Analysis of crystalline silica in bulk materials.
Ann Occup Hyg 2014 Jun; 58(5):657-658
We are writing concerning results presented in Annals of Occupational Hygiene as part of a manuscript by Radnoff and Kutz (2014). The manuscript presents the results of seven analyses without associated uncertainty or validated Limit of Quantitation (LOQ) for bulk crystalline silica content with values reported <1% down to <0.1%. In the Methods section, they state the samples were analysed according to NIOSH Method 7500 for the presence of quartz silica down to 0.1% w/w. This method includes protocols for analysing bulk or settled dust samples'. The NIOSH 7500 method (NIOSH, 2003) is designed to quantify respirable samples collected on a filter and does not include specific procedures for analyzing bulk or settled dust samples beyond using bulk samples to identify interferences in the air samples. The user of the method is directed from Paragraph 4a: Interference check. Prepare area dust sample or settled dust bulk sample for XRD analysis... to Paragraph 11: Obtain a qualitative X-ray diffraction scan of the area air sample (or bulk settled dust) to determine the presence of free silica polymorphs. The subsequent quantitative section (Paragraph 12) refers only to the air sample filter analysis. No evaluation data or LOQ estimate are presented in NIOSH 7500 to support bulk analysis and it is unlikely that the method could be used to measure down to or <0.1% in any case. Verma et al. (2002) evaluated an infrared method for bulk analysis between 1 and 75% and concluded that although it could be used to determine down to 1% in routine analyses, the method was ineffective <1% silica. A more recent evaluation of an X-ray diffraction method (Martin et al., 2012) suggests that it may be very difficult to go much <1%, even with the additional use of the Rietveld refinement in X-ray diffraction (LOQ = 0.76%). We are unaware of any method that has been published for the determination of crystalline silica in bulk materials that can measure down to 0.1%. If the laboratory used by Radnoff and Kutz has been able to modify NIOSH 7500 to achieve this goal, we would welcome publication of the details of the modification and methods validation as this would be of value to the occupational health community.
Silica-dusts; Silicates; Quartz-dust; Sampling; Respirable-dust; Respiratory-irritants; Respiration; Filters; Air-samples
Martin Harper, Exposure Assessment Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and Health (NIOSH), 1095 Willowdale Road, MS-3030, Morgantown, WV 26505
Annals of Occupational Hygiene