Geoscience Reference
In-Depth Information
the material; the bioaccessibility, biodurability, and bioreactivity of the material in
the body fluids; the body's physiological processes; and individual factors such as
genetics, personal habits (e.g., smoking), and nutritional status (Plumlee et al. 2006 ).
Although numerous PM studies have been conducted in an effort to identify which
components are responsible for the observed health effects (e.g., cardiovascular
or respiratory), there is little agreement among studies. Study inconsistencies
may be related to a number of factors including the following: PM sample and
source characterization, model design and parameters, and individual susceptibility.
Additionally, variation in the number of components to which people are exposed,
their concentrations in and on the surface of particles, and the complexity of surface
chemistry on small volume-to- high surface area particles magnifies the difficulty
of identifying specific etiological mechanisms responsible for observed adverse
effects. As the amount of literature on PM effects and toxicity is quite large and not
specific to MD, we provide here an overview of select factors and related research
likely to be associated with MD.
15.4.1
Particle Size and Composition
Early studies of “air pollution” often focused on particle size alone and ignored
composition. Studies that focus on size-dependent effects and ignore chemistry
address the concept of a “nonspecific” effect, meaning the effect is due only to
the particle's presence (Schlesinger et al. 2006 ). Crustally derived materials such as
MD are not often identified as components in many PM studies, though some studies
have used surrogates such as Al or Si to represent all crustal materials that may be
present in the sample (Schlesinger 2007 ). Results of studies designed to measure
the influence of particle size on adverse health effects have been inconsistent.
Some studies found no increase in health risk related to coarse PM exposure (Puett
et al. 2009 ), while other studies have demonstrated an association (Brunekreef and
Forsberg 2005 ; Malig and Ostro 2009 ). Also, as measures of PM concentration and
size are obtained from established monitoring sites, not necessarily where people
live or work, they may not provide representative exposures (Baxter et al. 2013 ).
Complicating the task of identifying the component or mechanism of action
responsible for illness or death is the variation in the numbers of components that
make up PM, their concentrations, and their chemical behaviors in the body. In
occupational settings, some trace metals have been recognized as inhalation toxi-
cants, such as iron, nickel, and hexavalent chromium. Studies indicate a variety of
metals likely contribute to air pollution toxicity, although no single physiochemical
property has emerged as a common etiological factor (Samet and Ghio 2007 ).
Several studies have associated metals with a variety of toxicity effects, such as
reactive oxygen species generation and oxidative stress (e.g., iron), production
of inflammatory or fibrotic responses (e.g., zinc), and cancer (e.g., hexavalent
chromium or nickel) (Adamson et al. 2000 ; Knaapen et al. 2002 ; Samet and Ghio
2007 ; Schaumann et al. 2004 ). Unfortunately, some studies assume that the metals
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