Particulate Size & Sampling: A Field Reference
Almost every airborne-contaminant question in industrial hygiene comes down to one number: how big is the particle. Size determines how it formed, where it deposits in the respiratory tract, which disease it's tied to, and which filter or device will actually catch it. This page walks that whole chain, then works backward from three real sample trains to the equations holding it together.
Particle size determines its generation mechanism (fume, smoke, mist/fog, dust, bioaerosol, or fiber), which respiratory region it deposits in (inhalable, thoracic, or respirable — each tied to a different disease outcome), and which sampling media can actually collect it. Three real sample trains tie the whole chain together: asbestos (fiber, counted by shape rather than aerodynamic size), respirable crystalline silica (dust, cyclone-selected), and VOCs (not a particulate at all — no filter, no size-selective inlet). It closes with the equations used to work out how much air was actually sampled, whichever instrument reading you're handed.
Particulate Size, By Generation Mechanism
Six generation mechanisms cover essentially every particulate an industrial hygienist samples for, and each one occupies a characteristic — if approximate — size band. The chart below plots all six on the same log scale, alongside the three size-selective sampling cut-points from the next section, so you can see directly which particulate types a respirable-only sample would catch and which it would miss entirely.
| Category | Generation mechanism | Physical size range | Regulatory fiber dimensions | Workplace operations & chemical examples |
|---|---|---|---|---|
| Fumes | Vaporization of solid metal followed by rapid cooling and condensation. | 0.001 to 1.0 μm | — |
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| Smokes | Incomplete combustion of organic, carbon-based materials. | 0.01 to 1.0 μm | — |
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| Mists / fogs | Mechanical atomization (splashing/spraying) or chemical condensation of liquids. | 0.05 to 50 μm | — |
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| Dusts | Mechanical processing (crushing, grinding, blasting, or drilling) of solid materials. | 1.0 to 100+ μm | — |
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| Bioaerosols | Living organisms or biological materials suspended in moving air currents. | 0.02 to 100+ μm | — |
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| Fibers | Elongated solid particles where length significantly exceeds width. | Width: 0.1 to 10+ μm Length: 5 to 200+ μm |
Aspect ratio > 3:1 Length > 5 μm Width < 3 μm (NIOSH 7400) |
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Ranges are approximate and vary by source, especially at the edges — fume nuclei can start below 0.001 μm before agglomerating, and mist/fog is a continuum of two related formation mechanisms (mechanical atomization vs. condensation) that some references split into separate bands.
Where Each Fraction Deposits — and the Disease It's Tied To
The size-selective sampling conventions aren't arbitrary — each cut-point marks a real anatomical boundary in the respiratory tract, and each boundary is associated with a different category of occupational disease.
Inhalable Fraction — D50 = 100 μm
Anywhere in the upper respiratory tract — nose, mouth, throat, pharynx. This is the broadest fraction: anything small enough to be inhaled at all counts.
Rhinitis, sinusitis, and upper-airway irritation from local deposition; systemic toxicity for substances that are absorbed through the nasal mucosa or swallowed after mucociliary clearance (e.g. wood dust and sinonasal cancer, or ingested lead dust).
IOM Sampler (2.0 L/min) or Button Sampler (4 L/min) — both follow the ISO/CEN inhalable convention, but at different design flow rates.
Thoracic Fraction — D50 = 10 μm
Bypasses the larynx and penetrates into the chest cavity, depositing in the conducting airways — trachea, bronchi, bronchioles — without necessarily reaching the alveoli.
Bronchitis, airway inflammation, and occupational asthma — conditions driven by irritation and immune response in the conducting airways rather than deep alveolar scarring.
Parallel Particle Impactor (PPI) head, typically 2 L/min — uses internal impaction plates to strip out everything above the 10 μm cut-point before it reaches the collection filter.
Respirable Fraction — D50 = 4 μm
Bypasses all upper-airway mucosal traps entirely and reaches the deepest, non-ciliated gas-exchange tissue — the alveoli.
Pneumoconiosis — silicosis, asbestosis, coal workers' pneumoconiosis — along with irreversible fibrosis and, for some analytes, fast systemic absorption straight across the alveolar-capillary membrane into the bloodstream.
