Dust and Human Health - Mineral Dust: A Key Player in the Earth System - page 383

that are lined with mucus-covered ciliated cells that provide two major clearance
mechanisms for particles: absorption of dissolved material and particle transport to
the larynx where they are swallowed or to the lymphatic system where they are
disposed of (Kreyling and Scheuch 2000 ). Finally, the bronchioles, alveolar ducts,
alveolar sacs, and alveoli make up the pulmonary region whose primary purpose is
gas exchange. Particles may be deposited there by impaction, sedimentation, and
Brownian diffusion. As airflow velocities are greater in the nasopharyngeal and
upper portion of the tracheobronchial tree, impaction plays a greater role in these
regions. In the midsize and smaller bronchioles and the alveolar region, gravitational
settling prevails. Diffusional transport is important for particles less than 0.5 m,
as their likelihood of contacting the airway walls is governed by where collisions
between gas molecules and particles cause small displacements of the particle
(Schultz et al. 2000 ).
Inhaled particles that are deposited in the different parts of the respiratory
tract can react chemically with the fluids and tissues (Plumlee and Ziegler 2007 ).
Deposited particles that are soluble in the respiratory tract fluids or that contain
loosely surface-bound material may dissociate or undergo dissolution upon depo-
sition, and surface-bound components may react or interact with airway fluids or
cells. Some constituents released from soluble particles may combine with fluid
components to reprecipitate as other insoluble phases; an example is calcium
released from dissolution of inhaled particles of calcium sulfate, which can repre-
cipitate as calcium phosphate phases (Plumlee et al. 2013a ). Insoluble deposited
particles are cleared by coughing, phagocytosis by alveolar macrophages, transport
along the mucociliary escalator, or retention in the tissues with sequestration by
collagen (Kreyling et al. 2007 ). Because they contain chemically aggressive, acidic
lysosomal fluids, alveolar macrophages can digest some types of particles that
escape dissolution in the lung fluids (Plumlee and Ziegler 2007 ). They then transport
particles that they cannot digest to the lymph system.

15.4 Characteristics That Contribute

to Observed Health Effects

Studies of occupational exposures throughout history have contributed significantly
to our understanding of respiratory exposure and pulmonary toxicology. Agricola
and Paracelsus, for example, documented the etiology and treatment of miner's
disease in the fifteenth century (Gallo 2008 ). Epidemiological studies examining
community risk have generally focused on anthropogenic sources of pollution and
related events such as the 1948 Donora, Pennsylvania incident or the great smog of
1952 in London.
A number of factors have been identified as contributing to the adverse health
effects of airborne dusts, other airborne Earth materials, and particulate matter
including the following: intensity and duration of the exposure; physical and
chemical characteristics of the material; microbial or other pathogens present in