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SLTC Logo 1997 OSHA Cyber-conference Silica, It's Not Just Dust Logo

- Perspectives from the Federal Occupational Safety and Health Administration, Salt Lake Technical Center (OSHA/SLTC)

- Including Documents from NIOSH, MSHA, OSHA, DOI, and other Participants at the Workshop #6, Sampling and Analysis, 1997,"Elimination of Silicosis Conference," Washington, D.C.
 
 

  1. Introductory Information
  2. Method Comparisons
  3. Sources of Crystalline Silica Reference Materials
  4. Communications with Your Industrial Hygiene (IH) Laboratory: Before You Sample, When You Submit Your Samples for Analysis, and After You Get Your Results
  5. Related Useful Information
  6. Frequently Asked Questions (FAQs)
  7. Responses to Frequently Asked Questions
  8. Follow-up of 1997 National Conference to Eliminate Silicosis (Washington, DC)


I. Introductory Information

What is silica?

Silicas are materials that all have the chemical formula SiO2. The SiO2 chemical formula indicates that there are twice as many atoms of oxygen (O) as silicon (Si). Both crystalline and amorphous silicas are chemically SiO2 , but they differ in the way the atoms are arranged three-dimensionally.

What is quartz?

Quartz is the most common form of crystalline silica. The flat surfaces of large quartz crystals, like those depicted above, attest to the underlying large-scale orderly molecular structure. Silicon atoms and oxygen atoms (shown below as silver and red spheres respectively) make up the three-dimensional crystal structure of quartz. Internal to the crystal, each Si atom is surrounded tetrahedrally by four O atoms. These SiO4 structural units common to silica and silicates can be seen in this picture by looking at the Si atoms just behind the surface layer.

If it's Silica, It's not just dust!
 

Silica, It's Not Just Dust Logo If it's Silica, It's not just dust!

Large 480 x 477 of the opening "If it's Silica, ..." artwork are available for downloading.
Silica 3D Lattice Silica 3D Lattice

If desired, a striking 3D effect can be achieved by viewing the image with the eyes gently crossed so as to overlap the repeating pattern; for a 15-inch monitor, view from about 1 meter with your eyes focused midway to the screen so the foreground silver spheres on the top left and top center combine to form one silver sphere. Change your distance to the screen for the most comfortable viewing. Now allow your gaze to drift down into the structure. It is the property of repeating units that helps to produce the 3D effect.


II. Method Comparisons

The three most popular analytical methods for crystalline silica are:

X-ray Diffraction (XRD)

The most common method is based on the diffraction of Xrays off the repeating layers of atoms in the crystalline structure. XRD is the most general but most expensive method. Because alternate analytical peaks are available, XRD is the method used by OSHA at the Salt Lake Technical Center (compliance) and at the Wisconsin State Lab (consultation). It is also used by MSHA for Metal and Non-metal Mining samples.  
 
X-ray Powder Diffraction (XRD)
X-ray Diffraction (XRD)
Infrared Spectroscopy (IR)

The next most common method (currently a close second in popularity) is based on the absorption of infrared light of frequencies that correspond to characteristic vibrations of the tetrahedral SiO4 structural units. One isolated tetrahedral unit is illustrated to the right. Compared to XRD, IR is a less expensive method choice when interferences are known and can be compensated for. The IR method is used by MSHA for Coal Mining samples.
 
Infrared Spectroscopy (IR)
Infrared Spectroscopy (IR)
Chemical Methods

The least common method is based in part on the differing Si-O bond strengths and densities of the various silicas compared to the silicate minerals. While the silicas are generally uncharged (except perhaps for SiO2.xH2O), in silicate minerals the tetrahedra are polarized or negatively charged relative to the positive counter ions these minerals contain. These chemical differences affect the speed at which each mineral dissolves in certain acids, bases, or molten fluxes. This differential solubility is often slight so chemical separation is sometimes incomplete. The separation is further complicated because the rate of dissolution is related to the surface to volume ratio; so small particles of the less soluble minerals sometimes dissolve faster than large particle of more soluble minerals. This is a good and inexpensive choice for samples that contain readily dissolved known interferences.
 
Chemical Methods
Chemical Methods
Chemical procedures

Chemical treatments used in the chemical methods are also used in the less common petrographic or microscopic methods. Also, variations on these chemical procedures can provide a useful adjunct to XRD and IR to chemically remove interferences that cannot be corrected for computationally (e.g., chemometrically).
 
Further Brief Method Comparisons
XRD IR or FTIR Chemical or Colorometric Other, e.g., Thermal Analysis
Universal Coal or specific evaluated matrix Specific evaluated matrix Thermal analysis not well suited for air samples.
Non-destructive Non-destructive to analyte destructive to matrix Destructive to analyte and matrix. Generally destructive towards matrix.
MSDS often useful MSDS needed for non-evaluated matrices. MSDS needed MSDS needed
Type or description of operation Type or description of operation Type or description of operation Type or description of operation
List specific crystalline interferences known to be present at worksite List chemical interferences known to be present at worksite List crystalline and chemical interferences known to be present at worksite. Interferences include ionic fluorides, e.g., CaF2 No likely interferences except perhaps alkaline materials.
Air volume and Sample mass Air volume and Sample mass Air volume and Sample mass (Bulks only)
Confirmation readily available. Confirmation possible. No confirmation generally available. Bulks may be confirmed by microscopy. No additional confirmation needed; very specific to quartz.
Can analyze quartz, cristobalite, and tridymite in diverse respirable dust matrices. Can analyze quartz, cristobalite, and tridymite in specific respirable dust matrices. Can analyze but not distinguish quartz, cristobalite, and tridymite in many respirable dust matrices. Can analyze quartz in many bulk matrices.


