The Future of Respirable 

Silica Monitoring 

Accurate Results Generated 

On-site in a Few Minutes 

By Emanuele Cauda, Ph. D., NIOSH 

senior research fellow, and 

Lauren Chubb, Ph. D., NIOSH 

physical scientist 

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I FEATURE 

SILICA DUST IS COMMON in many mining environments and there is usually 

a chance for that dust to become airborne. In aggregates operations, dust may 

be generated while drilling, blasting, mucking, hauling, crushing, screening, 

and transferring materials within the plant and to stockpiles. Dust may also 

be present at customer loadout points and inside quality control labs. 

22 STONE, SAND k GRAVEL REVIEW, September/ October 2017 
Operators should be fully aware that 

silica dust is a potential hazard in 

quarrying operations and workplace 

exposures should be assessed. Excess 

exposures can often be reduced using 

a variety of engineering control technologies and work practices. Controls 

may include mechanical ventilation, 

baghouses, water spray systems, 

enclosed crusher booths, and environmental cabs for vehicles. In addition, respirator use with appropriate 

employee training and workplace signage is often seen in areas of high 

dust concentrations. 

A well-vetted monitoring approach 

is essential to evaluating worker exposure and assessing the effectiveness 

of control technologies. Operators can 

and do engage in periodic workplace 

air monitoring, but this practice has 

its limits. Currently, the most common way to measure the airborne 

concentration of respirable silica is 

Figure 1. BELOW: Model of a common 

respirable dust sampler and a portable 

infrared analyzer. BOTTOM: model of 

a dust sampling cassette inside the 

analyzer. LEFT: A NIOSH researcher 

analyzes a dust sample using a portable 

infrared analyzer. 

Figure 2. Comparison of our silica estimation results with the reference 

method (NI0SH7500) TOP: for dust samples without mineral A; BOTTOM: 

for dust samples with mineral A. The presence of mineral A may induce an 

underestimation of silica by our method. 

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STONE, SAND &: GRAVEL REVIEW, www.nssga.org 23 
to collect one or more respirable dust 

samples and send the samples to a 

commercial laboratory for analysis. 

This practice can be troublesome for 

several reasons. 

First, it typically takes one to 

two weeks to get lab results back. 

Moreover, successive samples taken 

over a longer period than a single 

work shift may be required to describe 

exposures in dynamic environments. 

In the meantime, conditions may have 

changed where the samples were 

collected. 

What about modern, real-time dust 

monitors? They may be useful for estimating relative airborne dust concentrations, but they cannot distinguish 

silica from other dust components. 

Their value assessing absolute silica 

exposures is thus limited, particularly in significantly heterogeneous 

ore bodies. 

A new approach 

NIOSH researchers are developing an innovative silica monitoring approach that would generate 

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24 STONE, SAND &: GRAVEL REVIEW, September/ October 2017 

silica concentration data right on 

site, immediately after collecting a 

respirable dust sample. The research 

is well underway and the results are 

very promising. Once the approach 

is optimized, it will allow occupational health professionals to quickly 

evaluate silica exposures, adjust the 

control technologies or implement 

new technologies/interventions, 

and promptly evaluate the efficacy 

of the intervention - ultimately giving companies more power to focus 

resources where they are needed and 

better protect the health of their 

workers. 

The new approach uses a small, 

robust, portable infrared analyzer. 

Several comparable instruments are 

commercially available, and we're 

testing them in the lab and in the 

field to ensure they have the necessary technical and practical features. To perform the analysis of the 

sample, a standard dust-sampling 

cassette is inserted directly into the 

infrared analyzer. In under a minute, 

a sample can be analyzed specifically 

for its silica mass and silica concentration. Furthermore, after analysis 

in the field, t he same sample can 

be sent to a commercial laboratory 

for comparison by standard analysis. 

In creating an effective tool for 

measuring silica, we knew that - first 

and foremost - the data generated 

by the field-based approach must 

be accurate. When challenged wit h 

pure silica dust, our approach showed 

a limit of quantification of 16 M μ. 

The variability in the results generated by analyzing the same samples 

with two instruments was very low 

- less than 2 percent. Finally, we've 

verified that the approach produces 

results that are consistent over time. 

Our results proved that the analytical 

method is robust and it can be used 

in t he field. (MSHA still requires 

standard lab analysis for compliance 

samples.) 

We also knew that the presence 

of other minerals in t he dust sample can affect the accuracy of the 

silica results - potentially leading 

to under- or overestimation of silica - and this issue must also be 
Figure 3. Screen shot of the software under development by NIOSH to be used by occupational health professionals for the field-based 

silica monitoring approach. 

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addressed by the new monitoring 

approach. Therefore, our research 

is in progress to identify the different airborne minerals that are 

common in aggregates operations, 

and to determine how these minerals affect the accuracy of the silica 

analysis. We can then create appropriate calibration strategies to correct for their presence. 

With the help of NSSGA members, 

we've used this approach over the 

last two years to test the accuracy 

of more than 500 dust samples, from 

limestone, granite and sand and 

gravel operations. We compared our 

results with those produced by the 

standard NIOSH 7500 analysis for 

silica. Our results will be summarized in an upcoming peer-reviewed 

publication. From the preliminary 

analysis, it appears that some specific minerals may negatively affect 

the silica quantification (see Figure 

2). To correct for this confounding 

effect, we must design a specific 

calibration strategy. 

In addition to being accurate and 

accounting for the presence of other 

minerals in the dust sample, we know 

that the process must also be simple 

and quick. We are currently modifying 

the conventional sampling cassette 

for quick insertion into the portable 

analyzer - without opening the cassette first. 

Finally, with a user-friendly 

approach in mind, we are also developing software to assist with data 

analysis and data management to 

facilitate everyday use of the portable 

infrared analyzer. 

Next steps 

To ensure its scient ific integrity, 

our monitoring approach requires 

substantial testing in the field. We 

must verify that the final technique 

is as simple and as beneficial as possible for future users. The equations 

used to quantify silica concentration must be challenged and optimized with even more dust samples 

collected in aggregates operations. 

The new technologies involved in the 

monitoring approach, including the 

modified sampling cassette and t he 

NIOSH-developed software, need to be 

beta-tested and further refined based 

on user comments. 

Finally, feedback from occupational 

professionals in the mining industry 

- the intended users for the final technique - will be essential to ensuring 

that this novel monitoring approach 

provides accurate, accessible information that will ultimately better protect 

miners from exposure to respirable 

silica dust. Beta testing of the overall 

monitoring approach is scheduled in 

the next few months, and we look 

forward to sharing the results. 

Disclaimer 

The findings and conclusions in 

this article are those of the authors 

and do not necessarily represent 

the views of the National Institute 

for Occupational Safety and Health 

(NIOSH). Mention of any company or 

product does not constitute endorsement by NIOSH. 

For furt her information, please 

contact NIOSH researchers Emanuele 

Cauda, ecauda@cdc.gov, 412-386-4518 

or Lauren Chubb, lchubb@cdc.gov, 

412-386-6460. • 

STONE, SAND &: GRAVEL REVIEW, www.nssga.org 25