Airborne Particulates

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INDUSTRIAL HYGIENE AIRBORNE PARTICULATES
UNIVERSITY OF HOUSTON – DOWNTOWN
INTRODUCTION
Introduce the techniques available for Industrial
Hygienists to recognize, evaluate exposures to
particulates, and control in occupational settings.
Inhaled particles may react with or be absorbed
through tissues to cause adverse health effects.
Variables include:
size, shape, and density;
chemical properties;
airborne concentration and time of
exposure, and other factors, etc.; so,
health effects – irritation, illness, disease.
AEROSOLS
Aerosol – described as solid and/or liquid particles
dispersed in a gaseous medium. Range of > 50
um to microscopic particles invisible to naked eye.
For IH, gaseous medium is usually AIR.
Occupational aerosol hazards recognized by:
Pliny
Agricola
Ramazzini
Sir Percival Pott
Least toxic as “nuisance dusts”; ACGIH – PNOC
or OSHA with parallel term – PNOR.
PNEUMOCONIOSIS
Derived from Greek; means “dust in the lungs”.
Widely used to describe lung diseases. Recent
universally
accepted
definition
of
“the
accumulation of dust in the lungs and the tissue
reaction to its presence”.
Assumes dust is
insoluble/solid when deposited in alveoli of lung.
e.g. Asbestosis, Silicosis, Coal (black lung), dust.
Familiar with properties and techniques.
Evolving ultrafine and nano-sized aerosols.
Incidentally addresses bioaerosols or aerosols of
biological origin.
ASSESSMENT TECHNIQUES
Techniques for sampling and analysis
continue to evolve for characterization of
aerosols.
A single technique is not
appropriate; IH must be familiar with
properties and assessment techniques.
Evolving ultrafine and nano-sized aerosols.
Incidentally addresses bioaerosols or
aerosols of biological origin.
DEFINITIONS
Forms of Aerosols:






Dust (also crystalline materials)
Fumes
Mists
Fogs
Smokes
Fibers (length exceeds diameter)
DUSTS
Particulate aerosols produced by mechanical
processes such as breaking, grinding, and
pulverizing. Examples: mining; material handing;
dry material prep and packaging. Chemically
unchanged; smaller size and higher specific
surface area may enhance ability to be airborne,
inhaled, penetration, toxicity, solubility or explosion.
Less than 1 um up to 1 mm; regular in shape;
some crystalline materials; length to width ratio
less than 3:1 with some notable exceptions.
FUMES
Fine solid aerosol particles produced from
the re-condensation of vaporized material
that is normally solid at standard conditions
is melted and vaporized; condensation (0.01
um) occurs during cooling of vapor –
nucleation; coagulation – agglomerates at 1
um in diameter are nearly spherical. Small
enough to penetrate to deep lung areas;
chemically quite reactive.
Examples:
welding/smelting – “metal fume fever”.
MISTS
Spherical droplet aerosols produced from
bulk liquid by mechanical processes such as
splashing, bubbling, or spraying. Droplets
are chemically unchanged from the parent
liquid and range in size from a few microns
to over 100 um. Mist aerosol from spray
painting. Examples: mist aerosol from
spray painting; crop spraying operations
designed to produce mists; mist droplet
aerosols by coughing or treatment of
infected patients in health care settings.
FOGS
Droplet aerosols produced by physical
condensation from the vapor phase. Fog
droplets
are
typically
smaller
than
mechanically generated mist droplets, and
are of the order of 1 to 10 um. Whereas
mists may visibly settle toward the ground,
fogs appear to remain suspended in the air.
SMOKES
Complex mixtures of solid and liquid aerosol
particles, gases, and vapors resulting from
incomplete combustion of carbonaceous materials
and are formed by complex combinations of
physical
nucleation-type
mechanisms
and
chemical reactions. Examples: tobacco smokes;
smokes from other combustion (i.e. plastics,
synthetic fabrics, and petrochemical products –
toxic). Primary smoke particles are on the order of
0.01 to 1 um in diameters; but like fumes,
agglomerates containing many particles may be
much larger.
FIBERS
Elongated particles with length much greater than
width. May be naturally occurring or synthetic.
Examples: asbestos with convention to define a
“fiber” as a particle with a ratio of length or width
greater than 3:1; specific asbestos-related
diseases; synthetic fibers, etc. Fibrous aerosols
display aerodynamic and health effects behaviors
that differ in many respects from spherical or nearspherical particles of the same material and mass,
so aerosol characterization is more complex for
fibers than other aerosols.
AEROSOLS
Aerosol concentrations in air are often
assessed by mass per unit volume (mg/M3);
when using mass, large particles have the
most significant impact on total mass.