Last Updated on March 3, 1997 by Mike C. Rose


Table 3 - Methods Used to Detect Quartz in a Sample
Name Description of Technique Accuracy Remarks
Optical microscopy Samples are visually examined and the mineralogy is determined. Accurate to within a few percent. Requires considerable skill by analyst to identify the minerals present. Uses small samples. 
Electron microscopy Particle composition and morphology are determined. Crystal structure is determined with transmission electron microscopy. Resolves very small particles. Accuracy limited due to the nature of the analysis. Cannot differentiate crystalline and amorphous silica except when transmission electron microscopy is used. Methods are slow, expensive, and samples are very small.
Thermal analysis Measures mineral response to temperature changes. Accurate only for quantities over 1%. Can only be used on very small samples.
Selective dissolution Minerals are dissolved selectively using acids. Quartz generally is less soluble than other minerals so it remains in the residue. The residue is analyzed to determine the content of crystalline silica. Not very accurate. Particle size and sample composition affect the accuracy of this method. Fine-grained quartz, cristobalite, and tridymite may dissolve; other minerals may not dissolve.
Separation based on density A finely ground sample is suspended in a heavy liquid. The denser minerals settle faster than less dense minerals. By varying the density of the liquid, minerals with different densities can be separated from one another. Not satisfactory for routine analysis. Particle size, shape, and surface charge affect settling rates. The technique is slow and difficult to perform. Many of the heavy liquids used are highly toxic.
Infrared spectroscopy Minerals absorb infrared light at specific wavelengths. By examining how the light is absorbed by the sample, the analyst can identify the minerals in the sample. Accurate to about 1%. Requires very small samples, the analyst must be sure that samples are representative of the deposit.
X-ray diffraction X-rays are diffracted by the lattice planes of the minerals in the sample. By observing the intensity of the diffracted x-rays at different angles of incidence, the analyst can determine the identity and concentration of minerals in sample. Most accurate; typically, the limit is about 1% The degree of crystallinity (from amorphous to highly crystalline) and the presence of silicates can affect the accuracy of the quantitative analysis.


Last Updated on March 3, 1997 by Mike C. Rose
 


III. Sources of Crystalline Silica Reference Materials

Routine product quality control (QC) analyses (particularly packed-powder XRD and thermal analyses) can consume considerable amounts of primary standard reference material. To conserve precious primary reference materials we need to consider other approaches that will give equivalent or better analytical results. Some suggestions follow:

Bulk analyses

For analyses of crystalline silica analyses of bulk materials, the particle size range of the reference material should be comparable to that in the dust being analyzed. Bulk materials are often subjected to a reproduceable grinding technique and are ground and passed through a 325 mesh sieve (47 micron) in order to prepare a dust suitable for analysis. Due to agglomeration and the irregular particle shapes often produced by grinding, sieving with a 325 mesh size generally results in crystallites in the dusts that are much smaller than 47-microns. Different matrices have different properties and tendencies to agglomerate and fracture during grinding. The particle size of the resulting crystalline dust can vary considerably depending on the hardness of associated materials in the bulk material. A standard addition technique may be best for these analyses. One should consider adding a high purity quartz sand to the bulk material and carrying it through the full process as this may result in better recoveries than using a respirable crystalline silica reference material. (See also next section.) Alternately, if the manufactured material is already a dust, standard addition can be made using the appropriate size high-purity quartz flour such as Min-U-Sil 5, Min-U-Sil 10, or Min-U-Sil 20 (Pennsylvania Glass Sand Co., Berkley Springs, WV). The Min-U-Sils' sources vary geographically and particle size distribution varys from lot to lot, so the selection should be based on performance. The Min-U-Sils should be sieved before use to help homogenize the material and to remove "boulders."

Respirable Dust Analyses - Air samples and Bulks

When large quantities of respirable reference material are needed, e.g., for use in quality control in an industry, good analytical policy is to use the expensive primary respirable reference material to make comparisons with a similar material such as Min-U-Sil 5 which can be purchased in large quantity and can be used as a secondary respirable reference material. The Min-U-Sils should be sieved before use to help homogenize the material and to remove "boulders." It should then be blended before use to homogenize it further. To provide greater utility in dealing with interferences in XRD, the comparisons should be made on at least the three most intense peaks. The secondary reference material and the recovery factors obtained in the comparisons for the three peaks can then used be used in routine analyses.


Quartz

Some laboratories have used samples from previous rounds of the American Industrial Hygiene Association (AIHA)Quartz Proficiency Analytical Testing (PAT) Program as calibration standards (based on the consensus results from reference laboratories). This practice is not recommended as these samples can vary 10 - 20% in the aerosol generation process and the consensus value is an average of values having a wide spread with correspondingly 95% confidence limit. They are valuable tools to check proficiency, but not for calibration standards.

In the USA, the National Institute of Standards and Technology (NIST) Respirable Quartz Standard Reference Material, SRM 1878, has been the consensus standard for the analyses of respirable dusts containing quartz. This SRM has not been available for a couple years now. Fortunately, the replacement SRM 1878a is all packaged up and ready to ship pending clearance which is supposedly just days away. The previous SRM contained a few "boulders," so it was recommended that it be sieved before use. [Standard Reference Materials Program, Rm. 204, Bldg. 202, National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899. Phone: (301)975-6776 Fax: (301)948-3730]

For those methods using a muffle furnace of low temperature asher, SRM 2679a is available from NIST. This SRM consists of known amounts of SRM 1878 deposited on mixed cellulose ester (MCE) filters. [Standard Reference Materials Program, Rm. 204, Bldg. 202, National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899. Phone: (301)975-6776 Fax: (301)948-3730]


Cristobalite

Likewise the NIST Respirable Cristobalite Reference Material, SRM 1879, is the consensus standard for the analyses of respirable dusts containing quartz. It came into short supply, but a replacement is on the way. [Standard Reference Materials Program, Rm. 204, Bldg. 202, National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899. Phone: (301)975-6776 Fax: (301)948-3730]

Tridymite

In NIOSHMethod 7500 (XRD), NIOSH offers a suitable Tridymite SRM to those who need one. (NIOSH, DPSE, MRH, 4676 Columbia Pkway., Cincinnati, OH 45226. Technical Information Hotline Phone: (800)-35-NIOSH; Fax: (513)-533-8573)
 



IV. Communications with Your Industrial Hygiene (IH) Laboratory: Before You Sample, When You Submit Your Samples for Analysis, and After You Get Your Results
    NOTE The information in this section of the document is provided as an example. It should not be construed as representing OSHA requirements or policy.
What information should we exchange?

Good communication with your IH laboratory is important to get the right treatment for your samples and to get the most reliable results. As a consumer of analytical services, you need to address the whole analytical "life-cycle."

