* INDUSTRIAL HYGIENE SAMPLING AND SIZING
OF AIRBORNE
PARTICLES
UNIVERSITY OF HOUSTON - CLEAR LAKE
SPRING 2015
*INTRODUCTION
Techniques to evaluate exposures to particulates in
occupational workplace 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, time of exposure,
other factors;
- health effects – irritation, illness, disease.
*AEROSOLS
Aerosol – described as solid and/or liquid particles
dispersed in 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
Aerosol science and inhalation toxicology assist
with better understanding and development of
exposure characterization.
*
Sampling and analysis techniques for
aerosol characterization. Single procedure
is not appropriate; be familiar with
properties of aerosols.
Evolving ultrafine and nano-size aerosols,
also incidentally address bioaerosols.
*
Forms of Aerosols:
*Dust (also crystalline materials)
*Fumes
*Mists
*Fogs
*Smokes
*Fibers (length exceeds diameter)
*
Particulate aerosols produced by mechanical
processes (i.e. breaking, grinding, pulverizing).
Examples: mining; material handing; dry material
preparation and packaging.
Chemically unchanged; smaller size and higher
surface area enhances 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.
*
Fine solid aerosol particles produced from
re-condensation of vaporized material
normally solid at standard conditions.
Condensation (0.01 um) occurs during
cooling of vapor – nucleation; coagulation –
agglomerates at 1 um diameter nearly
spherical.
Small enough for deep lung
penetration; chemically quite reactive.
Examples: welding – “metal fume fever”.
*
Spherical droplet aerosols produced from
bulk liquid by mechanical processes (i.e.
splash or spray). Droplets are chemically
unchanged from the parent liquid and range
in size from a few microns to over 100 um.
Examples: mist aerosol from spray painting;
crop spraying operations designed to
produce mists; mist droplet aerosols by
coughing or health care treatment.
*
Droplet aerosols produced by physical
condensation from vapor phase.
Fog
droplets typically smaller than mechanically
generated mist droplets, at 1 to 10 um.
Fogs appear to remain suspended in air
compared to mists which visibly settle to
ground.
*
Mixtures of solid and liquid aerosol particles,
gases, and vapors resulting from incomplete
combustion of carbonaceous materials and formed
by complex combinations of physical nucleationtype mechanisms and chemical reactions.
Examples: tobacco smokes; smokes from other
combustion (i.e. plastics, synthetic fabrics, and
petrochemical products – toxic). Primary smoke
particles with diameters of 0.01 to 1 um, but like
fumes, agglomerates containing many particles
may be much larger.
*
Elongated particles with length much greater than
width. May be naturally occurring or synthetic.
Examples: asbestos with convention to define a
“fiber” as particle with ratio of length or width
greater than 3:1; specific diseases; synthetic
fibers.
Fibrous aerosols display aerodynamic and health
effects behaviors that differ in many respects from
spherical or near-spherical particles of 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 assessment methods include particle
counting (mppcf) and total surface area.
Can account for contribution of reactive
surfaces and considers smaller particles;
evaluation tools to assess risk of ultrafine
and nano-materials.
*
*Aerosol distribution – Figure 14.1
Refer to info regarding size ranges and general
definition of particle types.
*Monodisperse (single particle size)
vs. Polydisperse (range of particle sizes)
Figure 14.2
*Particle size distribution:
log normal !!!
Tail that extends out to larger particles.
*
*
*
*Isometric – length dimension independent of
particle orientation (e.g. dusts)
*Spherical – based on diameter (e.g.
bioaerosols)
*Singlet – single discrete particles
*Aggregate – coagulate or flocculate (i.e.
soot); large surface are per unit mass
*Morphology determination by optical or
electron microscopy.
*
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.
Influenced by physical aerosol
properties
(size,
shape,
density,
and
hygroscopicity [take up water]).
*
-
Sedimentation
-
Inertial Motion and Deposition
-
Diffusion
-
Interception
*
Refers to movement of an aerosol particle through
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 air density and viscosity.
Gravitational force opposed by gas viscosity.
Stokes’ Law and Diameter (Equations 14-2/14-4).
Aerodynamic equivalent diameter – “normalizes”
different aerosols to common basis for
comparison.
*
Discuss particle size in terms of 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, is diameter
of a unit density sphere (density = 1 g/cm3) that
would exhibit the same settling velocity.
*
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.
*
Inertia - defined as tendency to resist change in
motion;
important
for
human
inhalation/deposition and aerosol sampling.
Impaction on surface within distance traveled;
likelihood increased with mass and velocity of
particle and sharpness of change in direction.
Stokes Number = St. (Equation 14-7)
Inefficiency of impaction increases with increasing
St.
*
Diffusion - aerosol particles in a gaseous medium
collide with individual gas molecules that are in
random Brownian motion associated with
fundamental microscopic thermal behavior.
Diffusion coefficient is inversely proportional to
particle geometric size and independent of
particle density.
Favored by small particle diameter, large
concentration differences, and short distances for
diffusion.
*
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).
*
Other mechanisms are relevant to aerosols either
in terms of deposition in the respiratory tract or
sampling:
-
Centrifugal motion;
-
Electrical motion;
-
Thermophoretic particle motion.
