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 | — |
|
| Smokes | Incomplete combustion of organic, carbon-based materials. | 0.01 to 1.0 μm | — |
|
| Mists / fogs | Mechanical atomization (splashing/spraying) or chemical condensation of liquids. | 0.05 to 50 μm | — |
|
| Dusts | Mechanical processing (crushing, grinding, blasting, or drilling) of solid materials. | 1.0 to 100+ μm | — |
|
| Bioaerosols | Living organisms or biological materials suspended in moving air currents. | 0.02 to 100+ μm | — |
|
| 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) |
|
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.
In industrial hygiene and air sampling, D50 (also written d50) is the cut-point, or cut-off size, of a particulate sampling device, such as a cyclone or an impactor. It is the specific aerodynamic particle diameter that the sampler collects with exactly 50% collection efficiency.
In plain terms, D50 is the “cut-off” size for dust particles entering a sampling device. The device is exactly 50% good at catching particles of that one size: it will trap half of them and let the other half pass through.
For a respirable sampler, D50 = 4 μm means half of the 4 μm particles reach the filter. Smaller particles reach it more often and larger ones less often, so D50 is one point on a smooth curve, not a sharp wall.
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. Three ways to look at the same selection: by the general particulate category that generated it, by which size fraction is being sampled, and by the specific named contaminant sitting in front of you on a real job.
1. General Sample Media by Particulate Category
| 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) for spectroscopyWhy?Why this applies hereSpectroscopy measures light from free atoms of one element, so the metal must first be released into a clear liquid. A filter that dissolves completely in acid releases every atom it collected. Filter choice follows the lab method: MCE for metals.Base ideaEach element absorbs or gives off light at its own wavelengths. AAS measures the light absorbed, ICP-OES the light emitted, and ICP-MS counts the ions by mass.Learn the base idea: 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. |
2. By Size Fraction
| 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 remove most particulate above the 10 μm cut-point before air reaches the collection filter — a curve centred on that size, not a sharp pass/fail line. |
| Respirable (4 μm) | 10mm Nylon Cyclone or Higgins-Dewell Cyclone, PVC filter, 1.7 L/min. | Centrifugal force spins most of the heavier dust into a bottom grit pot, so the filter ends up dominated by fine particulate around and below the 4 μm cut-point. |
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.
3. By Common Workplace Contaminant — Particulates & VOCs
The two tables above sort media by general category or by size fraction. In the field, you're usually looking at a specific named contaminant instead — here's how that maps to real media and cassette choices, split the same way the hazard itself splits: particulate first, then gas/vapor.
| Hazard / contaminant | Physical form | Media / cassette | Why |
|---|---|---|---|
| Respirable Crystalline Silica (quartz, cristobalite, tridymite) | Respirable mineral dust | 37mm PVC filter (5.0 μm pore) in a 3-piece cassette, paired with a respirable cyclone. | PVC resists moisture absorption and won't interfere with XRD or FTIR lab analysis; the cyclone's centrifugal separation is centred on the 4 μm respirable cut-point, not a sharp pass/fail line. |
| Nuisance & general dust (wood, flour, coal) | Respirable or total dust | 37mm PVC filter (5.0 μm pore). | Pre-weighed, matched-weight PVC filters are what makes gravimetric (weight-based) testing accurate. |
| Metal dusts & welding fumes (lead, cadmium, iron oxide, copper, general welding fume) | Fine solid particulate / condensation fume | 37mm MCE filter (0.8 μm pore). | Acid-digestible, so the lab can extract pure metal atoms for elemental analysis (ICP-AES). |
| Hexavalent chromium, Cr(VI) | Fume / aerosol | 37mm PVC filter (5.0 μm pore). | Crucial: MCE can't be used here — the cellulose itself chemically reduces Cr(VI) to Cr(III), producing a false negative. |