A. Laboratory Selection. Twice annually, the AIHA publishes the geographical listing of 300+ accredited industrial hygiene laboratories along with any purchased advertisements describing the services. Of these approximately 70 are accredited to analyze crystalline silica see, Current List of (Industrial Hygiene Laboratory Accreditation Program  - IHLAP) Accredited Laboratories. Those performing silica analysis are indicated by "Silica" in the Analysis column.

The following items represent good science and include some of the accreditation requirements.
  1. Licensed and certified as needed in state.
     
  2. Qualified and trained personnel performing the analyses.
     
  3. Versed in the analytical method you need for your sample matrix.
     
  4. Provides a list of interferences for the analytical method that will be used.
     
  5. Quality Control program in place.

    a. Calibration standard reference materials traceable to NIST or NIOSH.

    b. Quality Control or Quality Assurance samples prepared and analyzed along with regular samples.

    c. Quality Control results charted. Policy for QCs that go out of control. Are results released regardless of QC results?

    d. Calibration check samples that bracket the sample results (re)analyzed along with regular samples.

    e. Written analytical procedure adhered to (modified only for good analytical reason.)

    f. Maintenance program.

    g. Turn-around time meets your time constraints.

    h. Data provided laboratory and results are secure and confidential. Trade secrets are not accessible.
B. Method Sampling Requirements.
  1. Proper leak-tested cyclone.
     
  2. Correct sampling medium.
     
  3. Calibrated pump with cyclone and assembled cassette in place.
C. Sampling.
  1. Consult the list of interferences provided by the laboratory. Note whether these interferences are known to occur in the worksite to be sampled.
     
  2. Collect air sample in the employee's breathing zone.
     
  3. Collect any bulks requiring analysis or provided for lab use if needed. The analysis of your manufactured product(s) may be needed in supporting the Hazard Communication (HAZCOM) standard. When the quartz content exceeds 0.1% quartz a MSDS may be required. Products containing low levels of quartz may pose a difficult analytical problem because it is often difficult to measure quartz content down to the 0.1% quartz level required for HAZCOM compliance. Different methods (and perhaps a different laboratory) than used for air samples may be needed for bulk material. Bulk analyses may require methods such as microscopy (refractive index and birefringence) and thermal analysis (heat released at the beta to alpha quartz transition near 573 deg. C). Manufactured articles, such as tiles and bricks, require MSDSs if the potential exists to produce respirable dust in normal use. Normal use of these articles in construction includes sawing. Prudent sawing requires precautions such as the use of wet sawing techniques and methods of cleanup that avoid creating suspended dust. It has been ruled that, logically, an employee cannot take the proper precautions to avoid exposure of self and others to respirable crystalline silica if that information is not provided.
     
  4. Ship according to laboratory's procedure including a list of any interferences you think may be present in the air samples.
D. Analytical Method is Performed at the Laboratory.
E. Results Reported.
  1. What method does the laboratory use.
     
  2. Are the results confirmed before releasing results to the customer? For example, if the lab does not confirm a result based on the most intense X-ray diffraction peak of quartz, would the lab could report mica or graphite contaminants as quartz?
     
  3. Are problems with QCs resolved before releasing results to the customer?
     
  4. What is the laboratory's estimate of the sampling and analytical error (SAE)?
     
  5. What is the laboratory's qualitative and quantitative detection limits (in % and microgram units)?
     
  6. For instrumental methods that can provide a graphic portrayal of the scan as in XRD, IR, and FTIR, are scans of your samples saved with the data?
     
  7. Can the method used reveal unknown interferences?
     
  8. What actions are taken when an interference is observed?
     
  9. For instruments that can provide scan peak data as in XRD, IR, and FTIR, are integrated (summed) peak areas or peak height instrument responses used to determine the amount of analyte present in the samples?
     
  10. Are scans, calculations, and results of the analyst checked independently by someone else trained on the method?
     
  11. How are results presented to the customer? Is the current laboratory SAE and DLs provided? Are result descriptors such as "less than" and "less than or equal to" explained adequately? Do these descriptors describe the qualitative or quantitative detection limit, lack of confirmation, or a "conservative" worst case result?


V. Related Useful Information
  • Crystalline Silica Primer [99 KB PDF, 29 pages]. US Department of Interior (DOI) / US Bureau of Mines.

  • Review of Quartz Analytical Methodologies: Present and Future Needs (Details strengths and weaknesses of various methods. )

Abstract:

A review of analytical methods for the qualitative and quantitative determination of crystalline silica is presented. The three prevalent forms of crystalline silica--quartz, cristobalite, and tridymite--are alluded to. Performance and popularity of present quartz analytical methods such as X-ray diffraction (XRD), infrared (IR) spectroscopy, and colorimetry are illustrated using Proficiency Analytical Testing (PAT) Program data. Although substantial improvements in performance have been made since the 1970s, quartz PAT sample results still continue to display high imprecision (>20% coefficient of variation). Past and present analytical methods are detailed in terms of theory and use, and strengths and weaknesses are discussed. Methods include gravimetric, chemical, microscopic, atomic absorption, XRD, IR, and colorimetry. Methods for determining bulk materials such as thermal analysis and nuclear magnetic resonance are briefly discussed. Results of a survey regarding international use of analytical methods for quartz are displayed. Both in the United States and internationally, the most popular methods are XRD and IR. Performance criteria such as detection limits, precision and accuracy, and potential future international trends are shown using international data. Future needs for enhancing quartz analysis, such as comprehensive evaluation of direct-on-filter techniques, an attempt at consensus for tridymite analysis, a consensus for defining analytical performance parameters for quartz in general, and a continued examination of methods for analyzing bulk materials for quartz, are suggested. MADSEN, F.A.; ROSE, M.C.; CEE, R.: REVIEW OF QUARTZ ANALYTICAL METHODOLOGIES: PRESENT AND FUTURE NEEDS. APPL. OCCUP. ENVIRON. HYG. 10(12):991-1002; 1995.

The December 1995 Issue of Applied Occupational and Environmental Hygiene (Volume 10, Number 12) was a special focus issue presenting the "Proceedings of the International Conference on Crystalline Silica Health Effects: Current State of the Art." This journal is published by Elsevier for the American Conference of Governmental Industrial Hygienists (ACGIH).



