3.2 Particulate and Aerosol Sampling

2.2 Particulate and Aerosol Sampling

Industrial aerosols comprise airborne suspensions of solid particles or liquid droplets, including dusts, mists, fumes, smokes, and bioaerosols. The toxicological impact of an inhaled aerosol is determined by its chemical composition, its mass concentration, and critically, its deposition location within the human respiratory tract.

Because deposition depends fundamentally on particle aerodynamic behavior, modern occupational health standards specify particle size-selective sampling criteria rather than collecting non-differentiated "total" particulate.

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Aerosol Physics & Aerodynamic Equivalent Diameter

Workplace particles possess irregular geometric shapes and variable material densities. To standardize physical behavior, industrial hygiene defines particle dimensions in terms of Aerodynamic Equivalent Diameter (dae).

[!NOTE] Definition of Aerodynamic Equivalent Diameter (dae): The diameter of a hypothetical spherical particle of unit density (ρ0 = 1.0 g/cm³ = 1000 kg/m³) that has the exact same gravitational terminal settling velocity (vts) in still air as the particle under consideration.

Irregular Particle                            Equivalent Aerodynamic Sphere
(Actual density ρp, Volume-equivalent dv)      (Standard unit density ρ0 = 1.0 g/cm³, dae)

        ******                                          .--------.
      **********                                       /          \
     ************      SAME TERMINAL SETTLING         |   ρ0 = 1.0 |  dae
      **********             VELOCITY (vts)            \   g/cm³  /
        ******         =======================>         '--------'

Terminal Settling Velocity (vts)

For particles in the continuum regime (0.5 µm < dae < 50 µm) governed by Stokes' Law with the Cunningham Slip Correction Factor (Cc):

$v_{\text{ts}} = \frac{\rho_{\text{p}} \cdot d_{\text{p}}^2 \cdot g \cdot C_{\text{c}}}{18 \cdot \eta} = \frac{\rho_0 \cdot d_{\text{ae}}^2 \cdot g \cdot C_{\text{c}}}{18 \cdot \eta}$

Where:

• ρp = Actual particle density (g/cm³)
• ρ0 = Standard unit density (1.0 g/cm³)
• dp = Physical or geometric diameter (cm)
• g = Acceleration due to gravity (980.665 cm/s²)
• η = Dynamic viscosity of air (1.81 × 10⁻⁴ poise or g/(cm · s) at 20°C)
• Cc = Cunningham slip correction factor (accounts for non-continuum slip as particle size approaches the mean free path of air molecules, λ ≈ 0.066 µm)

Equating both forms reveals the fundamental relationship between aerodynamic diameter, physical diameter, and particle density:

$d_{\text{ae}} = d_{\text{p}} \cdot \sqrt{\frac{\rho_{\text{p}}}{\rho_0} \cdot \frac{C_{\text{c}}(d_{\text{p}})}{C_{\text{c}}(d_{\text{ae}})}} \approx d_{\text{p}} \cdot \sqrt{\frac{\rho_{\text{p}}}{\rho_0}}$

(For dense particles such as lead dust with ρp = 11.34 g/cm³, a small 1.0 µm physical particle behaves aerodynamically like a 3.37 µm sphere, depositing higher in the respiratory tree than its physical size suggests).

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Particle Size-Selective Sampling Criteria (ACGIH / ISO / CEN)

The American Conference of Governmental Industrial Hygienists (ACGIH), International Organization for Standardization (ISO 7708), and European Standardization Committee (CEN) harmonize size-selective sampling into three anatomical fractions defined by their 50% cut point (d50):

+-----------------------------------------------------------------------------------------+
|                    RESPIRATORY TRACT DEPOSITION & SAMPLING CONVENTIONS                  |
|                                                                                         |
|   +---------------------------------------------------------------------------------+   |
|   | INHALABLE FRACTION (d50 = 100 µm)                                               |   |
|   | Enters nose and mouth; deposits throughout entire respiratory tract.           |   |
|   | Samplers: IOM at 2.0 L/min, Button Sampler at 4.0 L/min                        |   |
|   +---------------------------------------+-----------------------------------------+   |
|                                           |                                             |
|                                           v                                             |
|   +---------------------------------------------------------------------------------+   |
|   | THORACIC FRACTION (d50 = 10 µm)                                                 |   |
|   | Penetrates larynx into trachea and bronchial airways.                           |   |
|   | Target Hazards: Sulfuric acid mist, cotton dust.                                |   |
|   +---------------------------------------+-----------------------------------------+   |
|                                           |                                             |
|                                           v                                             |
|   +---------------------------------------------------------------------------------+   |
|   | RESPIRABLE FRACTION (d50 = 4.0 µm)                                              |   |
|   | Penetrates unciliated gas-exchange region (alveoli).                            |   |
|   | Target Hazards: Crystalline silica, coal dust.                                  |   |
|   | Samplers: Dorr-Oliver (1.7 L/min), SKC Aluminum (2.5 L/min)                      |   |
|   +---------------------------------------------------------------------------------+   |
+-----------------------------------------------------------------------------------------+