| Asbestos & synthetic vitreous fibers | Fibers | 25mm conductive black cassette with MCE filter (0.8 or 0.45 μm). | The 50mm cowl extension keeps static charge from pulling fibers onto the cassette walls instead of the filter. |
| Oil mists / liquid aerosols | Fine liquid droplets | 37mm PTFE or glass fiber filter. | Chemically inert media that won't absorb or react with the oil itself. |
| Hazard / contaminant | Physical form | Media / collection tube |
|---|---|---|
| General organic vapors / VOCs (benzene, toluene, xylene, gasoline, solvents) | Gas / vapor | Coconut-shell charcoal sorbent tube (standard 100mg/50mg dual-section) or an organic vapor diffusive badge. |
| Polar organic solvents (alcohols, ketones, glycol ethers) | Gas / vapor | Silica gel sorbent tube, or a synthetic polymer resin (Tenax, XAD-2). |
| Aldehydes (formaldehyde, acetaldehyde) | Gas / vapor | Silica gel coated with 2,4-DNPH (2,4-dinitrophenylhydrazine). |
| Acid gases (hydrochloric, sulfuric, nitric acid mists/gases) | Vapor / mist | Sorbent tube coated with sodium carbonate, or a treated 37mm quartz/PTFE filter. |
Charcoal is the default for nonpolar organic vapors, but polar solvents (alcohols, ketones) are prone to migrating between charcoal's two sections or desorbing poorly — silica gel or a resin like Tenax/XAD-2 is the usual fix. Aldehydes and acid gases both need a chemically reactive coating on the sorbent, not just physical adsorption, since the target reacts with the coating to form a stable, extractable compound.
4. Gas & Vapor Collection — Absorption vs Adsorption
The gas rows above rely on two different capture mechanisms, and the exam likes to swap them. The same split decides between a scrubber and a carbon bed when a gas is controlled instead of sampled.
| Feature | Absorption | Adsorption |
|---|---|---|
| What happens | The gas dissolves into a liquid, a bulk effect | Gas molecules stick on the surface or in the pores of a solid, a surface effect |
| Medium | Water or a reactive liquid | Activated carbon, silica gel, zeolites, resins |
| Sampling device | Impinger or bubbler holding a collection liquid | Sorbent tube (charcoal, silica gel) or diffusive badge |
| Control equipment | Wet scrubber (an absorber) | Carbon bed (an adsorber) |
| Humid stream | Tolerates it | Water fills the pores and ruins efficiency |
| Hot stream | Can handle it, and cools the gas | Needs cool gas; heat releases the pollutant again |
| After use | Liquid needs treatment or distillation | Bed can be regenerated by heating, or discarded |
Absorb = soak in, like a sponge. Adsorb = stick on, like tape. A humid or hot exhaust points to an absorber (scrubber); a question about pores, surfaces or activated carbon points to adsorption.
Spelling hook: aBsorption = Blended, it takes it all in. aDsorption = Dermis, it stays on the surface, like skin.
Running the Pump — Calibration, Blanks and Sample Size
The media only gives a trustworthy number if the air volume through it is known and the sample is big enough for the lab to measure. Rules below are from the OSHA Technical Manual, personal sampling chapter.
| Step | Rule | Why |
|---|---|---|
| Calibrate the pump | Before and after each day of sampling, with the same kind of media in line. Never submit the calibration media as a sample or a blank | Media resistance changes the flow; the pre/post average is used for the air volume |
| Primary vs secondary standard | Primary: measures volume and time directly (bubble-type calibrators). Secondary: a rotameter, which must itself be checked against a primary standard | A pump’s built-in rotameter only shows the pump is still running |
| Sample long enough | Minimum time = LOQ ÷ (OEL × flow rate) | Below the limit of quantitation, the lab cannot report a reliable number |
| Field blanks | Unused media opened briefly at the site with no air drawn, then shipped with the samples; about 10% of samples, never fewer than two. Asbestos needs at least two even for a single sample | Shows contamination from the media lot, handling, shipping or storage |
| Representative sampling | Full-shift samples for an 8-hour TWA; any unsampled time is counted as zero exposure | Short samples can under-state the TWA |
Two calculations that come with every sample
Equation cards: Concentration (ppm and mg/m³).
