Question: Hello LCS Laboratory. We’d like to try silica sampling by NIOSH 7602 as a less expensive sampling option, but we are not sure how it will affect our historical data that we collected using the NIOSH 7500 method. Can you please explain the practical differences between the NIOSH 7500 and NIOSH 7602 methods? Thank you.
Answer: Thank you for asking. We have been offering the NIOSH 7602 method for 16 years now, and can say that they both are great methods. NIOSH 7500 and 7602 are both developed for chemical analysis of quartz, cristobalite and tridymite. The methods utilize two very different analytical techniques but they provide identical results with some minor pros and cons:
Sampling technique: Silica is regulated as respirable dust, and both methods require the same sampling procedure and sampling media: dust is collected on 37 mm PVC filter using air sampling cyclone.
Analytical Technique:
- NIOSH 7500 is based on X-Ray Diffraction (XRD), and
- NIOSH 7602 utilizes Fourier Transform Infrared Spectroscopy (FTIR)
Method Sensitivity: Most laboratories offer identical sensitivity (Reporting Limit) for both methods. Typically it is between 5 and 10 ug/sample. This is typically more than sufficient for the assessment of occupational exposure. If you need to compare your results with the ACGIH guidelines, look for a lab that can achieve a sensitivity of 5 ug/sample or better.
Ability to detect different polymorphs of silica. Quartz is the most common form of silica, and is likely the most common mineral on the Earth. Cristobalite and Tridymite can be found in volcanic rocks and man-made ceramic materials that have been exposed to high temperatures. Although they are relatively rare in industrial samples. Less than 1% of silica samples tested by LCS Laboratory resulted in a detectable amount of Cristobalite or Tridymite.
- NIOSH 7500 can distinguish all 3 polymorphs of Silica: Quartz, Cristobalite and Tridymite.
- NIOSH 7602 reports Quartz separately and Cristobalite and Tridymite as total.
Interferences:
- NIOSH 7500: micas, potash, feldspars, zircon, graphite, aluminosilicates may cause positive interference, and Iron reduces sensitivity of the test.
- NIOSH 7602: some phosphates and sulfates may cause positive interference, high level of silicates reduces sensitivity of the test
Third party comparison of the methods: The American Industrial Hygiene Association conducted side-by-side comparison of the 1000’s of samples tested simultaneously by NIOSH 7500 (XRD) and NIOSH 7602 (FTIR). No statistical difference is detected. J Occup Environ Hyg. 2014; 11(10): D157–D163. doi: 10.1080/15459624.2014.930561, PMCID: PMC4698134; NIHMSID: NIHMS747102; Assessment of Respirable Crystalline Silica Analysis Using Proficiency Analytical Testing Results from 2003–2013; Martin Harper, Khatchatur Sarkisian, and Michael Andrew
Legal Status of the Methods. Both methods are developed by The National Institute of Occupational Safety and Health (NIOSH, USA) and are widely accepted as industry standards.
Cost and Availability in Canada:
- NIOSH 7500: 2-4 laboratories offer the test. Typical cost per sample:$80-$140
- NIOSH 7602: more than 10 laboratories offer the test. Typical cost per sample:$60-$70
If you need more information or would like to order sampling supplies, please email us at your convenience.
