04.11.2026
Cutting concrete, tipping powder bags and grinding metal can expose workers to particles with different sizes, properties and health effects. A visible cloud reveals little about concentrations reaching a worker's breathing zone or smaller particles remaining after it disperses. Understanding inhalable vs respirable dust helps safety teams distinguish what enters the respiratory system from what can reach the deep lung, where gas exchange takes place.
The distinction also matters when interpreting monitoring results: PM1, PM2.5, PM4 and PM10 do not automatically correspond to occupational sampling fractions. This article explains those relationships, the strengths of established sampling methods and the information available from real-time personal monitoring. The aim is clearer exposure decisions, centred on workers' activities and respiratory health. Discover the Dustlight exposure article and connect particle fractions with practical prevention.
Inhalable vs respirable dust and thoracic fractions
Inhalable dust is the fraction entering the nose and mouth; respirable dust can penetrate to the gas-exchange region of the lungs. Thoracic dust penetrates beyond the larynx and includes the respirable fraction. These are overlapping fractions, rather than separate particle bins. Penetration describes access to a respiratory region, not the proportion actually deposited there or the dose retained by an individual worker.
Is respirable dust part of inhalable dust?
Respirable dust is part of inhalable dust, but its share varies with the material and activity. Larger particles can contribute substantial inhalable mass without making the same contribution to the respirable fraction. An inhalable result therefore cannot be converted reliably into a respirable result using one fixed percentage across different processes and conditions.
Particle size and respiratory penetration
These overlapping fractions are defined through aerodynamic diameter and size-dependent sampling curves. Aerodynamic diameter is the diameter of a sphere with a density of 1 g/cm³ that has the same settling velocity as the particle under specified conditions. Shape and density therefore matter alongside physical size. Breathing patterns and individual differences affect respiratory penetration, so the conventions provide reproducible measurement targets rather than an individual lung-dose prediction.
Can particles larger than 4 µm contribute to respirable dust?
The occupational respirable convention is a target curve for samplers: at an aerodynamic diameter of 4 µm, it assigns about 50% of airborne particles to the respirable fraction. Smaller particles contribute more strongly, but particles above 4 µm are not all excluded. NIOSH optical-monitor guidance explains this curve and its relevance to direct-reading measurements. The thoracic convention has a nominal 50% cut-point around 10 µm, while inhalability is approximately 50% at 100 µm. The established inhalable convention is undefined above 100 µm, according to HSE MDHS 14/4, paragraph 44.
PM1, PM2.5, PM4 and PM10 classifications
PM classifications use nominal size references in micrometres, whereas the occupational fractions describe respiratory penetration conventions. PM2.5 and PM10 are established ambient-air classifications; the implementation of PM1 and PM4 outputs must be checked in the instrument documentation.
| PM fraction | Nominal size reference | Smaller-particle contributions | Meaning for exposure monitoring |
|---|---|---|---|
| PM1 | 1 µm | Smallest of these four reported ranges | Tracks smaller-particle mass; does not provide an ultrafine particle count. |
| PM2.5 | 2.5 µm | Includes the PM1 range | Ambient-air metric; does not represent the complete occupational respirable convention. |
| PM4 | 4 µm | Includes smaller-size contributions | Often labels a respirable output; the actual response curve determines its meaning. |
| PM10 | 10 µm | Includes smaller-size contributions | Broader size coverage; does not represent the complete occupational inhalable fraction. |
The table describes cumulative mass outputs on a consistent measurement basis, not a universal specification for every monitor. The EPA PM overview explains nominal cut-points for PM2.5 and PM10. Optical instruments infer particle size from scattered light, so a displayed PM label alone does not establish the response of a standard aerodynamic sampler.
Do PM10 readings include PM2.5 and PM1?
For consistently defined cumulative outputs, PM10 includes smaller-particle contributions represented by PM2.5 and PM1. Adding these values would double-count some mass, whereas non-overlapping size bands describe separate contributions. Different response curves and measurement uncertainties mean subtraction is not automatically a validated measurement of another occupational fraction. PM1 is also distinct from ultrafine particles, generally smaller than 0.1 µm or 100 nm. A PM1 mass value gives neither their number nor their chemical identity.
Occupational fractions and PM measurement differences
These differences determine whether a reading represents the fraction relevant to an exposure assessment. PM10 is not the entire inhalable fraction, and a PM4 label alone does not demonstrate agreement with the respirable convention. PM10 and thoracic sampling have related nominal cut-points but different specifications; PM2.5 is also distinct from the occupational respirable convention.
Is PM4 the same as respirable dust?
Some instruments use PM4 as shorthand for a respirable output, but the full size response matters, rather than the 4 µm reference alone. Safety teams should establish whether physical selection or algorithmic weighting produces the reported fraction and whether the documented method is suitable for the intended assessment.
