6.3 Aerosol Deposition, Clearance, and Respiratory Tract Dosimetry

Aerosol Deposition, Clearance, and Respiratory Tract Dosimetry

The BGC subject area definition for Toxicology names aerosol deposition and clearance in the respiratory tract explicitly. It is the hinge between air sampling and toxicology: a filter measures what is in the air, but disease is caused by what is deposited, where it deposits, and how long it stays. Two aerosols with identical mass concentrations can produce completely different doses to the alveolar region.

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1. Deposition Mechanisms

Five mechanisms remove particles from inhaled air. Three dominate.

Impaction occurs when a particle's momentum carries it straight on while the airstream turns — at the back of the throat, at the carina, and at every subsequent bifurcation. It is why high inspiratory flow rates (heavy work, mouth breathing) shift deposition upward and increase total deposition of coarse particles.

Sedimentation requires residence time. It dominates in the small airways where air velocity is low, and it is enhanced by breath-holding.

Diffusion dominates for ultrafine particles, whose random thermal motion carries them to a wall during the residence time of a breath. Diffusional deposition increases as particle size falls, which is why nanoparticles deposit efficiently in the alveolar region.

The deposition minimum

Because impaction and sedimentation both fall as size decreases while diffusion rises, total deposition efficiency passes through a minimum near 0.3 to 0.5 µm:

 Deposition
 efficiency |*                                       *
            | *                                    *
            |  *                                 *
            |   *                              *
            |     *                         *
            |        *                  *
            |            *          *
            |                *  *
            +----------------------------------------- Aerodynamic diameter
             0.01    0.1    0.3-0.5    1      5     10   (um)
                            MINIMUM

The same physics explains why 0.3 µm is the standard filter test particle size: it is the most penetrating particle size, so a filter certified at 0.3 µm performs at least as well everywhere else. An N95 is 95% efficient at its worst case, not on average.

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2. Regional Anatomy and the Size-Selective Conventions

The ACGIH and ISO/CEN size-selective sampling conventions are direct expressions of this physiology:

This is why a respirable silica sample uses a cyclone with a 4 µm cut and a lead sample does not: the toxicological target differs.

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3. Clearance

Mucociliary clearance. The conducting airways are lined with ciliated epithelium covered by a mucus blanket. Coordinated ciliary beating moves the blanket upward at roughly 1 to 10 mm per minute, carrying deposited particles to the pharynx where they are swallowed. Most material deposited in this region is cleared within about 24 hours. Cigarette smoke, ozone, sulphur dioxide, and cold dry air all impair ciliary function and prolong retention.

Alveolar macrophage clearance. The alveolar region has no cilia. Particles are phagocytosed by alveolar macrophages, which migrate to the mucociliary escalator or into the lymphatic system. This is slow — a half-time of months to years — and it is the reason alveolar-deposited dusts produce chronic rather than acute disease.

Dissolution and translocation. Soluble particles dissolve and are absorbed directly into blood or lymph. Insoluble ultrafine particles can translocate across the alveolar epithelium into the interstitium and systemic circulation.

Particle overload

At high dust burdens the macrophage clearance mechanism saturates: macrophages become engorged, mobility falls, and clearance slows dramatically. Retained dose then rises faster than exposure, producing a non-linear dose-response. This overload phenomenon underlies the pulmonary responses seen in high-dose rodent studies of poorly soluble low-toxicity dusts, and it complicates extrapolation from those studies to workplace exposures.

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4. Fibres Behave Differently

For a fibre, aerodynamic diameter is governed almost entirely by the fibre's diameter, not its length — approximately three times the physical diameter for a long fibre. A 20 µm long, 0.5 µm diameter asbestos fibre has an aerodynamic diameter around 1.5 µm and therefore reaches the alveolar region readily, while a 20 µm compact particle would impact in the upper airway.

This is the basis of the fibre pathogenicity paradigm: long (greater than about 20 µm), thin, and biopersistent fibres are the dangerous ones. They deposit deep because they are thin, they cannot be fully phagocytosed because they are long — producing frustrated phagocytosis and sustained inflammation — and they are not cleared because they do not dissolve. Interception, not impaction, is the mechanism that brings fibres into contact with airway walls.

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5. Why This Governs Sampling and Control Decisions

• Selecting a cyclone versus a total dust cassette is a toxicological decision about where the agent acts, not a convention.
• A mass concentration says nothing about particle size distribution; two welding operations at the same mg/m³ can deliver very different alveolar doses.
• Ultrafine and engineered nanoparticle exposures deposit efficiently in the alveolar region and may translocate systemically, so mass concentration alone is a poor exposure metric for them — surface area or number concentration is more relevant.
• A worker doing heavy work breathes faster and deeper and shifts from nasal to oral breathing, which increases both total deposition and the fraction reaching the deep lung at the same airborne concentration.