VI. Frequently Asked Questions (FAQs)

The following examples deal with common types of sampling and analytical questions received by members of the OSHA X-ray Team from industrial hygiene professionals in laboratories and the field. These include persons employed in foreign, federal, state and local governments; private laboratories; manufacturers; industry; construction; and insurance. Topics addressed in the answers often diverge from the question; therefore, if you have a question that does not appear in the list below, try using your browser's "Find" feature (usually a control F command) to locate key words on this page. 


1. How does the OSHA Permissible Exposure Limit (PEL) of 10/(%Quartz + 2) compare to the American Conference of Government Industrial Hygienists (ACGIH) old Threshold Limit Value (TLV) of 0.1 mg/m3?


2. How does a PVC filter with a 5-micron pore size capture submicron size dust?


3. Can I sample crystalline silica using low-ash 0.4-micron pore size AA (MCEF) filters instead of low ash PVC filters?


4. Nylon has a problem with developing a surface static charge during use. I've been told that I can use a metal cyclone operated at a specific flow rate to give the desired 50% cutpoint at 3.5 microns. Why do we still use nylon cyclones rather than the newer metal cyclones or conductive plastic cyclones?


5. How do I perform a crystalline silica mixture calculation when the sample contains both quartz and cristobalite?


6. How do I perform a leak test of the Dorr-Oliver cyclone?


7. I am in the construction industry and can't locate any lab that analyzes quartz in terms of millions of particles per cubic foot (mppcf) using microscopy. How should I sample and analyze for quartz?


8. How do I interpret a company's crystalline silica sampling data when it doesn't list the percentage quartz and all I have are the TWA exposure mg/m3in terms of quartz and the corresponding TWA exposure mg/m3for the mixture of respirable dust containing quartz? For example, say I'm told that the quartz exposure was 0.066 mg/m3and the mixture exposure was 3.0 mg/m3.


9. How do I use the percentage quartz results from the analysis of the bulk samples to determine whether the OSHA PEL for respirable dust containing quartz has been exceeded on the air samples?


10. The material safety and data sheet (MSDS) for a product consisting of food-grade calcium silicate indicates that (for example) it contains:

19% CaO
67% SiO2
6 to 8% H2O and
less than 0.1% free silica.
How can that be; isn't SiO2 "free" silica? Should I sample for quartz?



11. We need OSHA's sampling and analysis error (SAE) for the analysis of crystalline silica in order to apply it to the results we get from our private contract lab. Where can we obtain OSHA's SAE?


12. We need OSHA's detection limit (DL) for the analysis of crystalline silica in order to apply it to the results we get from our private contract lab. Where can we obtain OSHA's DL?


13. Can AIHA Proficiency Analytical Testing (PAT) data for a laboratory from recent rounds be used to estimate the analytical error portion of the sampling and analysis error (SAE)?


14. How can we resolve possible misidentification of opal C or other materials that mimic cristobalite in the analysis of cristobalite?


15. The process I need to sample involves silica (quartz, fused silica, diatomaceous earth, etc.) that reaches a temperature of only 1100 C – too low for the formation of cristobalite. Why should I analyze for cristobalite?


16. We are removing some old fire brick, fiber glass (or rock wool) heat insulation from a furnace. Can these materials contain crystalline silica?


17. XRD is not as sensitive to submicron crystalline silica. Is this an important consideration?


18. I need to sample a sand-blasting operation in a room that has no forced ventilation. The blasting lasts only 20 minutes each day. The employee wears a hood only during the operation. The employee spends 8 hours a day in the same room with other employees performing other activities. How long should I sample?


19. Just what is "free silica?"



VII. Responses to Frequently Asked Questions

Topics addressed in the answers often diverge from the question; therefore, if you have a question that does not appear in the list above, try using your browser's "Find" feature (usually a control F command) to locate key words on this page. If you have any question dealing with sampling and analysis of crystalline silica call us and we will try to assist. (Voice: 801-233-4930; FAX: 801-233-5000)



1. How does the OSHA Permissible Exposure Limit (PEL) of 10/(%Quartz + 2) compare to the American Conference of Government Industrial Hygienists (ACGIH) old Threshold Limit Value (TLV) of 0.1 mg/m3?
 

Most OSHA PELs are set values for a single air contaminant such as cadmium or a related family of contaminants such as the polyaromatic hydrocarbons (PAHs). The PEL for respirable dust containing quartz differs considerably in that it is a function that varies between a value of 0.1 mg/m3(when the material is pure quartz) up to a value approaching the Particulates Not Otherwise Regulated (PNOR) OSHA PEL of 5 mg/m3. The PEL does not apply below 1% quartz, so the highest it can get is 3.3%. (When the concentration drops below 1%, the PNOR PEL of 5 mg/m3applies.) It can be shown mathematically that when this PEL function for respirable dust containing quartz is divided into the TWA exposure, the resulting standardized concentration (or exposure severity) is the sum of the standardized concentrations for the separate quartz and PNOR exposures. This derivation requires only simple algebra, but is available in the literature [Frank J. Hearl, "Mixture Formula Justified," Letters to the Editor, AIHA Journal (57) (1996, June), p 575 and also in Frank J. Hearl, "Guidelines and Limits for Occupational Exposure to Crystalline Silica," in Silica and Silica-Induced Lung Diseases, V. Castranova, V. Vallyathan, and W. E. Wallace Eds., CRC Press Inc. pp.15-22.] [Note that the derivations count the quartz twice because it is not subtracted from the dust exposure in determining the PNOR exposure. If the quartz were subtracted, the quartz standard of 0.098 mg/m3would result which is only very slightly more restrictive than the 0.1 mg/m3 proposed in the (now vacated) Final Rule.] Through the same approach, it is easy to derive the PEL for the mixture of respirable dust containing quartz and cristobalite [ PEL = 10/(%Quartz + 2(%Cristobalite) + 2) ]. The use of these PELs achieves the same result as using the mixture calculation specified in 29 CFR 1910.1000 for exposures to substances having an additive effect on the body or target organ system.

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2. How does a PVC filter with a 5-micron pore size capture submicron size dust?
 
The low-ash 5-micron PVC filter is the preferred sampling medium for respirable dust containing crystalline silica and is used in the American Industrial Hygiene Association (AIHA) Performance Analytical Testing (PAT) Program. The 5-micron pore size filters reduce problems associated with sample loading and back pressure. This condition is important in maintaining a constant sampling rate in dusty work environments.

