09.09.2026
Respirable crystalline silica is strictly regulated in every major jurisdiction, and safety teams increasingly ask whether a wearable monitor can identify it directly. The answer determines which method carries legal weight and which one guides daily prevention. A real-time dust monitor cannot detect crystalline silica as a distinct substance. It measures the concentration of respirable dust in the air, while silica-containing particles contribute to the reading without being distinguishable from other particles of similar size. Laboratory analysis remains the only route to substance identification. What real-time measurement adds is the exposure pattern behind the laboratory value: when concentrations rose, which task produced them, and how widely exposure varied between individual workers.
Ability to detect crystalline silica in respirable dust
Optical dust monitors measure the concentration of respirable dust in the air, not chemical composition. A device reporting respirable dust in mg/m³ registers silica particles within that figure, but cannot confirm whether they are silica, limestone, gypsum or another mineral of similar size.
The distinction between detection and identification contains the whole of the answer.
Respirable crystalline silica (RCS) is not a separate size fraction. It is a constituent of the respirable fraction defined in EN 481 and ISO 7708, which covers particles small enough to reach the alveolar region of the lung. An optical instrument responds to every particle in that fraction, so silica is measured in the sense that it adds mass to the reading, but it is never identified as silica. The same holds in reverse: aerosols and liquid droplets in the same size range are registered alongside dust, because the instrument counts and sizes particles rather than analysing them.
Can a real-time dust monitor detect crystalline silica?
Not as a distinct substance. The measurement principle rules it out, because light is scattered by a particle according to its size, shape, and refractive index, and several unrelated materials scatter light in comparable ways. The same reading may correspond to a large silica content or to almost none, and the instrument alone cannot separate the two.
Some instruments claim to distinguish crystalline silica optically, using principles other than conventional light scattering. Independent evaluation of those claims is ongoing rather than concluded. Dustlight makes no such claim: the device measures the respirable fraction, and composition is established in a laboratory.
Learn more about how silica dust can be made visible.
Gravimetry as a reference method
Gravimetric sampling remains the reference method for assessing compliance with limit valuesin every major jurisdiction. A pump draws air through a size-selective cyclone onto a pre-weighed filter, the filter is reweighed, and the collected mass is divided by the volume of air sampled.
The strength of the method lies in what happens afterwards: the collected sample can be analysed. X-ray diffraction (XRD) or infrared spectroscopy (FTIR) determines how much of the respirable dust consists of a specific substance. The approach is applied by OSHA method ID-142, by NIOSH methods 7500 and 7602, and by MDHS 101 in Great Britain. No optical technique substitutes for that analysis.
The analytical step is what connects a measurement to a legal limit. The OSHA permissible exposure limit for respirable crystalline silica is 50 mg/m³ as an eight-hour time-weighted average, with an action level of 25 µg/m³. The workplace exposure limit under COSHH is 0.1 mg/m³. The EU limit value under Directive 2004/37/EC is 0.1 mg/m³; the ACGIH value of 0.025 mg/m³ is a technical recommendation without legal force. When a value refers to quartz, the assessment depends on the laboratory determining the quartz content of the sample.
The procedure is standardised as well as analytical: EN 482 sets performance requirements for measurement procedures, EN 689 the strategy for demonstrating compliance. Results acquire a documented basis that a screening measurement does not have. A fuller treatment appears in the comparison of gravimetric and real-time methods.
Blind spots in a single gravimetric result
A gravimetric result reports one average value, for one measurement period, at one location. It does not show how exposure developed over time, nor conditions elsewhere in the workplace.
The temporal limitation is the more consequential of the two. An identical shift average can arise from:
- steady low-level exposure
- several short peaks
- one extreme event lasting minutes
All three produce the same figure on the filter, yet each calls for a different control measure, and averaging removes the information needed to choose between them.
The spatial limitation follows from the sampling design. The filter records the exposure of the person who wore it, or the concentration at the point where a static sampler stood. Colleagues working nearby may experience conditions that differ considerably.
A further constraint appears as exposure limits fall: a filter must collect enough mass for the laboratory to report a value with confidence. Where the limit of quantification approaches or exceeds the applicable limit, the measurement can neither confirm nor rule out compliance, a situation examined in detail under the limit of quantification.
Combined use of gravimetric sampling and Dustlight
Dustlight complements gravimetric sampling rather than replacing it. The laboratory supplies the reference concentration and, where analysed, the share of a specific substance in the collected dust, while Dustlight supplies the exposure pattern in real time.
The two can be brought together in a defined workflow. A gravimetric result obtained alongside a parallel real-time measurement can be used to calculate a customer-specific correction factor. That factor adjusts the optical reading towards the mass concentration the reference method established for the material and process in question. From September, the material share determined in the laboratory can also be added to this customer-specific material configuration. Readings and alerts then relate to the constituent of concern rather than to the mixture as a whole. Both steps bring the measurement closer to the material and process actually present on site.