Centrifugal and electrical
effective for larger particles.
forces
are
more
*
Action of forces between particle and surfaces:
London-van der Waals (by molecular
interactions);
electrostatic attraction (charge
differences);
capillary forces (adsorption of water
{or other liquid} film between …).
Smaller particles are more difficult to dislodge
than larger ones.
*
For given exposure situation, amount of aerosolized
material actually inhaled; fraction of inhaled aerosol
depositing in different respiratory tract regions, and fate
of deposited material are functions of:
- Physical and chemical nature;
- Exposure conditions;
- Individual characteristics.
*
Definition of “Breathing Zone”:
*Nasopharyngeal (NP): hygroscopic; absorb water;
humid; inertial impaction most significant.
*Tracheobronchial (TB): conducting airways
distribute inhaled air 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 dominant mechanism.
Respirable fraction.
*
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
critical issue about performance.
Aerosol mass per unit air volume (mass
concentration) based on size fractions and
respiratory system penetration by inhalation.
*
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.
*
*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.
*Rudimentary
sampler theory, but useful for
sampler selection for application and assess losses
and apply correction factors.
*
Aerosol particle size greatly influences where
respiratory tract deposition occurs, and site of
deposition determines degree of exposure hazard.
Sampling techniques to measure aerosol as:
- inhalable,
- thoracic, or
- respirable fractions.
*
-
-
Study aerosol mass concentration; numbers;
particle morphology; radioactivity; chemical
content; and biohazard potential.
Choice of media depends on aerosol
characteristics and analytical technique used.
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.
*
-
-
-
Fiber filters: cellulose, glass, or quartz.
Porous membrane: cellulose ester gels, PVC;
high porous mesh microstructure with
convoluted flow paths.
Efficiency by pore size (i.e. 0.1 to 10 um).
Capillary membrane: PC or polyester film
with direct pores of nearly uniform size
and distribution.
Polyester foam media
Also treated filters; sorbent/filter combo.
*
-
Fiber: low pressure drop at high flow rates;
high loading capacity; inexpensive; not
adequate for submicron size; water!
-
Porous: also “depth” filters as above due to
matrix deposition; higher flow resistance and
lower loading capacity than fiber filters.
-
Capillary: high pressure drop and low loading
capacity; susceptible to static charge build-up
that can affect particle capture and retention;
microscopy advantage [particles > pores are
captured at smooth and flat surface to view].
*
Cyclones use centrifugal forces for capture.
*
Cut size indicates aerodynamic diameter of
particle for which penetration through 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/2.5.
Others: electrostatic or thermal precipitators.
*
Widely used to characterize aerosols by size.
Impactor performance by 50% cut point size as d50, which
is particle size captured by impactor with 50% efficiency.
Single stage – DPM or PEM.
Multi-stage – used in cascade configuration – cumulative
mass distribution.
Different analysis – gravimetric; chemical, etc..
Airborne Particulate Matter fractions – PM 2.5 / 10.
Liquid impingers for mists; particle counting.
*
Measurement
employs
scattered
light
to
characterize aerosol concentration and/or particle
size distribution.
Examples: count concentration; count particles
and measure size; estimate aerosol mass conc.
Miniaturized personal samplers vs. real-time
aerosol measurements; datalogging, etc..
IH application of single-particle optical counters is
Condensation Nucleus Counter (CNC) – range of
0.005 to 1 um; component of respirator
quantitative fit-test systems (i.e. TSI PortaCount).
Nephelometer
*
Examination of the aerosol particles after deposition on a
suitable substrate by microscopy:
-
Phase Contrast Microscopy (PCM) – fibers; look at
count regarding concentration or size distribution;
-
Polarized Light Microscopy (PLM) – fiber ID;
-
Scanning Electron Microscopy (SEM); and,
-
Transmission Electron Microscopy (TEM).
Particle size characteristics by method, and resolution
power of microscope. “Representative” sample needed!
*
Significant interest in ultrafine and nanometer size
particulates. Engineered nano-particles (particles with at
least one dimension less than 100 nm) may have designed
physical, chemical, or biological properties.
-
Tapered-Element Oscillating Microbalance (TEOM)
-
Electrical Aerosol Detectors (EAD)
-
Denuder systems and diffusion batteries – research
tools used to collect ultrafine or nano-particles
between 1 m and 0.1 um.
*
Particle sizes are often approximately lognormally distributed; e.g., logarithms of particles
sizes follow 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.
*
*
CMD is taken as the particle size corresponding to
the 50% probability of occurrence, and the GSD is
calculated as either ratio of CMD to 15.75%
particle size or ratio of 84.13% particle size to the
CMD (both give the same GSD value).
If a single straight line cannot fit the data, then
distribution is not lognormal, as for mixtures of
aerosols from different sources (i.e. multimodal).
Cascade impactors are examples of mass-based
measurement instruments that characterize the
mass fraction rather than count fraction in
specified particle size intervals. MMAD>CMD.
*
* Airborne Concentration
* Air Volume
* Unit Conversions – mg/M3 to/from ppm
* Temperature/Pressure Corrections
* Statistics and Confidence Limits
* Time-Weighted Averages
* Potential Work Shift Adjustments
NOTE: Corrections needed for comparisons to
published occupational exposure limits.