Mass = Volume x Density
Other methods to assess aerosols include
particle counting (mppcf) and total surface
area. Can account for contribution of
reactive surfaces and give consideration to
smaller particles; good evaluation tools to
assess risk of ultrafine and nano-materials.
CHARACTER/MORPHOLOPY








Aerosol distribution – Figure 8-1
Monodisperse vs. Polydisperse; modal
Particle size distribution: log normal !!!
Isometric – length dimension independent of
particle orientation (e.g. dusts)
Spherical – based on diameter
Singlet – single discrete particles and remain
Aggregate – coagulate or flocculate (i.e.
soot); large surface are per unit mass
Morphology – optical or electron microscopy
DEPOSITION
For a given exposure situation, the amount
of aerosolized material actually inhaled; the
fraction of inhaled aerosol depositing in the
different regions of the respiratory tract, and
the fate of the deposited material are
functions of:
Physical and chemical nature;
Exposure conditions;
Individual characteristics.
DEPOSITION MECHANISMS
-
Inertial impaction
Interception
Sedimentation
Electrostatic attraction
Diffusion (i.e. Brownian movement)
INERTIAL MOTION/DEPOSITION
Inertia - defined as tendency to resist a
change in motion; important for human
inhalation/deposition as well as aerosol
sampling.
Impaction on surface within distance
traveled; likelihood increased with the mass
and velocity of particle and the sharpness of
change in direction.
Stokes Number = St.; inefficiency of
impaction increases with increasing St.
AED
INTERCEPTION
Interception – flow of an aerosol past a
surface may produce particle deposition.
Deposition process does not depend on
particle motion across fluid stream lines, as
for inertial impaction. Depends on particle
coming close enough to a flow boundary (by
any means) that it may be collected by
virtue of its own physical size.
Significant to elongated particles (i.e. fibers).
SEDIMENTATION
Refers to movement of an aerosol particle through
a gaseous medium under the influence of gravity.
The rate of settling depends on particle size,
shape, mass, and orientation (for non-spherical
particles) and on the air density and viscosity.
Gravitational force opposed by gas viscosity.
Stokes’ Law and Aerodynamic Equivalent
Diameter (AED).
Aerodynamic equivalent diameter – “normalizes”
different aerosols to common basis for
comparison.
SURFACE PARTICLE RETENTION
Action of various forces:
London-van der Waals (by molecular
interactions between particle and
surfaces);
electrostatic attraction (charge
differences between particle and
surface);
capillary forces (adsorption of water
{or other liquid} film between the particle
and the surface).
Smaller particles are more difficult to dislodge than
larger ones.
DIFFUSION
Diffusion - aerosol particles in a gaseous
medium collide with individual gas
molecules that are in random Brownian
motion associated with their fundamental
microscopic thermal behavior.
Diffusion coefficient is inversely proportional
to particle geometric size and is
independent of particle density.
Favored by small particle diameter, large
concentration
differences,
and
short
distances for diffusion.
DEPOSITION BY INHALATION
Definition of “Breathing zone”:
 Nasopharyngeal (NP): hygroscopic; absorb water;
humid; inertial impaction is most significant.
 Tracheobronchial (TB): conducting airways
distribute the inhaled air quickly and evenly to
deeper portions of lung; therefore, lower velocities
and higher residence times favor sedimentation
and diffusion. Thoracic fraction of < 10 um.
 Pulmonary (P): depending on particle size, either
sedimentation or diffusion is the dominant
deposition mechanism. Respirable fraction.
PARTICLE DEPOSITION
-
-
-
Head/airways/nasopharyngeal
region:
nose,
nasal turbinates, throat – 5 to
100 um by IMPACTION
Thoracic/bronchial
region:
trachea,
bronchi
–
1
to
10
um
by
SEDIMENTATION, INTERCEPTION,
AND IMPACTION
Alveolar/Gas exchange region: terminal
bronchioli and alveoli – 0.01 to 10 um by
DIFFUSION
CRITICAL EXPOSURE FACTORS





Chemical/biological composition
Crystalline, structural, and isotropic forms
Shape of particles
Size of particles
Dose: concentration of particles in the work
environment and exposure duration
 Pre-existing health or generic status of
worker
 Concurrent exposure to other toxic agents.
STOKES AND AERODYNAMIC
DIAMETER
Discuss particle size in terms of the diameter of a
spherical particle of the same density that would
exhibit the same behavior as the particle in
question = Stokes diameter (dST).
Aerodynamic equivalent diameter, dae, which is the
diameter of a unit density sphere (density = 1
g/cm3) that would exhibit the same settling velocity
as the particle in question.
Aerodynamic equivalent diameter “normalizes”
different aerosols to a common basis so that
behaviors may be directly compared. Particle
Stokes and aerodynamic diameters are important
for inertial and gravitational deposition, collector
design, and data interpretation.
