19.08.2026
As occupational exposure limits are tightened, gravimetric sampling meets a physical floor — and the calibration factor becomes the mechanism that lets a real-time reading take its place. The conventional method of measuring particulate matter draws air through a filter and weighs the dust collected on it. The balance needs a minimum mass before it can return a reliable figure. As exposure limits decrease, less dust reaches the filter in the same sampling time: a measurement that once took two hours now takes a full shift, and some samples still come back below the detection limit with no usable result.
A direct-reading instrument sidesteps that floor entirely, because it needs no collected mass. What it needs instead is a way to convert a light-scattering response into a mass concentration — and that conversion depends on the material. Dustlight from Latai supplies it through material configurations, each carrying the calibration factors for a specific dust along with the exposure limit that applies to that material in your country. Seven are available in the Dustlight app: General Dust, silica dust, welding fume, wood dust, oil mist, aluminium oxide ceramics and diesel particulate matter. For materials not on that list, custom calibration factors can be created from gravimetric reference measurements.
The Dustlight measuring principle
Dustlight is a wearable dust monitor that draws ambient air past a laser and measures the light scattered by the particles carried in it, which yields both particle number and particle size. A processing algorithm converts those two quantities into a mass concentration in µg/m³, recorded at the worker level.
Air first passes a removable close-meshed grille that retains coarse particles, then enters a settling chamber where anything too large to be relevant is deposited before measurement. Part of the airflow is filtered and reintroduced as sheath air, which prevents particles from settling on the optics. Within the measuring chamber a laser illuminates each particle, and a photodiode positioned at a defined angle converts the scattered light into an electrical signal. A self-cleaning cycle runs every two hours, briefly increasing fan speed so that any deposits are loosened before they can affect the reading. It runs only when ambient concentration is low and is skipped otherwise, so cleaning never coincides with a period of raised exposure.
The device derives concentrations for PM1, PM2.5, PM4 and PM10, then applies the convention defined in EN 481 and ISO 7708 to output respirable dust — the size-dependent weighting that describes how likely each fraction is to reach the deep lung. Each sensor is compared against a reference instrument and calibrated to match it, so that readings are consistent across devices.
Dustlight is a particle counter rather than a material detector. It does not identify the composition of a particle, and it registers aerosols and liquid droplets falling in the same size range. Gravimetric sampling combined with laboratory analysis therefore remains the reference method for formal exposure assessment and for legally binding compliance evidence. Optical monitoring is intended to work alongside it rather than in place of it.
Density, refractive index and the calibration factor
Because the only unknown in the calculation is the conversion from a light-scattering response to a mass, that conversion can be resolved in advance for a given material. Two properties determine it. Density governs how a particle count becomes a mass. Wood particles are considerably lighter than lead particles of the same size, so an identical optical signal may represent a very different mass concentration. Refractive index governs how strongly a material scatters light. A quartz crystal is highly reflective and returns a strong signal, whereas a dark coal particle returns almost none. Two clouds of identical particle size and number may therefore be read very differently.
NIOSH describes this behaviour directly, noting that "the response of the sensing element can be different for different aerosol materials", and reports that concentrations from factory-calibrated monitors can diverge from filter-based samplers by as much as an order of magnitude (NIOSH Manual of Analytical Methods, 5th edition, Chapter AM, Measuring respirable aerosol with real-time optical monitors, December 2021). A photometric calibration factor is the mechanism by which that divergence is reduced. Each is derived from side-by-side comparison against gravimetric and laboratory-grade optical references, so the reported concentration reflects the material actually processed.
Most workplaces do not present a single substance, since cutting, grinding and powder handling generate mixed dust whose composition shifts with the task. The standard configuration of Dustlight was therefore calibrated on a representative mixture at roughly 2.7 g/cm³ — largely crystalline silica-containing dust with other materials included — and validated against both gravimetric and laboratory-grade optical instruments. Supplied on every device as General Dust, it provides a sound orienting measurement across most environments without prior knowledge of the material. Its warning thresholds default to the general dust limit and should be lowered where carcinogenic or toxic substances are handled.
Where a calibration factor matters — and where it does not
Any workplace in which powdered or particulate material becomes airborne falls within scope. Matching the calibration factor to the dominant material converts an orienting figure into one a safety team can defend in a risk assessment.
Once the material is matched, a time-weighted average (TWA) can be compared meaningfully against the applicable limit — a permissible exposure limit (PEL) under OSHA in the United States, a workplace exposure limit (WEL) under COSHH in Great Britain, or the equivalent national OEL elsewhere. Warning thresholds then trigger at the intended point.