There are several reasons generally attributed as to why these filters work well in this application.

The pore size in this type of membrane filter is just one description of the filter medium. The 5-micron size for this filter is not the physical dimension of holes at the surface of the filter; it is based on the pressure drop across the filter. The electron microscope shows the surfaces to be a maze of PVC strands presenting passageways much smaller than 5 microns. The "smooth" surface appears as a spongy mat on which impaction can occur. Pore paths are not linear so particles do impact surfaces.

Smoothside .

 
Dullside
 
This is the smooth shiny side of clean 5-micron filter (Omega P503700)   This is the rough dull side of the same clean filter.
Such filters do not behave merely like sieves and have good collection efficiencies for air suspended dusts well below the pore size. The Omega Specialty Instruments 5-micron PVC membrane filter P503700 is used in the MSA 800019 cassette in the OSHA automated weighing program. In capture efficiency tests, the P503700, for example, retained 99.9% of respirable coal dust. Particles are captured primarily by impaction and diffusion (Morris Katz, Ed., Methods of Air Sampling and Analysis, 2nd Ed., American Public Health Association, Washington, DC 1977, p 199 and University of Minnesota Short Course on Aerosol and Particle Measurement, Aug. 22-24, 1994). Dust particles tend to agglomerate (clump or stick together) due to Van der Waals, polar and ionic surface features, and hydrogen-bonding forces. [Crystalline silica surfaces exposed to the air rapidly become covered with silanol (Si-OH) groups that can ion exchange and hydrogen bond.] Through agglomeration, smaller particles join together and with larger particles to be more readily captured on the filter than isolated particles. Filter loading also increases filter efficiency; as the filter begins to pick up dust, the effective pore size decreases. Dusts are rarely spherical. For example, quartz dust produced by abrasive and grinding processes generally appears under the microscope like shards of broken glass. So in some orientations they often present a larger cross section for capture than their equivalent spherical diameter may indicate. Most aerosols in the workplace carry a static electrical charge. When they contact the filter they induce an opposite charge to form at the surface, enhancing the surface attraction. Also, uncharged particles can develop charge; when different materials (the aerosol and the membrane) come in contact they become oppositely charged and attracted to one another. [Note: Currently available PVC filters of the smaller pore size (0.5 micron) do not dissolve properly in tetrahydrofuran (THF) and therefore cannot be analyzed by OSHA method ID-142.]

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3. Can I sample crystalline silica using low-ash 0.4-micron pore size AA (MCEF) filters instead of low ash PVC filters?

Generally speaking, no.

The weights of MCEF filters are affected greatly by slight variations in humidity and are not suitable for the gravimetric portion of the analysis of respirable dusts containing crystalline silica.

Additionally, MCEF filters do not dissolve properly in THF and therefore the crystalline silica collected on MCEF cannot be readily analyzed by OSHA method ID-142.


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4. Nylon has a problem with developing a surface static charge during use. I've been told that I can use a metal cyclone operated at a specific flow rate to give the desired 50% cutpoint at 3.5 microns. Why do we still use nylon cyclones rather than the newer metal cyclones or conductive plastic cyclones?

Manufacturers often refer to the 50% cutpoint of 3.5 microns as being the critical distinction, but the other cutpoints specified in 29 CFR 1910.1000 Table Z-3 are equally important. These sampling efficiencies are at various aerodynamic particle sizes that compare well with both the then-current model for respirable dust deposition in the human respiratory tract and with the sampling efficiency of the AEC instrument consisting of a 1-cm Dorr-Oliver cyclone made with a nylon body. (The nylon body is not to be confused with the sampler holder which may have a metal protective tube surrounding the cyclone body.) Currently the only cyclone we are aware of that has the characteristic sampling efficiencies listed in 29 CFR 1910.1000 Table Z-3 is the Dorr-Oliver nylon cyclone. A device that does not develop a static charge would probably give more reproducible sampling and represent an improvement, but until an equivalent conductive device becomes available, we use the Dorr-Oliver nylon cyclone. Please inform the Salt Lake Technical Center if you are aware of another cyclone that matches the characteristics in 29 CFR 1910.1000 Table Z-3.

From 1910.1000 Table Z-3:


(e) Both concentration and percent quartz for the application of this limit are to be determined from the fraction passing a size-selector with the following characteristics:

Aerodynamic diameter (unit density sphere) Percent passing selector
2 90
2.5 75
3.5 50
5.0 25
10 0

The measurements under this note refer to the use of an AEC (now NRC) instrument. The respirable fraction of coal dust is determined with an MRE; the figure corresponding to that of 2.4 mg/m(3) in the table for coal dust is 4.5 mg/m(3).

(Note: Aerodynamic diameter unit is in terms of microns. The aerodynamic diameter (unit density sphere) is often referred to as the equivalent aerodynamic diameter. This measure differs from the size that would be measured under a microscope; a quartz sphere with an equivalent aerodynamic diameter of 3.5 micron would be about 2.2 micron in diameter under the microscope. M. C. Rose)


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5. How do I perform a crystalline silica mixture calculation when the sample contains both quartz and cristobalite?

Reproduced here is Figure I:1-14 from TED 1.15 Chapter 1 Appendix I:1-5. This example also shows how to combine the results of two samples for a TWA exposure calculation. Excel 4* and Excel 7* spreadsheets to perform this calculation are available for CSHOs or may be modified for other IHs.

Please note: An error was found in the previous spreadsheet version where the cristobalite and tridymite cells were improperly used in the calculation. The current spreadsheets fix that bug. (Revised 10/28/97 MCR)

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6. How do I perform a leak test of the Dorr-Oliver cyclone?


[The Cincinnati Technical Center is putting together a kit with instrumentation and instructions for USDOL/OSHA CSHOs.]

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7. I am in the construction industry and can't locate any lab that analyzes impinger samples collected for quartz in terms of millions of particles per cubic foot (mppcf) using microscopy. How should I sample and analyze for quartz?


