust

Respirable dust sampling was performed for two months during the excavation of a large disposal pit at the Los Alamos National Laboratory Solid low-Level Radioactive Waste Disposal Facility. The facility is located on a mesa formed through erasion of volcanic ash deposition, Three regulated silica polymorphs are naturally present ill the soil in combined concentrations as high as 65 percent, During the excavation, approximately 64,000 cubic meters of earth was removed by heavy equipment, including scrapers, graders, backhoes, and dozers. Samples were collected with a modified NIOSH 7500 method, in which aluminum cyclones following the British Medical Research Council curve were substituted for nylon cyclones. Over 350 samples were collected to assess personal exposure. Gravimetric analysis of the filters was performed on site. The calculated permissible exposure limit (CPEL) for dusts containing silica was established at 0.182 milligrams per cubic meter (mg/m(3)) in accordance with the Code of Federal Regulations (29 CFR 1910.1000, Table Z3). Respiratory protection was required for all equipment operators until a minimum of 15 samples had been collected for operators of each type of equipment, the samples had been evaluated to assess personal exposure, and the evaluation had confirmed that operator exposure was below the OSHA-calculated permissible exposure limits. Sample analysis revealed respirable-dust concentrations that ranged from 0.014 to 3.169 mg/m(3). For various types of equipment, operators received average gross personal exposures as follows: 0.694 mg/m(3) for dozers; 0.070 mg/m(3) for a pressurized-cab scraper; 0.321 mg/m(3) for an enclosed, nonpressurized cab scraper; and 0.257 mg/m(3) for graders. Dust control measures included use of an 8,000 gallon water wagon to apply water to the site while work was being performed. This article also discusses variations in dust concentrations and additional control measures.