Quantifying the Exposure of Heavy-Equipment Operators to Respirable Crystalline Silica Dust

ABSTRACT 

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 erosion of volcanic ash deposition. Three regulated silica polymorphs are naturally present in 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/m3) in accordance with the Code of Federal Regulations (29 CFR 1910.1000, Table 23). 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/m3. For various types of equipment, operators received average gross personal exposures as follows: 0.694 mg/m3 for dozers; 0.070 mg/m3 for a pressurized-cab scraper; 0.321 mg/m3 for an enclosed, nonpressurized cab scraper; and 0.257 mg/m3 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.


METHODS AND MATERIALS

To collect all respirable-dust samples, a modified National Institute of Occupational Safety and Health (NIOSH) 7500 method was used. Aluminum cyclones from SKC, Inc., designed to adhere to the British Medical Research Council (BMRC) curve, were substituted for the NIOSH-specified cyclone. The substitution was made for three reasons:

1. a higher flow rate,

2. less likelihood of static charge interference, and

3. a sharper collection efficiency curve around the sampling cut point.

In addition, the aluminum cyclone proved easier to clean and very durable under the rugged field conditions experienced.

Both personal and equipment samples were collected to determine airborne concentrations of respirable dust during earth-disturbing activities. Personal samples were collected with accepted sampling protocol. Equipment samples were collected with the cyclones and sampling pump attached outside the cab at approximately the same height as the operator's breathing zone. Pump vibration was reduced in equipment samples with a G-inch-thick felt pad placed between the pump and the cab.

Gravimetric analysis of the 37 millimeter (mm) diameter, 5.0 micrometer (µm) pore size polyvinyl chloride (PVC) filters was performed on site. Each filter was exposed to an alpha-emitting source prior to being weighed to eliminate static-charge interference. A microbalance capable of 1 microgram (µg) resolution was used for gravimetric analysis of all filter media. The accuracy of the microbalance was verified daily with a series of five reference weights certified to national standards. Throughout the excavation project, balance accuracy was maintained within 3 µg of the reference weights.

RESULTS

Over 350 acceptable respirable-dust samples were collected during excavation of the disposal pit: 175 personal samples and 151 equipment samples. Quality assurance was maintained by the collection and analysis of 38 total-dust laboratory samples and 52 field blanks. Gross concentrations calculated from 175 personal respirable-dust samples collected from the operators of eight types of heavy equipment are shown in Table 1. The comparative statistics given are arithmetic mean (µ) and standard deviation (s). Gross concentration is defined as the net respirable-dust loading divided by the volume of air sampled for the sample duration only. No adjustment is made to a time-weighted average value. Operators in pressurized cabs received the lowest mean respirable-dust exposure, 0.072 mg/m3. Operators in enclosed, nonpressurized cabs received mean respirable-dust exposures ranging from 0.234 to 0.323 mg/m3, and operators in open cabs received the highest mean exposures, ranging from 0.426 to 0.837 mg/m3.

The maximum values presented in Table 1 are clearly above the established CPEL of 0.182 mg/m3 for the eight-hour workday. These values appear to indicate that operators of heavy equipment were overexposed to crystalline silica. Three factors, however, refute that indication. First, all heavy equipment operators were required to wear half-faced respirators until sufficient exposure data confirmed that their exposures were acceptable. Second, the data presented in Table 1 are gross exposures, not time-weighted averages. Finally, the data are purely gravimetric. Each personal sample collected with a filter loading of 100 µg or more was submitted for XRD analysis. The XRD results mandated respiratory protection, thereby ensuring that no personnel were overexposed to respirable crystalline silica during the construction of Pit 38.

Dozer operators consistently received the highest exposures. For all heavy equipment operators, respirable-dust exposures were matched with pit depth. This process indicated a trend of increased dust concentrations at greater pit depths; however, the Pearson product-moment correlation between pit depth and exposures was calculated to be 0.352. The large standard deviations of the respirable-dust concentrations indicate exposure variability based on operator, task, and environmental conditions.

Table 2 shows gross concentrations calculated on the basis of 151 respirable-dust samples taken from samplers placed on heavy-equipment cabs. Dozer activities generated the highest mean concentrations; they ranged from 0.628 to 0.971 mg/m3, with standard deviations ranging from 0.212 to 0.352 mg/m3. As with personal samples, standard deviations were very high compared with the means. The relatively low mean dust concentrations generated by backhoe and grader activities were attributed to the lower intensities of their respective activities.
