Industrial Hygiene Survey of Respirable Crystalline Silica Dust Exposure in the Ready-Mixed Concrete Industry

ABSTRACT

As part of the U.S. Department of Health and Human Services initiative, "Health People 2000," the National Institute for Occupational Safety and Health (NIOSH) is investigating respirable crystalline silica exposure in industries where silica is a constituent material. Ready-mix concrete was identified as an industry to investigate because of the potential for respirable crystalline silica exposure from the silica sand which comprises 10%-20% of concrete.

The Occupational Safety and Health Administration's Integrated Management Information System (OSHA's IMIS) listed 64 instances of personal and/or area sampling for respirable silica dust at ready-mixed concrete plants (SIC 1 3273) between September 1980 and December 1993. Fourteen (22%) were listed as measuring above the OSHA Permissible Exposure Limit (PEL) for respirable crystalline silica. The exposures measured ranged up to 7 times the PEL.

As a pilot project, six local ready-mixed plants were contacted and site visits arranged to learn how extensively NIOSH should investigate the silicosis potential in the industry. Personal and area sampling for respirable dust was conducted to gauge exposures to respirable crystalline silica. Area samples were collected at areas with the highest visible dust, which was generally below the loading point for the cement component. Personal samples were collected for employees who were willing to participate.

Dust concentrations were calculated by weighing the dust collected on the filters for personal and area samples. Filters with significant weight gain were submitted for crystalline silica analysis. None of the airborne dust samples contained enough crystalline silica for a quantitative measurement. Two of these dust samples had a "detectable" but not a "quantifiable" amount of crystalline silica. Although most of the measurements were below detectable levels during our survey, day-to-day variability in work practices and weather conditions could produce higher levels of exposure. The presence of crystalline silica in two airborne dust samples suggests a "potential" for higher exposures. Road dust from around the plant's lots was the only apparent source of respirable crystalline silica. Operators might consider wetting-down dusty roadways to suppress potential exposures on their properties.

METHODS AND FINDINGS

Personal and area sampling were conducted to gauge potential exposures to respirable crystalline silica. The sampling included respirable dust samples along with bulk samples of the cement, sand, and fly ash (where used). The respirable dust samples were collected in dusty areas around the plants, according to NIOSH Method 7500. Personal samples for respirable dust were also collected by this method for plant employees who were willing to participate.

When the gravimetric analysis demonstrated sufficient dust weight on the filter (0.05 mg), those filters were submitted for quartz analyses using x-ray diffraction according to NIOSH Method 7500, with minor modifications. Three-pound bulk samples of sand were collected from the sand piles at three of the sites visited, and sieve analyses were requested to learn the percent of the material that was of respirable size (smaller than 10 micrometers) on delivery to the ready-mix plants 10. Twenty-five (25) micrometers was the smallest sieve that would pass enough of the material to quantitate, and even then the weight percentages (see Table 2) were much less than one percent.

Smaller bulk samples of sand, cement, and flyash were collected from all six plants surveyed and analyzed to determine the percent of silica in each. These results are also shown in Table 2.

The analytical method for the respirable dust samples requires a minimum amount of silica to be present in order to state with certainty that it was present in the sample. The Limit of Detection is 0.015 mg. Additional silica material is required in the sample in order to state quantities. The Limit of Quantitation is 0.030 mg.

Based on visual observation of ready-mix plants visited during this study, the dustiest area was the loading point as the cement ingredient was being added. As shown in Table 2, only one sample of cement was found to contain even a small amount of silica. From these analyses, it is likely that a significant respirable crystalline silica exposure does not occur at the loading point.

Personal and area respirable dust sample data are shown in Table 3 (next page). Exposure data given as ND indicates the exposure was below the Limit of Detection - None Detected. Exposure data given in brackets indicates that the value falls between the Limit of Detection and the Limit of Quantitation. This means that although silica was found in the sample, there was not enough for the analytical laboratory to say exactly how much was present. The samples which did not contain sufficient dust weight to proceed with the silica analysis are not reported in Table 3.