AIRBORNE PARTICLE MOTION
Dose – drives biological response; result of
exposure history; deposition efficiency;
target organs; depends on exposure history;
pharmacokinetics of clearance process and
intrinsic toxicity. Bioaccumulation.
Dose rate – rate at which substance arrives
(inhaled or deposited) and exposure may be
measured by IH. Influenced by physical
properties of aerosol (size, shape, density,
and hygroscopicity [take up water].
BIOLOGICAL REACTIONS
-
Acute sensory/pulmonary irritation
Lung edema
Allergic sensitization
Fibrosis
Emphysema
Toxicity – systemic, lymphatic
Oncogenesis
Infection
Metal fume fever
AEROSOL SAMPLING THEORY
Sampling objective is to obtain information
about aerosol properties at a given location
over a specified length of time. Therefore,
nature of air flow and particle motion both
inside and outside of sampling device is a
critical issue regarding performance.
Aerosol mass per unit air volume (mass
concentration) based on size fractions and
respiratory system penetration by inhalation.
Breathing zone definition.
FILTRATION TECHNIQUES
-
-
-
Study aerosol mass concentration; number
concentration;
particle
morphology;
radioactivity; chemical content; and biohazard
potential.
Choice of media depends on the aerosol
characteristics and the analytical technique to be
used. Various types of filter media. Table 8-E.
Gravimetric analysis: mass/volume; mg/M3.
Open–face vs. Closed-face filter cassettes.
“Total” particulates; under-sample inhalable
fraction of larger particle sizes.
Best is IOM sampler for inhalable fraction.
PARTICLE SIZE SELECTION
 Size selective aerosol sampling.
 External and internal sampling losses.
 Sampler efficiency is a complex function of:
sampler geometry; sampling rate; flow
external to sampler; and sampler orientation
with respect to direction of air flow.
 Personal vs. area sampling.
 Clearance vs. aggressive techniques
depending on specific application.
SAMPLING THEORY
The intention of “total dust” sampling is that
all particles in the air should be collected
with equal efficiency without respect to any
particular particle size fraction. By contrast,
particle size-selective sampling is intended
to separate the aerosol into size fractions
based on health rationale.
Exercise caution regarding sampler choice
and insure that the particle size fraction of
interest is properly collected.
PARTICLE SIZE-SELECTIVE
SAMPLING
Aerosol particle size greatly influences where
deposition occurs in the respiratory tract, and the
site of deposition often determines the degree of
hazard represented by the exposure. Sampling
techniques to measure aerosol as:
inhalable,
thoracic, or
respirable fractions.
SEDIMENTATION TECHNIQUES
Cyclones use centrifugal forces to effect particle
capture.
Cut size indicates aerodynamic diameter of
particle for which penetration through the cyclone
is 50% (d50 at 4 um for respirable fraction).
Efficient for large particle sizes and IH use as preseparators in respirable aerosol samplers:
Dorr-Oliver nylon – 1.7 lpm
Casella and SKC cyclones – 1.9 lpm.
Others: electrostatic or thermal precipitators.
IMPACTION TECHNIQUES
Among
most
widely
used
in
aerosol
characterization in relation to particle size.
Impactor performance by 50% cut point size as
d50, which is the particle size captured by the
impactor with 50% efficiency.
Single stage – DPM or PEM.
Multi-stage – used in cascade configuration –
cumulative mass distribution; Andersen or Marple.
Different analysis – gravimetric; chemical, etc..
Airborne Particulate Matter fractions – PM 2.5 / 10.
Liquid impingers for mists; particle counting.
AIRBORNE PARTICULATES
-
Microbial – culturable vs. non-culturable
Radon/radon progeny
Diesel exhaust
Fibers – e.g. asbestos
Total vs. respirable mass
SAMPLING TECHNIQUES
-
Size-Selective
Dual-Phase Monitoring
Isokinetic Sampling
Surface Sampling
Dermal Monitoring
AEROSOL SIZE DISTRIBUTION
ANALYSIS
Particle sizes in an aerosol are often
approximately lognormally distributed; that is , the
logarithms of the particles sizes follow a Gaussian,
or normal, frequency distribution.
Therefore, “statistics” include geometric mean (or
median) size and geometric standard deviation
(GSD). Distribution expressed using either the
count median diameter (CMD) and GSD or the
mass median aerodynamic diameter (MMAD) and
GSD, depending on how the measurement data
were obtained.
PARTICLE SAMPLE ANALYSIS
-
Metals
Free Crystalline Silica
Asbestos
Radioactive Particles
Gravimetric Analysis
Biological Organisms
Organic Particles
Direct-Reading Particle Detectors
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