That methodology is obsolete. The impinger samples need to be counted within 24 hours. Errors associated with the analyses are unknown and the method does not provide information about the particulate composition. Separate samples must be taken to determine the % crystalline silica in the samples. So the microscopy method is impractical. Instead, resample using the cyclone and collect on tared low-ash PVC filters. Analyze by XRD, IR, or by a Chemical Method as appropriate for your sample matrix and apply the equivalent OSHA PEL for respirable dust containing quartz, 10/(%Quartz + 2) in terms of mg/m3.

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8. How do I interpret a company's crystalline silica sampling data when it doesn't list the percentage quartz and all I have are the TWA exposure mg/m3 in terms of quartz and the corresponding TWA exposure mg/m3 for the mixture of respirable dust containing quartz? For example, say I'm told that the quartz exposure was 0.066 mg/m3 and the mixture exposure was 3.0 mg/m3.


There are two mathematically equivalent ways you can determine the standardized exposure of the mixture:

a) From the response to FAQ #1 above, one could obtain the sum of the standardized exposure to quartz at an exposure limit of 0.100 mg/m3 and the standardized exposure to the mixture at a PEL of 5.0 mg/m3. In the example exposures given above, the summed standardized exposure is 0.066/0.100 + 3.0/5.0 = 0.66 + 0.60 = 1.26. This method may be how the company tracks exposures. The practical advantage is that the component standardized exposures are determined separately, which may help in evaluating the engineering actions needed to bring the exposure to the mixture under control. The company may be using the 0.050 mg/m3 respirable quartz REL or TLV as a more restrictive exposure limit; it is still important that they consider the additive effect of the exposures to both quartz and PNOR.

b) For compliance purposes calculate the percentage quartz by taking 100 times the ratio of the TWA quartz exposure divided by the TWA mixture exposure. (The equal volumes of air in the concentration units cancel out leaving a weight ratio.) In the example exposures given above, the percentage quartz is 100 (0.066/3.0) = 2.2%. From this the PEL is 10/(2+2.2) = 2.38 and the standardized exposure would be 3.0/2.38 = 1.26, which is the same result as in (a) above.

Note: Calculating the standardized exposure both ways can provide a check on your math, but results from methods (a) and (b) may differ slightly due to round off of intermediate results.

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9. How do I use the percentage quartz results from the analysis of the bulk samples to determine whether the OSHA PEL for respirable dust containing quartz has been exceeded on the air samples?


Please Don't. Analytical results on the quartz content of the air samples are necessary to evaluate whether the OSHA PEL is exceeded. Results for bulk materials do not necessarily represent the composition of the respirable dust suspended in the air at the time of sampling. Bulk materials are analyzed primarily to establish the potential for exposure or to assist the laboratory when interferences are encountered or might be present in the air samples.

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10. The material safety and data sheet (MSDS) for a product consisting of food-grade calcium silicate indicates that (for example) it contains:

19% CaO
67% SiO2
6 to 8% H2O and
less than 0.1% free silica.
How can that be; isn't SiO2 "free" silica? Should I sample for quartz?


No and no.

The description indicates that the SiO2 is not "free silica." "Free silica" includes amorphous silica and the crystalline forms that are not chemically combined with any other elements. That is to say, silica that is not "free" is chemically bound in another compound. In this material, the element silicon is present in the form of silicate compounds and is not in the form of a compound consisting of SiO2 as implied by the analytical data. The confusion here is due to the use of the mineralogist's convention in representing the chemical composition of the product as component oxides. This use is a common and often practical way to report analytical results and, in this case, does not represent a mixture of the compounds CaO, SiO2, and H 2O. Users of MSDSs should be aware of this common convention, but the distinction may not be emphasized even in undergraduate chemistry courses. The material was probably analyzed for Ca, Si, and weight loss on heating. When the results of the analysis of the product were reported, the gravimetric factors for the oxides were used with oxygen making up the remainder of the composition. This assumed oxide composition satisfies the chemical combining capacity of Ca and Si for compounds formed under normal conditions. The practical value of using oxides can be demonstrated by looking at the mineralogist's oxide representation of some of the various calcium silicate compounds that might be present in this product:

CaSiO3 or CaO.SiO2
Ca2SiO4or 2CaO.SiO2
Ca3SiO5or 3CaO.SiO2

The downside is that the convention does not always address health concerns.

(a) In addition to the ambiguity problem noted in the case of crystalline silica, the mineralogist convention also causes difficulty in interpreting some MSDSs for compounds containing pentavalent vanadium e.g., manganese vanadate Mn3(VO4)2analytical data may report percentages of MnO and V2O5when in fact the compounds MnO and V2O5may not be present in detectable amounts. Mn3(VO4)2does not have an OSHA PEL whereas the compound V2O5does.

(b) On the other hand, compounds containing hexavalent chromium such as PbCrO4 may be represented analytically in terms of PbO and CrO3. The current (March 1997) OSHA PEL (0.100 mg/m3 as CrO3) for hexavalent chromium is as CrO3 using a gravimetric factor of 1.923 to convert Cr(VI) results to be in terms of CrO3. This factor is applied regardless of whether oxygen is in the chemical formula, so in this case, the mineralogist's convention has applied a gravimetric factor that happens to be useful to the health professional.


Note: The term, "free silica" is often used loosely to refer to quartz; quartz is a "free silica," but not all "free silica" is quartz.

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11. We need OSHA's sampling and analysis error (SAE) for the analysis of crystalline silica in order to apply it to the results we get from our private contract lab. Where can we obtain OSHA's SAE?


This request indicates some confusion with regard to industrial hygiene analyses and the meaning of SAE. Generally speaking, neither NIOSH nor OSHA mandate either specific analytical procedures or analytical equipment. Good science should prevail, and when permitted, other methods may be used. Each IH laboratory should determine its own SAE values based on the analysis of quality control samples, spiked samples or other tests of recovery. The private contract lab should be contacted for its SAE estimates for the time period of the analyses.

The SAE is not a universal constant but is a time-varying function of instrument and analyst performance for particular analytes. The OSHA PEL for dust containing quartz is given by PEL = 10 / (%Quartz + 2) , where the %Quartz in the respirable dust is calculated from the analytically determined mass of quartz in the gravimetrically determined mass of the dust. The standardized concentration is a measure of exposure severity to which the SAE is applied to confidently determine whether an overexposure has actually occurred. The standardized concentration is: Standardized Concentration = Exposure / PEL, where the Exposure term refers to the gravimetrically measured time-weighted average concentration (mg/m3) of respirable dust containing quartz. So the application of the SAE for respirable dust containing quartz involves two analyses: Gravimetric and quartz analyses.