1. Inhalable Particulate Mass (IPM)

• Anatomical Definition: Particles that enter the nose and mouth during breathing and deposit anywhere in the respiratory tract (head airways, tracheobronchial, and pulmonary alveolar regions).
• Cut Point: d50 = 100 µm.
• Collection Efficiency Curve: $E_{\text{I}}(d_{\text{ae}}) = 0.50 \cdot \left[ 1 + \exp(-0.06 \cdot d_{\text{ae}}) \right] \quad \text{for } d_{\text{ae}} \le 100\text{ }\mu\text{m}$
• Standard Collection Devices:
  • IOM Sampler: Institute of Occupational Medicine conductive plastic or stainless steel cassette operated at 2.0 L/min. The entire internal filter plus cassette housing is weighed as a unit to capture wall deposits.
  • Button Aerosol Sampler: Porous curved-screen inlet operated at 4.0 L/min, minimizing ambient wind direction sensitivity.
• Limitation of Traditional 37-mm Closed-Face Cassette (CFC): The traditional 37-mm CFC severely undersamples particles larger than 30 µm, underestimating true inhalable particulate exposure by up to 50% in wood, grain, or metal processing environments.

2. Thoracic Particulate Mass (TPM)

• Anatomical Definition: Particles that bypass the head airways (extrathoracic region) and penetrate through the larynx into the conducting airways (trachea and bronchi).
• Cut Point: d50 = 10 µm.
• Target Hazards: Sulfuric acid mist (ACGIH TLV-TWA 0.2 mg/m³ thoracic), cotton dust, berylliosis agents.
• Sampling Devices: Thoracic cyclones (e.g., BGI GK2.69 at 1.6 L/min) or Parallel Particle Impactors (PPI).

3. Respirable Particulate Mass (RPM)

• Anatomical Definition: Particles that penetrate deep into the unciliated gas-exchange region of the lungs (alveoli), where macrophage clearance mechanisms operate.
• Cut Point: d50 = 4.0 µm (ACGIH/ISO/CEN criterion; note that the historical British Medical Research Council / BMRC curve used d50 = 5.0 µm, and older 1968 ACGIH guidelines used 3.5 µm).
• Target Hazards: Respirable crystalline silica (α-quartz, cristobalite, tridymite), respirable coal dust, talc.

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Respirable Size-Selective Samplers: Cyclones & Impactors

A cyclone is a centrifugal separator that uses high-speed rotational airflow to remove coarse particles. Air enters tangentially at the cyclone inlet, forming a high-velocity downward spiral. Particles with high inertia (> d50) are thrown against the outer cylinder wall by centrifugal force, lose kinetic energy, and fall into the grit pot. The inner vortex reverses and carries respirable particles (< d50) upward onto the collection filter.

Tangential Air Inflow                                Filter Stage (Respirable Fraction)
======================> [ Cyclone Body ]            +--------------------------------+
                        |              |            |          Filter Paper          |
                        |   Downward   |            +--------------------------------+
                        | Outer Vortex |                           ^
                        |              |                           | (Upward Inner Vortex)
                        |              |                           |
                        \              /            ----------------------------------
                         \   Taper    /
                          \          /
                          +----------+
                          | Grit Pot | (Coarse particles > 4.0 µm discarded)
                          +----------+

Major Cyclones and Mandatory Calibrated Flow Rates

Operating a cyclone at an incorrect flow rate alters the internal centrifugal acceleration, drastically shifting the cut point:

• Higher Flow Rate: Increases centrifugal force arrow lowers d50 cut point arrow underestimates respirable mass.
• Lower Flow Rate: Decreases centrifugal force arrow raises d50 cut point arrow allows non-respirable particles onto filter arrow overestimates respirable mass.

[!WARNING] Field Cyclone Protocol: Cyclones must always remain in a vertical, upright orientation during and after sampling. If an active cyclone is inverted or tilted past horizontal, non-respirable coarse particles from the grit pot will dump directly onto the filter face, invalidating the gravimetric and chemical analysis.