Data quality: what makes a result defensible
| Item | Rule | Why |
|---|---|---|
| Four goals | Air-sampling data should be scientifically valid, legally defensible, of verified accuracy and of characterized precision | A result that fails any one of these can be challenged |
| Accuracy target | A validated method should give results within ±25% of the true value at least 95% of the time, over roughly 0.5 to 2 times the exposure limit | Accuracy combines bias (systematic error) and precision (random error). It is a method-validation target, not a tolerance for every sample |
| Accredited laboratory | Use a lab accredited for the specific method, for example under the AIHA Laboratory Accreditation Programs (IHLAP), which accredit to ISO/IEC 17025 and expect regular proficiency testing (PAT) | An independent check of the lab's competence, plus blind samples that show whether it gets the right answer |
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.
Equation cards: Fiber density · Asbestos fiber concentration.
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.
If the sampling flow runs higher than the cyclone's specified 1.7 L/min, is the measured respirable concentration over- or under-estimated?
Answer: Under-estimated — higher flow lowers the cyclone's cut-point (D50), so it throws out more of the larger respirable-range particles by centrifugal force before they ever reach the filter, leaving less mass collected than the true respirable concentration.
| Actual flow vs design flow | Cyclone D50 | What happens in the cyclone | Respirable result |
|---|---|---|---|
| Higher than design | Smaller than 4 μm | Spins more of the larger respirable-range particles out into the grit pot | Under-estimate |
| At the design flow | 4 μm | Follows the ACGIH/ISO respirable curve | Correct |
| Lower than design | Larger than 4 μm | Lets larger, non-respirable particles reach the filter | Over-estimate |
Remember the direction: more flow, smaller D50, less mass collected.
| Respirable cyclone | Flow for a 4 μm D50 | Note |
|---|---|---|
| 10 mm nylon (Dorr-Oliver) | 1.7 L/min | MSHA coal-dust sampling runs this cyclone at 2.0 L/min against its own curve. For the ACGIH/ISO convention the flow is 1.7 L/min. |
| Higgins-Dewell | 2.2 L/min | Stated design flow. |
| Aluminum | 2.5 L/min (manufacturer) | Research found 2.2 L/min gives the least bias. Running it at 2.5 L/min under-estimates respirable dust against the ISO convention. |
| GS-1 (SKC), same cyclone at two flows | 2 L/min → 4 μm 3 L/min → 3.5 μm | A real example of the rule above: 50% more flow, a smaller D50. |
Design flows per NIOSH 0600/7500 and manufacturer specifications; GS-1 cut-points from SKC’s GS-1 cyclone instructions; aluminum-cyclone finding from the CDC Stacks study “Recommended Flow Rate of the Aluminum Cyclone for Improved Exposure Assessment.” Always use the flow the cyclone’s own instructions give for the convention you are sampling to.
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.
Direct-Reading Instruments — Real-Time, but Less Specific
Everything above is an integrated sample: collected over time, analysed later in a lab. Direct-reading instruments answer a different question: what is the level right now? They suit screening, finding sources, confined-space entry and IDLH checks, and most are less specific than a lab method. From the OSHA Technical Manual, direct-reading instruments chapter.