The OSHA SAE for quartz analysis is provided to the OSHA compliance officer in the same report as the analytical results. Assuming a 5% error in the flow rate, the SAE for quartz analysis is typically around 20% for 95% confidence.

So strictly-speaking, the SAE for such analyses contains weighted components for the errors associated with the flow rate, gravimetric, and quartz portions of the analyses. The function for the overall SAE (at 95% confidence) that can be derived for the combined errors of the two analyses varies between about 10% for respirable dust with little quartz to 20% for respirable dust that is nearly pure quartz. Using the SAE for quartz given with the analytical report, therefore, provides a better than 95% confidence level. The PEL calculation for respirable dust containing both quartz and cristobalite is given by:

PEL = 10 / [2 + %Quartz + 2(%Cristobalite)],

so a similar situation arises.


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12. We need OSHA's detection limit DL for the analysis of crystalline silica in order to apply it to the results we get from our private contract lab. Where can we obtain OSHA's DL?


As discussed in FAQ # 11 above re. SAEs, contact your contract lab for the detection limit data it has determined for their analyses.

The qualitative detection limit is usually defined as the amount of analyte equivalent to three times the standard deviation of the instrument response for low level spikes on sampling media using the slope of the calibration curve of low level calibration standards. The low level spikes may be made either at a single low level to obtain the standard deviation, or the standard error of estimate of the regression for several low level spikes bracketing the detection limit may be used instead of the standard deviation at a single level. Ten or more spiked samples are recommended depending on whether spikes are at a single or several levels.

The quantitative detection limit is usually defined as 3.33 times the qualitative detection limit defined above.


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13. Can AIHA Proficiency Analytical Testing (PAT) data for a laboratory from recent rounds be used to estimate the analytical error portion of the sampling and analysis error (SAE)?


No.

The PAT program is designed to help consumers select laboratories that are proficient. In the PAT program analyses of quartz, the "true" values against which a laboratory's results are compared are based on results from reference laboratories that are a subset of the participating laboratories. Assuming that the PAT samples were made from accurately delivered consensus reference material and that the participants all used the same techniques, instrumentation and methodology, and the samples are not otherwise flawed to introduce bias, the best accuracy that can be achieved by consensus analyses is limited by the standard error of the precision of that analysis [SD/(n), where SD is the standard deviation in the results among the n reference labs].

But the PAT program is not suited to achieve the best accuracy: In the case of crystalline silica analyses, the analytical equipment and methods vary between labs. X-ray diffraction, infrared spectroscopy (IR), and colorimetric (chemical) analyses are all in use. The reference material used in these analyses may also differ in homogeneity and quality from the National Institute of Standards and Technology (NIST, formerly the NBS) standard reference material SRM 1878. NIST SRM 1878 is strongly recommended as a consensus standard for calibrations and tests of analytical accuracy.

The current method of PAT quartz sample generation is by aerosol generation using "5 micron" Min-U-Sil 5 without cyclones. In addition to any errors in the generation process, this "total dust" approach introduces a sampling error that may not duplicate the sampling error associated with the use of a cyclone.

In the PAT program, these generation and sampling errors are recognized as significant and are evaluated in statistical tests conducted on sub-batches and batches of PAT samples by the contract laboratory that prepares them. Part of each batch is sacrificed for characterization. The criterion of acceptability at each concentration level (sub batch) of PAT quartz samples permits the characterization consisting of 7 samples to have a maximum CV of 30% at each concentration level of quartz. The corresponding 95% confidence range would be about 42%. Additionally, a maximum average CV of 20% (for 28 samples) each set of 4 concentration levels (batch) is permitted. These criteria are currently under review by the AIHA and NIOSH. The results obtained by participants in the PAT program therefore include both the analytical error the participating laboratories introduce and an unknown but potentially large amount of error introduced in the generation and sampling of the aerosol. These latter errors may vary batch to batch.

Therefore, more appropriate data for monitoring analytical recovery and variability in the analysis of respirable quartz might be obtained by labs using SRM 1878 (or comparable reference material) to prepare blind QCs for their in-house quality control program. Quality control samples are typically prepared from aliquots of well-dispersed suspensions of the SRM in a solvent or are prepared gravimetrically. NIST prepares its SRM 2679a Quartz on Filter Media by delivery of a water suspension of SRM 1878 using a thickening agent to ensure homogenous suspensions. The true value is determined spectroscopically with a demonstrated precision of 4 to 11% depending on spike level. The deposit is overlain with a layer of clay. The filter medium is well suited for some IR methods, but unfortunately does not dissolve in the solvent used for XRD analyses.

Gravimetric QCs may give more accurate delivery and may be prepared by weighing the SRM or equivalent material onto tared filters or by delivering SRM to a filter as an aerosol (as in the case of the direct-on-filter methods). (Particle size effects can have opposite effects on recoveries in XRD and IR. The effect of particle size on the colorimetric method also strongly depends on analyst technique.)

See also, Sources of Crystalline Silica Reference Materials.


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14. How can we resolve possible misidentification of opal C or other materials that mimic cristobalite in the analysis of cristobalite?


One resolution of this problem requires a combined approach using a modified Talvitie chemical treatment (see description and reference in OSHA ID-142 method). In order to optimize the conditions for removal of the interferent, recovery tests using cristobalite-spiked bulk material is needed to ensure that cristobalite is not being removed along with the opal-C.

This procedure is tedious and is only performed when absolutely necessary. At the OSHA SLTC, the efficient use of resources is brought about by the procedures for analyzing compliance air samples for cristobalite given in method ID-142. The type of industrial operation determines whether cristobalite is analyzed. Air samples suspected of containing cristobalite are first screened on the most intense X-ray diffraction peak (JCPDS 11-695 an undistorted cristobalite). Based on the screening, if the exposure is above the detection limit and possibly over a significant fraction of the OSHA permissible exposure limit (PEL), an attempt is made to confirm on the next 2 most intense peaks. A consensus Standard Reference Material (SRM) for respirable quartz analyses is currently manufactured by National Institute of Standards and Technology (NIST, formerly the NBS). The best confirmation is achieved both quantitatively and qualitatively. We calibrate the instrument to provide quantitative results on all four quartz diffraction peaks – typically three are used. The silver membrane used to support the dust is used as an external 2-theta calibration; the peaks should agree within 0.05 degrees 2-theta of the location of the NIST Standard Reference Material. If the presence of cristobalite is confirmed, a wide angle scan is performed on the bulk material(s) that may be supplied with the air samples to look for potential interfering phases. We discuss any unusual situations that arise with the compliance officer who performed the inspection.