How is a correction factor derived from a gravimetric sample?
Dustlight and a gravimetric sampler are operated in parallel, under identical conditions, over the same period. The laboratory result is then entered into Dustlight app,; which calculates the factor and applies it to subsequent measurements.
One condition keeps the factor reliable: it applies as long as material and process remain comparable to those in force during the parallel measurement. Where the material changes, a fresh reference measurement restores the match — a short step that keeps readings aligned with the material actually in use
Correction factors and material configurations are explained in detail in this article.
Limits of optical substance identification
Dustlight measures respirable dust optically and cannot determine whether individual particles are silica or another specific substance. Laboratory analysis remains necessary to establish composition, and Dustlight does not replace gravimetric sampling as the reference method for compliance assessment.
Two consequences follow for daily use, and both concern assumptions the device cannot check for itself. The silica configuration, for instance, is calibrated on silica-containing Arizona Road Dust, and its limits are set to 100 % silica content. Readings must therefore be read against the composition actually present. Where a substance share is entered from a laboratory report, the share is applied as supplied and is not confirmed by the optical measurement.
This is a scope boundary rather than a shortcoming. Direct-reading instruments are positioned the same way in the OSHA Technical Manual. That guidance describes them as tools which inform a sampling strategy and identify where exposure potential is highest. Validated filter-based methods provide the defensible quantitative result.
Two figures are worth keeping apart. Device accuracy for respirable dust is documented at ±25 µg/m³ below 100 µg/m³ and ±25 % above it, established under laboratory validation against reference instruments. Agreement between an optical reading and a gravimetric sample in a running process is a different quantity, since it also depends on how closely the configuration matches the dust present. A correction factor narrows that second gap, bringing the two methods measurably closer together; the validated device figure stands on its own.
The key value of Dustlight
Dustlight supports the entire measurement process, not a single step within it:
- Before gravimetric sampling, it indicates where exposure is highest and where a reference measurement will be most useful.
- During sampling, the parallel measurement supplies the data needed to derive the correction factor — and, unlike the single average the filter returns, it shows how the concentration developed across the sampling period.
- 3. Afterwards, it continues to monitor the process between formal campaigns and shows whether conditions change over time.
Why can the same shift average mean different exposures?
Because averaging removes the time axis, a gravimetric measurement may report an identical shift average for entirely different exposure situations. Dustlight shows when the exposure actually occurredand whether the average arose from continuous exposure, from repeated peaks, or from one major event. Identifying the task or incident responsible becomes considerably easier.
The distinction matters because it changes which control measure is appropriate:
- Repeated short peaks point towards a specific task, and often towards local exhaust ventilation or a change of method.
- Repeated brief spikes indicate a specific activity, often suction at the source or another process.
- A single event points towards an incident, a failed control or an unplanned activity.
A filter result alone rarely separates these cases.
How much can exposure differ between workers standing metres apart?
Substantially, and often by more than a site layout would suggest. An operator, an assistant and a third worker several metres away may experience very different concentrations, depending on distance from the source, air movement and the direction in which dust travels. Body position and task timing introduce further variation across a shift.
Measuring several workers or locations therefore produces a much fuller picture than a single gravimetric point can provide. Personal exposure, rather than room concentration, is what the exposure limits refer to, and personal exposure is what protective measures have to address.
The two roles complement each other: additional measured positions describe the workplace far more completely, while the formal assessment rests on the reference method and its laboratory analysis.
Would you like to see how Dustlight can be used in your current compliance strategy?
Not as primary evidence. Compliance with a crystalline silica limit is demonstrated through sampling and laboratory analysis, following EN 482 and EN 689 or the equivalent national procedure. OSHA permits a performance option combining air monitoring data with objective data, provided exposures are characterised accurately, but the reference route remains filter-based.
These terms are not interchangeable and apply only within a specific legal jurisdiction:
– In Germany, the limit for quartz is 0.05 mg/m³. For A-dust in general, TRGS 900 specifies an AGW of 1.25 mg/m³.
– The EU limit value under Directive 2004/37/EC is 0.1 mg/m³.
– In Australia, a value of 0.05 mg/m³ as an 8-hour average has been in effect since 2020. As of December 1, 2026, it will be considered the Workplace Exposure Limit.
– WEL refers to the COSHH value in the United Kingdom, which is also 0.1 mg/m³.
– The ACGIH limit of 0.025 mg/m³ is a technical recommendation with no legal force.
– PEL refers to the OSHA limit in the U.S., 50 µg/m³ as an 8-hour average, with an action limit of 25 µg/m³.
Frequency is governed by the applicable regulation and the risk assessment, and continuous monitoring does not alter a required interval on its own. It can show where and when a reference measurement will be most informative, and the resulting evidence base may support a case for a revised interval agreed with the responsible body.