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Filter Media Selection Guide

Selecting the correct analytical filter membrane is governed by chemical compatibility, gravimetric stability, digestion properties, and the required instrumental finishing technique.

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Particle Size Distribution: MMAD and GSD

Aerosol populations in occupational settings are rarely monodisperse (all particles having identical size). Instead, they exhibit lognormal particle size distributions.

Cumulative Mass % (Probability Scale)
99.9 +
     |
84.1 +--------------------------------------------o (d84.1)
     |                                           /|
     |                                          / |
50.0 +---------------------------------o (MMAD)/  |
     |                                /|      /   |
15.9 +-----------------------o (d15.9)/|     /    |
     |                      /|       / |    /     |
 0.1 +---------------------+--------+------+------+-----> Log Particle Aerodynamic
                           d15.9    MMAD   d84.1          Diameter (dae, µm)

Definitions & Formulae

1. Mass Median Aerodynamic Diameter (MMAD): The aerodynamic diameter (d50) at which exactly 50% of the total aerosol mass is contained in particles smaller than this diameter, and 50% in particles larger.
2. Geometric Standard Deviation (GSD, σg): The measure of aerosol polydispersity (breadth of the particle size spread), determined from log-probability plots:

$\sigma_{\text{g}} = \frac{d_{84.1\%}}{d_{50\%}} = \frac{d_{50\%}}{d_{15.9\%}} = \sqrt{\frac{d_{84.1\%}}{d_{15.9\%}}}$

• Monodisperse Aerosol: σg < 1.2 (typically generated only in laboratory aerosol research).
• Polydisperse Aerosol: σg ≥ 1.2 (virtually all real-world occupational aerosols, with typical workplace GSD values ranging from 1.5 to 3.5).

Cascade Impactors

Cascade impactors (e.g., the 8-stage Andersen Cascade Impactor or personal Marple impactors) measure particle size distributions directly. Air is drawn through successive stages with progressively smaller nozzles. At each stage, air jets accelerate; particles exceeding the stage cut point (d50) possess sufficient inertia to break out of air streamlines and impact onto an impaction substrate, while smaller particles remain entrained and pass to the next stage.

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Worked Step-by-Step Example: Respirable Silica Exposure Calculation

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WORKED EXAMPLE: Respirable Crystalline Silica Compliance Assessment
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Scenario:
An industrial hygienist conducts personal exposure monitoring on a concrete grinding
worker. A 37-mm PVC filter (5.0 µm pore size) mounted in an SKC Aluminum Cyclone is
operated at 2.50 L/min for 420 minutes (7.0 hours).

Laboratory Analytical Results (NIOSH 0600 / NIOSH 7500 XRD):
- Pre-sampling filter tare weight (W1)  = 15.420 mg
- Post-sampling filter gross weight (W2) = 16.295 mg
- Field blank net weight change (W_blank) = +0.015 mg
- Crystalline quartz mass via XRD (m_silica) = 48.0 µg

----------------------------------------------------------------------------------------
Step 1: Calculate Total Air Volume Sampled (V)
  V = Q × t = 2.50 L/min × 420 min = 1050 L
  V = 1050 L / 1000 L/m³ = 1.050 m³

Step 2: Calculate Net Respirable Dust Mass on Sample Filter
  m_gross_dust = W2 - W1 = 16.295 mg - 15.420 mg = 0.875 mg
  m_net_dust   = m_gross_dust - W_blank = 0.875 mg - 0.015 mg = 0.860 mg (860 µg)

Step 3: Calculate Respirable Dust Concentration
  C_dust = m_net_dust / V = 0.860 mg / 1.050 m³ = 0.819 mg/m³ (819 µg/m³)

Step 4: Calculate Respirable Crystalline Quartz Concentration
  C_silica = m_silica / V = 48.0 µg / 1.050 m³ = 45.71 µg/m³ (0.0457 mg/m³)

Step 5: Calculate Percentage Quartz in the Respirable Dust
  % Quartz = (m_silica / m_net_dust) × 100%
  % Quartz = (48.0 µg / 860 µg) × 100% = 5.58%

Step 6: Evaluate Compliance against OSHA Standards
  OSHA Permissible Exposure Limit (PEL) for Respirable Crystalline Silica = 50.0 µg/m³
  OSHA Action Level (AL) for Respirable Crystalline Silica = 25.0 µg/m³
  
  Finding: The worker's 8-hour TWA exposure is 45.71 µg/m³. This is below the PEL (50 µg/m³)
  but exceeds the OSHA Action Level (25 µg/m³), triggering mandatory periodic exposure
  monitoring, medical surveillance, and inclusion in the written exposure control plan.
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