10mm Nylon Cyclone or Higgins-Dewell Cyclone, 1.7 L/min for the standard nylon cyclone — centrifugal force spins out the larger, non-respirable particles into a grit pot before the filter.
These three cut-points explain why the same dust can be a minor nuisance or a fatal exposure depending on particle size alone — a coarse dust cloud that's 90% inhalable-fraction by mass might carry almost none of its mass in the respirable fraction that actually reaches the alveoli, and vice versa.
Sampling Media — Matching Filter to Particulate
Media selection isn't just about physically catching the particle — it's dictated by whatever happens to the filter after it comes off the pump. The lab method downstream is usually the real constraint.
| Category | Media | Why |
|---|---|---|
| Fumes | Mixed Cellulose Ester (MCE) filter, 37mm or 25mm in a cassette. | MCE dissolves completely in acid during digestion, letting the lab extract pure metal atoms (lead, manganese, hexavalent chromium) for spectroscopy. |
| Smokes | Quartz fiber filter or PTFE membrane filter. | For diesel particulate matter, quartz is required because the lab heats the filter to extreme temperatures to thermally separate elemental from organic carbon — quartz won't melt or burn. |
| Mists / fogs | PVC or PTFE filter, sometimes backed by a sorbent tube if the liquid vaporizes easily. | Oil mists and mineral acids damage paper/MCE media. PVC and PTFE are hydrophobic and chemically inert, so they won't absorb humidity or alter the sample's weight. |
| Dusts | Polyvinyl Chloride (PVC) filter. | PVC's very low moisture pickup (low hygroscopicity) is what makes accurate gravimetric weighing — pre-weight vs. post-weight — possible. |
| Bioaerosols | Agar culture media plates (inside an impactor, e.g. an Andersen sampler) or gelatin/polycarbonate filters. | Agar keeps collected bacteria or mold spores alive to incubate, grow into colonies, and be identified under a microscope. |
| Fibers | MCE filter inside an elongated, static-conductive cowled cassette. | For Phase Contrast Microscopy, a chemical clearing agent (acetone vapor) turns the opaque MCE filter transparent, leaving only the dark fibers visible and countable. |
| Size fraction | Device & media | How it works |
|---|---|---|
| Inhalable (100 μm) | IOM Sampler (2.0 L/min) or Button Sampler (4 L/min), 25mm PVC or MCE filter. | The whole cassette is weighed by the lab, not just the filter — large particles stick to the inner walls and have to be included in the final mass. |
| Thoracic (10 μm) | Parallel Particle Impactor (PPI) head, multi-stage PVC filter, 2 L/min. | Internal impaction plates strip away everything larger than the 10 μm cut-point before air reaches the collection filter. |
| Respirable (4 μm) | 10mm Nylon Cyclone or Higgins-Dewell Cyclone, PVC filter, 1.7 L/min. | Centrifugal force spins heavier dust into a bottom grit pot, letting only fine ≤4 μm respirable dust pass through and deposit on the filter. |
Note: the IOM's 2.0 L/min and the Button Sampler's 4 L/min are both real, but different, design flow rates for the same inhalable convention — don't assume every inhalable device runs at the same speed.
Putting It Together — Real Sample Trains
Three worked examples, chosen specifically because one is a fiber, one is a dust, and one isn't a particulate at all.
Asbestos
Fiber. Not the inhalable/thoracic/respirable convention at all — a countable fiber under NIOSH 7400 is defined purely by shape: length > 5 μm, width < 3 μm, aspect ratio > 3:1, regardless of aerodynamic diameter.
Open-face, conductive, cowled 25mm cassette; MCE filter; flow rate chosen (commonly 0.5–16 L/min) to land the fiber density on the filter within the countable range for PCM.
Phase Contrast Microscopy (PCM) after acetone-vapor clearing; Transmission Electron Microscopy (TEM, NIOSH 7402) when fiber type needs to be confirmed, since PCM can't distinguish asbestos from other fibers of the same shape.
Respirable Crystalline Silica
Dust, respirable fraction specifically — OSHA's silica PEL (29 CFR 1910.1053) is defined against respirable mass, not total dust.
10mm Nylon Cyclone; PVC filter; 1.7 L/min — the calibrated flow rate that gives that specific cyclone the ACGIH respirable sampling curve.