| Instrument | How it works | Use it for | Watch out |
|---|---|---|---|
| Photoionization detector (PID) | A UV lamp ionizes molecules whose ionization potential is at or below the lamp energy (typically 9.5, 10.6 or 11.7 eV) | Hydrocarbons and many VOCs; finding sources and concentration gradients | Not specific: cannot identify which chemical; sensitivity drops with lamp age and a dirty window |
| Infrared analyzer | Measures absorption of infrared light at a chosen wavelength | A broad range of organic and inorganic gases | Other chemicals absorbing at the same wavelength interfere |
| Oxygen sensor | Electrochemical (galvanic) cell | Confined spaces, inert or steam atmospheres | Alarm below 19.5% (deficient) or above 23.5% (enriched); below 12% is IDLH |
| Combustible gas meter | Reads flammable gas as a percentage of the LEL of the calibration gas | Fire and explosion risk | Needs normal oxygen to read correctly; silicones and sulfur compounds poison the sensor (reads low); LEL levels are thousands of ppm, far above toxic limits |
| Toxic gas sensors | Electrochemical cells: current proportional to concentration | CO, H2S and other specific gases | Cross-sensitivity to other gases |
| Colorimetric detector tubes | A known air volume is pulled through a reagent that changes colour; read the stain length | Quick screening of over 200 gases and vapors | Use only with the same brand of pump; accuracy, interferences, temperature, humidity and shelf life all matter |
| Aerosol photometer (nephelometer) | Light scattered by particles is proportional to particle count | Dusts, smokes, mists and fumes in mg/m³; some read the respirable fraction, some count fibers | A real-time trend, not a replacement for a gravimetric compliance sample |
| Condensation particle counter | Grows very small particles with alcohol vapor, then counts them by laser | Quantitative respirator fit testing; HEPA filter testing | Counts particles; it does not measure mass |
Test in this order: oxygen → flammability (LEL) → toxics. Many combustible gas sensors need oxygen to read correctly, and fire or explosion is usually the more immediate threat.
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? Five formula groups cover essentially every case.
Group 1 — Active Sampling, From a Velocity Pressure Reading
Equation cards: Velocity (from velocity pressure) · Airflow (Q = VA).
Group 2 — Active Sampling, From a Hood Static Pressure Reading
Equation cards: Hood airflow (standard air) · Coefficient of entry.
Group 3 — Passive (Diffusive) Sampling
Group 4 — Combining Multiple Sample Periods Into a Shift TWA
Group 5 — Gravitational Settling Velocity (Stokes’ Law)
Equation cards: Terminal settling velocity (Stokes) · Reynolds number.
Master Every CIH Equation
All 98 equations from the CIH exam — Radiation, Ventilation, Noise, Science & Statistics, Hood Airflow, and Heat Stress — with worked practice problems, dimensional-analysis breakdowns, and a built-in scientific calculator.
Try the CIH Equation Master →Radiation equations are free to try — no sign-up required
CIH Exam Hint — Key Patterns to Memorize
Two shortcut lists worth having cold going into the exam — media selection by what a chemical actually is, and the toxicology signatures that let you identify a substance from its health-effect description alone.
1. Sampling Media Shortcut
- General organic vapors → coconut shell charcoal
- Polar solvents (alcohols/glycols) → silica gel
- Reactive aldehydes → DNPH-coated silica gel
- Acid-digestible metals → Mixed Cellulose Ester (MCE)
- Gravimetric / XRD / sensitive metals (Cr(VI), silica) → Polyvinyl Chloride (PVC)
2. Toxicological Signature Shortcuts
- Peripheral neuropathy: n-hexane, acrylamide, carbon disulfide.
- Ototoxicity (hearing loss + noise): toluene, styrene, xylene, trichloroethylene.
- Sensitizers (asthma/dermatitis): isocyanates (TDI/MDI), glutaraldehyde, beryllium, nickel.
- Biological Exposure Indices (BEI): lead (blood), benzene (urine), carbon monoxide (exhaled air/blood), toluene (urine).
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 find a duct or hood airflow without a flow meter?
Yes — if a direct flow 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.
What is the difference between a primary and a secondary calibration standard?
A primary standard, such as a bubble-type calibrator, measures volume and time directly. A secondary standard, such as a rotameter, must itself be checked against a primary standard. Sampling pumps are calibrated before and after each day of sampling with the same kind of media in line.
In what order should a confined space atmosphere be tested?
Oxygen first, then flammability as a percentage of the LEL, then toxic gases. Many combustible gas sensors need normal oxygen to read correctly, and fire or explosion is usually the more immediate threat.
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.