A considerable part of the operational value requires no calibration factor at all, because comparing a device against itself cancels the factor from the ratio.
Three applications therefore remain valid on the standard setting:
- Verification of an engineering control, by measuring the same task with local exhaust ventilation (LEV) running and then switched off.
- Location of a source, by walking the area against the live measurement trace.
- Attribution of exposure to a task, which frequently shows that a short step, often only a few minutes, accounts for a disproportionate share of a shift's dose, and identifies it as the place where a control will have most effect.
No shift-long filter can reveal that distribution. Because the measurement is available during the shift, work method or respiratory protective equipment (RPE) can be adjusted while exposure is occurring rather than weeks afterwards. Measurement volume carries no marginal cost: the cost sits with the device, not with each measurement, so a device can be worn daily without additional charge per measurement.
Material configurations available from Dustlight
Six configurations are included at no extra charge in the Business and Corporate software plans, alongside the General Dust standard. Each was developed through side-by-side comparison against gravimetric and laboratory-grade optical references for that material.
- General Dust
- Silica dust
- Welding fumes
- Wood dust
- Oil mist
- Alumina Ceramics
- Diesel particulate matter (DPM)
Each configuration also loads the country-specific exposure limit for that material wherever one is established in law, so the reading is assessed against the value in force where the device is used.
For respirable crystalline silica that value varies widely:
| Jurisdiction | Applicable value, 8-hour TWA |
| EU limit value, Directive 2017/2398 | 0.1 mg/m³ |
| United Kingdom | 0.1 mg/m³ |
| Australia | 0.05 mg/m³ |
| Germany | 0.05 mg/m³ |
| OSHA, U.S.A. | 0.05 mg/m³, with a 0.025 mg/m³ action level |
| ACGIH Threshold Limit Value (TLV) | 0.025 mg/m³ |
A further eight configurations cover silica-based powders and are available to any customer for an annual fee:
- Silica gel: fine and standard grades
- Fumed silica: 150, hydrophobic and HMDS variants
- Precipitated silica: hydrophobic, LS and granulated grades
Selection takes about a minute. The device is connected over wireless or USB, and the material is chosen in the device settings of the app or web app.
Custom calibration factors in an upcoming release
From September 2026 a further step becomes available. Alongside selecting a material configuration, the share of the dust accounted for by one specific substance can be entered in the Dustlight app, so the assessment reflects the substance that actually carries the risk at that site.
The route to that figure follows the order in which most sites already work. Where a configuration exists for the material processed, selecting it is the whole task, and the result is already considerably closer than the general setting. Where none exists, General Dust is the place to begin. It shows which areas are worst, how concentrations behave over time, and how conditions differ between winter and summer — which is what identifies where a problem sits.
Gravimetric sampling is then directed at those locations rather than spread across the site: where exposure appears highest, where the material is toxic or carcinogenic, or where compliance looks doubtful. A device runs alongside the sampler so that both receive the same dust under the same conditions. The laboratory result is entered against the Dustlight average, which the software already holds, and the configuration is named and stored.
Where that sample is also sent for composition analysis, the laboratory reports the share of the substance that matters — 20 per cent silica, for instance, or 5 per cent chromium. Entering that share tells Dustlight to report and raise warnings against that constituent alone, against the limit for silica rather than the limit for the mixture. Dust mixtures can therefore be assessed on the substance that carries the risk.
Where no laboratory analysis is available, the share can be set directly, supported by reference bands in the application for common materials such as concrete, granite and fibre cement. At a setting of 35 per cent, a total of 120 µg/m³ is displayed as 42 µg/m³. Leaving the value at 100 per cent keeps the assessment conservative where the share is genuinely unknown.
Book a demo to see how Dustlight can enhance your dust measurement strategy.
| Specific configurations are created in cooperation with the |
Optical instruments measure scattered light rather than mass, and the strength of that response depends on the density and refractive index of the material. A calibration factor supplies those properties so the light-scattering response can be converted into a mass concentration comparable with an exposure limit.
General Dust is appropriate when multiple materials are present together, when the composition is unknown, or when the goal is a broad, preliminary assessment. It is also sufficient for any comparison of a device with itself, such as a before-and-after test of a dust extraction system, because the correction factor cancels out based on the ratio.
That depends on the jurisdiction, and the values differ by a factor of four. The EU binding limit and the British WEL are both 0.1 mg/m³; Australia and the United States apply 0.05 mg/m³, with a 0.025 mg/m³ action level under OSHA; the ACGIH threshold limit value is 0.025 mg/m³. A material configuration loads the limit in force in the country where the device is used.