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15. The process I need to sample involves silica (quartz, fused silica, diatomaceous earth, etc.) that reaches a temperature of only 1100 C – too low for the formation of cristobalite. Why should I analyze for cristobalite?

Contrary to the usual diagram for the conversion of silica to tridymite (the diagram shows cristobalite requiring higher temperatures), silica in the temperature regime of 867 and 1470 C will most often convert to a disordered cristobalite phase favored by free-energy considerations rather than tridymite. The extent of disordering depends in part on the presence of trace contaminants and on the thermal history of the material (sequence and time of heating at the various temperatures encountered in the process.

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16. We are removing some old fire brick, fiber glass (or rock wool) heat insulation from a furnace. Can these materials contain crystalline silica?
 
The hazards in removal need to be evaluated – if no cutting, breaking, grinding, etc. is involved there may be no hazard.

In these materials devitrification (conversion of the glassy to the crystalline state) may have occurred. Devitrification can occur in fire brick, obsidian, pumice, fused silica, fiber glass, and rock wool held at elevated temperatures producing disordered crystalline silica forms. These materials should be analyzed for the type and amount of crystalline silica present before removal so proper protection can be provided to the employee. Even without good XRD quantitative confirmation, crystalline silica may be qualitatively identified.


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17. XRD is not as sensitive to submicron crystalline silica. Is this an important consideration?

No. For at least two reasons:

a. The smallest quartz particles are generally less significant in relation to the total weight of quartz in a sample. The larger particles in an aerosol generally represent most of the mass of the aerosol because the mass varies as the cube of the diameter. It takes 125,000 particles, 0.1 micron in size, to equal the mass of a single particle 5 microns in size.

b. Due to the presence of an amorphous silica layer on crystalline silica particles, a smaller proportion of the mass of small particles is crystalline silica. The amorphous layer is estimated to be approximately 0.03 micron thick (N.J. Elton, P.D. Salt, and J.M. Adams, "The Accurate Determination of Quartz in Kaolins by X-ray Powder Diffraction," Issues and Controversy: The Measurements of Crystalline Silica, International Symposium, August 20-21, 1992, Cambridge, MA. CMA Crystalline Silica Panel.) A 0.03-micron layer on a spherical 1-micron diameter quartz particle represents 17% of the volume of the particle leaving about 83% crystalline. The relative volume of the amorphous layer increases rapidly with decreasing diameter; a 0.5-micron diameter quartz particle would be about 68% crystalline, and a 0.1-micron diameter "quartz" particle would be about 6.4% crystalline. These are high estimates because dusts are rarely spherical. As a result, the amount of crystalline silica decreases rapidly as particles become smaller.

 
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18. I need to sample a sand-blasting operation in a room that has no forced ventilation. The blasting lasts only 20 minutes each day. The employee wears a hood only during the operation. The employee spends 8 hours a day in the same room with other employees performing other activities. How long should I sample?

Sample the whole 8-hour work day.

If respirable silica dust is generated during the blasting, it will remain suspended long afterwards. The air is rarely still enough for the dust to completely settle. The activities of the employee and his/her coworkers would also resuspend dust that has settled out on their clothes. Dust can also be resuspended when employees pass by or over dusty surfaces such as the factory floor and when they perform any inappropriate cleanup operations such as dry sweeping or using an ordinary shopvac. While IHs are often tempted to sample only during the dust generation operation, doing so will underestimate the actual exposure – the result of such sampling may easily show up as non-detected, whereas the exposure may be much larger. For example, if the 20 minute sampling gave a result just below the detection limit (say, 10 micrograms of quartz) and the air concentration averaged just half that level throughout the rest of the day, the total quartz exposure would be about 0.150 mg/m3 – well over the TLV. If this dust was a typical % quartz composition of say, 20%, the total weight collected over the day would have been 0.77 mg/m3 giving a severity of exposure of 0.77/[10/(2+20)]=1.7 or 1.7 times the OSHA PEL. The IH should consider such consequences of short-duration sampling for any air contaminant.


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19. Just what is "free silica?"

The term, "free silica" is often used loosely to refer to quartz; quartz is a "free silica," but not all "free silica" is quartz. Amorphous silica and cristobalite are also examples of "free silica." See FAQ #10 for distinctions.

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VII. Follow-up of 1997 National Conference to Eliminate Silicosis (Washington, DC)

Observations

Workshop #6, Sampling and Analysis, 1997,"Elimination of Silicosis Conference," Washington, D.C., was a very successful workshop session.

Of course, a fully successful conference will be judged by whether silicosis is eradicated. The presenters have done their part at the 1997 National Conference to Eliminate Silicosis and are continuing to do their part in supporting that endeavor. We're in it for the long haul.

The Sampling and Analysis Workshop (#6) was well attended. Two sessions had been originally planned to meet early projections of interest; but as the conference approached, demand had increased pushing the number of sessions to four. By the beginning of the conference, the number of registered participants blossomed to well over 600, making it difficult to assign later registrants to their first choices. So it was announced at the plenary session that participants could attend those that met their needs if room occupancy restrictions were not exceeded. This worked very well. Workshop #6 attendance appeared to peak during the scheduled breaks and presentations continued non-stop to meet those needs.

Workshop #6 was set up as poster presentations to provide focus on issues expected to be important to the broad audience. The handouts were continuously replenished. Experts in the field alternated between brief expositions of the issues and one-on-one responses to questions. This approach adapted well to the ebb and flow of traffic in the room. As in all such interchanges, learning was two way, and ideas and an enhanced appreciation of the problems encountered by others was brought back to the respective agencies of the presenters. Extra copies of the handouts were placed in the resource area after the last session.


For more information on crystalline silica, see OSHA's Safety and Health Topics Page:

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