Gravimetric weighing (NIOSH 0600) for total respirable mass, plus X-ray diffraction (NIOSH 7500) or infrared spectroscopy to quantify the crystalline silica fraction specifically within that mass.
VOCs — the Contrast Case
Not a particulate at all — gas/vapor phase. No aerodynamic diameter, so none of the size-selective conventions above apply.
Charcoal (or XAD-2) sorbent tube, low flow rate — typically well under 1 L/min, method-dependent (e.g. NIOSH 1500-series aromatic hydrocarbon methods).
Solvent desorption (commonly CS2) followed by GC/FID or GC/MS. A passive diffusive badge is the pump-free alternative — governed by Group 3's equation below, not a flow rate at all.
If the chart at the top of this page were extended to include VOCs, there'd be nowhere to put them — they don't have a size. That's the actual reason gas/vapor sampling is a fundamentally different sample train from every particulate example above it, not just a different filter.
The Equations Behind It
Whichever train above you're running, and whichever instrument reading you're actually handed, the same question always needs answering: how much air — or how much diffusive uptake — really reached the media, and what does that add up to across the whole shift? Four formula groups cover essentially every case.
Group 1 — Active Sampling, From a Velocity Pressure Reading
Group 2 — Active Sampling, From a Hood Static Pressure Reading
Group 3 — Passive (Diffusive) Sampling
Group 4 — Combining Multiple Sample Periods Into a Shift TWA
All four groups, plus the full Concentration/Sample Volume/Mass Collected chain, are built into the CIH Equation Master as an interactive cross-note and practice question — worth running through if you want to work a few by hand.
Particulate Size & Sampling — Frequently asked questions
What's the difference between inhalable, thoracic, and respirable particulate?
They're size-selective sampling conventions, each with its own 50% cut-point: inhalable (100 μm) is anything that can deposit anywhere in the nose, mouth, or throat; thoracic (10 μm) penetrates past the larynx into the trachea and bronchi; respirable (4 μm) is fine enough to reach the alveoli, the deep gas-exchange tissue of the lung. Each is sampled with a different device — an IOM or Button sampler, a Parallel Particle Impactor, and a nylon or Higgins-Dewell cyclone, respectively.
Why doesn't asbestos fiber counting use the respirable/thoracic/inhalable convention?
Because fibers aren't spheres, so aerodynamic diameter conventions built around settling velocity don't describe them well. Instead, NIOSH Method 7400 defines a countable fiber by shape alone — length greater than 5 μm, width less than 3 μm, and an aspect ratio greater than 3:1 — sampled on an open-face MCE filter rather than through a size-selective inlet.
Why do different particulate types need different filter media?
Media selection follows the downstream lab method, not just what physically catches the particle. MCE filters dissolve completely in acid, so metals can be extracted for spectroscopy. Quartz fiber filters survive the extreme heat of thermal-optical analysis, needed to separate elemental from organic carbon in diesel particulate matter. PVC filters have very low moisture pickup, which is what makes accurate gravimetric pre/post weighing possible for dust mass.
Can you determine a sample flow rate without a rotameter?
Yes — if a rotameter reading isn't available, a duct or hood's actual flow rate (Q) can be derived from a pressure reading instead. A pitot-tube velocity pressure reading converts to velocity and then to Q via Ventilation's own formulas; a hood static pressure reading converts directly to Q through the Coefficient of Entry relationship, Q = 4005·Ce·A·√|SPh|.
What's the difference between active and passive (diffusive) sampling?
Active sampling uses a pump to pull a known volume of air through a filter or sorbent over a measured time (sample volume = flow rate × time). Passive, or diffusive, sampling has no pump at all — the contaminant reaches the sorbent by molecular diffusion alone, governed by Fick's Law: uptake rate R = D·A/L. R plays the same role Q does in active sampling, letting mass collected be calculated as M = R × C × t.
Why doesn't a VOC sample train use a filter at all?
Because a VOC is gas-phase, not particulate — it has no aerodynamic diameter and won't be captured by any filter or size-selective inlet. Instead it's collected by adsorption onto a sorbent (typically activated charcoal), then recovered by solvent desorption and analyzed by GC/FID or GC/MS, or collected passively on a diffusive badge with no pump at all.