Gravimetric sampling, by contrast, remains the reference method for formal exposure assessment: a suitable sampler collects particles from a measured air volume, and weighing determines their mass without relying on optical scattering properties. Compatible collection media can also support separate laboratory analysis for specific substances, including crystalline silica; weighing alone does not identify the composition.
HSE MDHS 14/4 describes the sampling and gravimetric procedures. Appropriate size selection, flow rate, sample handling and laboratory quality control remain essential. A filter result represents the sampling period; continuous monitoring adds the timing of concentration changes within that period.
Explore how Dustlight turns changing particle concentrations into personal readings and alerts, with the measurement capabilities explained in one place.
Particle properties and workers' respiratory health
Particle size, composition, concentration and exposure duration jointly influence risk, so selecting the right fraction is only part of an assessment. Inhalable dust should not be treated as harmless merely because respirable dust can penetrate further. Respirable crystalline silica can damage lung tissue; other substances may cause irritation, sensitization or systemic effects. Particle size alone cannot establish which substance is present.
Workers doing the same task may encounter different concentrations as extraction, tool position and material handling change. Nearby workers may also be exposed without performing the activity themselves. Control decisions therefore need to consider the people affected, rather than visible emissions or distance alone.
Personal exposure measurements during workers' shifts
Personal exposure monitoring addresses this variation by measuring in the worker's breathing zone, near the nose and mouth. A fixed monitor may not represent the air encountered as a worker moves between tasks. The inlet should be unobstructed and its position documented, as the breathing-zone monitoring guide explains. Measurements elsewhere on the body are not automatically representative of the breathing zone.
Dustlight records concentration changes during activities such as bag emptying, cutting and cleaning. These time-resolved readings can help locate recurring peaks and investigate control performance while the shift is still running. A time match remains evidence of an association, not proof of the source or chemical identity.
Occupational exposure limits across jurisdictions
A personal reading can be compared meaningfully with an occupational exposure limit only when the substance, fraction, averaging period and units match. For example, an instantaneous PM2.5 value cannot establish compliance with an eight-hour respirable limit, even when both use the same concentration units. 1 mg/m³ equals 1,000 µg/m³, but converting units cannot correct a mismatch in fraction or averaging period.
Great Britain uses workplace exposure limit (WEL), OSHA uses permissible exposure limit (PEL), and NIOSH publishes recommended exposure limits (RELs). ACGIH threshold limit values (TLVs) are guidance unless adopted into law. New Zealand uses workplace exposure standard (WES). Australia is transitioning from WES to WEL on 1 December 2026, according to current regulator guidance. Canadian requirements vary by jurisdiction; South Africa distinguishes maximum limits (OEL-ML) and restricted limits (OEL-RL), with mining considered separately. EU provisions require national implementation. Ambient standards do not replace occupational limits, although worker-protection rules can use PM2.5, as California does for wildfire smoke.
Dustlight measurements and their interpretation limits
Dustlight determines PM1, PM2.5, PM4 and PM10 (the app currently displays PM1, PM2.5 and PM10); its technical documentation separately describes a respirable dust output calculated using the EN 481 / ISO 7708 convention. The output is an optical estimate whose suitability depends on the application and documented performance. Dustlight derives mass concentration from laser scattering, with density, shape and refractive index affecting the result. The Dustlight calibration process explains reference comparisons; optical response remains dependent on the particles present.
Dustlight material configurations and correction factors adapt the conversion for an application, while an entered constituent share can support an estimate of that constituent's concentration. The material configuration guide explains these settings. A field factor should be based on a parallel reference comparison covering the same fraction and period. Determining actual composition or crystalline silica content requires separate laboratory analysis; a bulk-material percentage is not automatically representative of the airborne respirable fraction.
Dustlight combines concentration records with visual and acoustic alerts, supporting decisions while work is underway. The practical sequence remains substance, relevant fraction, suitable method and representative personal measurement. Dustlight does not:
- establish chemical identity,
- measure an individual's deposited lung dose
- replace the methods and assessment procedures required for legally binding verification of occupational exposure limits. Its readings are indicative measurements.
See how Dustlight can bring task-related exposure changes into view. Book a demonstration to explore personal monitoring and the settings relevant to the application.
Frequently asked questions
Personal monitoring can support exposure management, but alarm settings, maintenance and substance-specific assessment remain separate responsibilities. These distinctions support interpretation within a monitoring programme.
A green indicator relates to the configured warning conditions, not a guarantee that all relevant hazards are controlled. Interpretation depends on the settings, measurement limitations and substances present.
A correction factor established for one particle mixture may become less representative when materials or processes change, so its suitability should be reassessed when exposure conditions differ substantially.
Checks should follow the manufacturer's instructions, application and assessment requirements, while a functional response check cannot demonstrate measurement accuracy or replace prescribed maintenance and calibration.