Asbestos Fibers and Other Elongate Mineral Particles: State of the ...

or statistical power to permit a conclusion regarding
the presence or absence of a causal association,
or no data on cancer in humans are
available” [IARC 1997]. On the basis of studies
in rats, palygorskite (attapulgite) fibers longer
than 5 mm were determined to be possibly carcinogenic
to humans (Group 2B) [IARC 1997].
In experimental animals the evidence was limited
for the carcinogenicity of long sepiolite fibers
(>5 µm) and inadequate to assess carcinogenicity
of nonerionite fibrous zeolites (including
clinoptilolite, mordenite, and phillipsite)
and wollastonite (Group 3) [IARC 1997]. These
Group 3 determinations highlight the need for
additional research on nonasbestiform EMPs.
2.9 Determinants of Particle
Toxicity and Health Effects
Current recommendations for assessing occupational
and environmental exposures to asbestos
fibers rely primarily on dimensional
and mineralogical characteristics. Dimension,
which influences the deposition of EMPs in the
lung, lung clearance mechanisms, and retention
time in the lung, is an important determinant
of toxicity. However, other particle characteristics,
such as durability in lung fluids, chemical
composition, and surface activity, may also
play important roles in causing respiratory diseases.
Research to elucidate what roles these
EMP characteristics play in causing biological
responses may help to provide better evidence-based
recommendations for asbestos fibers
and other EMPs.
2.9.1 Deposition
Deposition of airborne particles in the respiratory
system is defined as the loss of particles
from the inspired air during respiration.
Clearance pertains to the removal of deposited
NIOSH CIB 62 • Asbestos
particles by diverse processes over time, whereas
retention is the temporal persistence of particles
within the respiratory system [Morrow 1985].
The deposition of inhaled particles in the respiratory
tract is a function of their physical characteristics
(dimension and density), the anatomical
and physiological parameters of the airways, and
the rate and depth of respiration [Yu et al. 1986].
Although particle chemical composition does
not play a role in deposition, respiratory clearance
of all particle types is dependent on both physical
and chemical characteristics of the particle.
In addition, surface charge and hydrophilicity, as
well as adsorbed materials (e.g., coatings on synthetic
fibers) and other physical and chemical factors,
determine whether small particles and fibers
will agglomerate into larger, nonrespirable masses
[ILSI 2005].
Depending on their physical characteristics, inhaled
particles are differentially deposited in one
of the following three respiratory system compartments:
the extrathoracic region, consisting
of the anterior and posterior nose, mouth, pharynx,
and larynx; the bronchial region, consisting
of the trachea, bronchi, and bronchioles down
to and including the terminal bronchioles; and
the alveolar-interstitial region, consisting of the
respiratory bronchioles, alveolar ducts, and alveolar
sacs.
Important parameters for the deposition of
airborne particles are their aerodynamic and
thermodynamic properties. Below a particle
size of 0.5 µm aerodynamic equivalent diameter
(AED), thermodynamic properties prevail.
The AED of EPs is mostly determined by
their geometric diameter and density. Deposition
of EPs in an airway is strongly related to
the orientation of the particles with respect
to the direction of the air flow and is affected
by the interrelationship of four major deposition
mechanisms: impaction, interception,
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