Respirable Crystalline Silica in Underground Metal and Nonmetal Mining

Most delays, qualified results, and re-sampling in respirable crystalline silica (RCS) programs don’t originate in the lab - they originate during sampling.

In underground metal and nonmetal (MNM) mining, getting defensible exposure data depends on collecting the correct respirable fraction and sending a sample the lab can actually analyze.

From a laboratory perspective, RCS results are only as reliable as the sample that arrives.

The respirable fraction of dust is the very small particles (often described in practical terms as roughly ~4 microns and smaller) that can penetrate deep into the lungs. From an environmental testing laboratory perspective, the fastest path to usable and defensible results is helping ensure samples are (1) collected as the respirable fraction (not just “dust”), and (2) likely to arrive at the lab intact and not overloaded (typically over 7 mg of dust collected). The following overview details what RCS is and why “respirable” matters – so you can avoid common sampling issues that can cause delays, qualified data or re-sampling.

What is respirable crystalline silica?

Crystalline silica is silicon dioxide (SiO2) in crystalline form. The most common forms referenced in occupational sampling programs and lab reporting are:

• Quartz
• Cristobalite
• Tridymite.

Different mine areas can contain very different proportions of these minerals. That variability is one reason we encourage clients to think about where and when samples are collected, and not just how many.

At the laboratory level, respirable crystalline silica is typically quantified using X-ray diffraction or infrared spectroscopy methods that differentiate crystalline silica polymorphs such as quartz, cristobalite and tridymite from the remaining dust matrix.

The accuracy of that measurement depends on:

• Proper respirable size selection during sampling
• Adequate air volume (liters) to meet required airborne reporting limits
• Filters that are not overloaded, damaged or compromised
• Sample media that are compatible with the selected analytical method.

If any of these elements are not aligned with method requirements, results may require qualification, reanalysis, or resampling.

What does “respirable fraction” mean (and why do we care)?

Not all airborne dust behaves the same in the body. RCS evaluations focus on the respirable fraction.

That’s why respirable sampling typically uses size-selective collection (commonly a cyclone + filter approach or a parallel particle impactor (PPI): the intent is to collect a particle size fraction consistent with how occupational exposure limits are evaluated.

Lab takeaway – If the sampler setup or flow rate is not correct for collecting the respirable fraction, the lab may still be able to analyze the filter, but the airborne concentration result may not represent a true respirable exposure.

Why underground MNM conditions matter over a shift

In underground MNM mines, conditions can shift during a normal production day. Since RCS results are commonly compared to an 8-hour time-weighted average (TWA), within-shift changes can materially affect the shift average. Also, airborne reporting depends on both the lab’s capability and the air volume collected.

Lab takeaway: This is simply why, from the lab side, we ask a few practical questions before sampling to help prevent avoidable problems and return results faster.

For instance:

• Air volume directly affects the airborne reporting limit that the laboratory can achieve. If the air volume is too low, the airborne detection limit will not be low enough to compare to the threshold limit.
• Conversely, excessive dust loading can overload filters and compromise result accuracy.

Geology can change (silica percentage in dust can change)

Even when “dustiness” looks similar, the silica content of that dust can change as headings advance or crews move between areas. Common underground MNM scenarios include:

• Moving into a different rock unit (some zones are more quartz-bearing than others)
• Crossing a mixed zone/vein/dike with different mineral content
• A worker splitting time across multiple headings/areas in one shift.

If your goal is full-shift exposure information, it helps to sample the actual mix of areas/tasks the worker experiences: not just a short window in one location.

Controls can drift (control performance changes during the shift)

Dust controls don’t always perform identically throughout the day. Practical drivers of variability seen across mining projects include:

• Ventilation variability: auxiliary fan/duct moves, damaged ducting or changes in air controls that change airflow at the face
• Water suppression variability: clogged nozzles, pressure drops, intermittent supply or inconsistent use that changes fine dust generation
• Equipment/tooling condition worn bits/picks or degraded dust controls that can increase respirable dust.

These shifts can also affect sample quality, because overloaded or wet filters are common sources of delay or data qualifiers.

MSHA’s silica rule framework calls out evaluating impacts from changes in production, processes, equipment, engineering controls and geologic conditions – because these are known drivers of exposure variability.

Practical pre-sampling questions to avoid delays and rework

When clients contact us before sampling, asking these questions most often helps prevent poor samples and re-sampling:

1. What decision are you supporting? (screening vs control check vs investigation)
2. Personal full-shift or task/area sampling? (results need to match the exposure time basis)
3. Where will the worker spend the shift? (one heading vs multiple areas)
4. Any risk of overloaded or wet filters? (high dust, wet drilling or heavy water spray)
5. What air volume are you targeting? (air volume can drive airborne detection limits)

How ALS helps

As an environmental testing laboratory, our role is to help you confirm the fit of methods and media, reduce preventable sampling issues and return clear results as quickly as possible. If you share your mine type (underground MNM), the work being evaluated and your timeline, we can help you pressure-test the sampling plan before you collect.

Phone: 801-266-7700

Email: ALSLT.Project.Managers@alsglobal.com