bahaya aerosol
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Generation and Behaviorof Airborne Particles (Aerosols)
Paul Baron
Division of Applied Technology
National Institute for Occupational Safety and HealthCenters for Disease Control and Prevention
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What is an AEROSOL?
Simply defined- tiny particlesor droplets suspended in air.
The haze in the picture on the
right is caused by light
scattering from numerous
water/oil droplets and mineral
particles released into the airfrom the drilling of rock.
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Are Aerosols dangerous?
The air we breathe always contains solid particlesor droplets and is therefore an aerosol.
These aerosol particles can be from naturalsources or man-made sources
Sometimes the particles are of type that, atsufficient concentration, are toxic to our body.
The organ in our body most sensitive to particle
exposure is the respiratory system
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Toxic Aerosols!?Our respiratory system is efficient at removing aerosols, but if they fallwithin particular size ranges, are highly concentrated, or toxic, they maycause adverse health effects. They may also deposit on skin or eyes,generally only causing irritation, though more toxic effects may occur.Very small particles may pass through the skin and enter the body that
way. Soluble particles may dissolve and pass through the skin.
Read on for more details on aerosol generation and behavior
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Overall Scenario:
Evaluation of Exposure in WorkplacesAerosol Transport
Based on Air FlowAerosol Sampling/
Measurement
Aerosol Inhalation
Aerosoleneration from,
e.g., Grinding
AerosolLosses
to Surfaces
SecondarySources
(Resuspension)
Loss Mechanisms
Settling,
Diffusion,Impaction,
Electrostatic
Deposition
Filter
Samplers
Direct
Reading
Instruments
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Aerosol Assessment in the
Workplace: Types of Measurements
Sampling, usually with a filter and pump, providesa sample that can be analyzed in the lab for
specific chemicals, quantity of dust, particle shape
(fibers), etc. Direct reading instruments allow continuous
observation of dust concentrations, e.g., mass or
concentration or size distribution, but do notusually provide specifics of the aerosol type.
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Aerosol Assessment in theWorkplace: Types of Measurements
The most accurate assessment of workersexposure is measurement with a personal sampler,
i.e, a collection or measurement device placed on
the workers chest. Techniques for control of exposures can use either
personal samplers or (fixed) area measurement
devices. Direct reading devices allow rapidassessment of the effectiveness of dust control
devices or strategies.
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I. Aerosol Size RangeParticle size is often determined by the process that generated the particle.
Combustion particles usually start out in the 0.01-0.05 m size range, but combine
with each other (agglomerate) to form larger particles. Powder is broken down
into smaller particles and released into the air; it is difficult to break down such
particles smaller than ~0.5 m. Biological particles usually become airborne fromliquid or powder forms, so these particles are usually larger than ~0.5 m.
VirusesBacteria
Spores
Mechanical Generation
(dust or mist)
Combustion Particles
(fume)
0.001 10 100
Particle Diameter (m)0.01 0.1 1
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II. Respiratory System DepositionICRP Model, averaged
over males, females,
several respiration rates
Particle Diameter (m)
0
0.6
0.8
1.0
0.4
0.2
Total
Head Airways
Head Airways
Alveolar
Alveolar(Gas exchange)
Tracheo-
bronchial
Tracheo-
bronchial
0.001 0.01 1010.1 100
DepositionFra
ction
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III. Aerosol Particle Behavior
Settling Impaction
Charge effects
Release from surfaces
Agglomeration/
Deagglomeration
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Particle Settling in Still Air
Time to settle 5 feet by unit density spheres
0.5 m 1 m 10 m 100 m3 m
41 hours 12 hours 1.5 hours8.2 minutes
Aerodynamic diameter definition:
diameter of a unit density sphere that
settles at the same velocity as the particle
in question
5.8 seconds
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Particle Settling in a Closed RoomStagnant air Turbulent air
Time
Conc. Conc.
Time
Particles of the same size will settle
at the same speed in still orstagnant air
Particles passing close to a
horizontal surface can settle, butthe rest will continue to be stirred.
Concentration
profiles using a
direct
measurementinstrument
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Particle Settling in Turbulent Air
Half-life of particles in 8 foot high room
0.5 m 1 m 10 m 100 m3 m
41 hours 12 hours 1.5 hours8.2 minutes
Particles settling in turbulent air will
have an exponential decay rate as
indicated in the previous slide 5.8 seconds
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Particle Transport in Buildings
Most large particle losses by settling Most small particle losses by exchange
with outdoor air
Complex flow systems
Turbulence production
Doors, people, fans. ventilation
Ventilation system
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III. Aerosol Particle Behavior
Settling Impaction
Charge effects
Release from surfaces
Agglomeration/
Deagglomeration
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Particle Impaction Impaction depends on
particle size, airvelocity, jet diameter
Large particles depositmore easily
Even larger particlescan bounce fromsurface
Impaction surface canbe modified to improvecollection, e.g., add oil
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Cascade Impactors
Pump
Used for size distributionmeasurement
Commercial impactors
Andersen
MOUDI
>8
>4
>2
Filter>1
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Virtual Impactors
Used to reduceparticle bounce
Used to concentrate
larger particles Commercial virtual
impactor up to 100:1
Contains smallerparticles in minorflow
Q0
0.9Q0
0.1Q0
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III. Aerosol Particle Behavior
Settling Impaction
Charge effects
Release from surfaces
Agglomeration/
Deagglomeration
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Electrostatic Effects
+
+
++
+
+
+
+
+
-
Particle-particle interaction small
Particle-surface interaction large Particle charge depends mostly on
generation process, surface energy,
humidity, time in the air Airborne particle charge gradually
decreases due to ions in air(particles are nearly neutral afterabout 30 min)
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Particle Charge Imparted DuringGenerationLiquid Droplets
In conductive solution,ions equally distributed
In nonconductivesolution, fewer ions
Droplet chargegenerally low
When liquidevaporates, the finalparticle may haverelatively high charge -
+
-
-
+
++
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Particle Charge Imparted DuringGenerationSolid Particles
Difference in surface
energy levels
Separation energy
Humidity creates
bridge betweenparticle and surface+ + + + + + + +
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Space Charge
Expansion of Aerosol
High aerosol concentration Particles are highly charged
All particles have same polarity
Aerosol will expand because of particle-particlerepulsion
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III. Aerosol Particle Behavior
Settling Impaction
Charge effects
Release from surfaces
Agglomeration/
Deagglomeration
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Generation from Carpet
Particles deposited incarpet; acts as a sink
Footstep crushes
fibers against each
other
Footstep compressescarpet, creating high
velocity air flow
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Particle Transport from Sources
Small particles
through
ventilationsystem Transport by
local turbulence
Direct settling
(larger particles
and clumps
Resuspension
by activity
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Asbestos Fiber GenerationEffect of humidity on particle chargeand particle generation efficiency
Mean
ElectricalMobility
Relative
Concentration
0 10 20 30
Relative Humidity (%)
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Particle Removal from Surfaces
by Air Flow
Boundary layer near
surfaceproducedby motionlesssurface
Factors affecting
release: Air velocity,particle attraction tosurface versusparticle cross
section Water (humidity)
can increaseadhesion
< 0.1 m virtually impossible
> 20 m relatively easy
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Vacuum Removal
Suction
forces air
near surface
to remove
particles
Variable
removal
efficiency
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III. Aerosol Particle Behavior
Settling
Impaction
Charge effects
Release from surfaces
Agglomeration/
Deagglomeration
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Agglomeration/ Deagglomeration
Particles in a powder are in close
contact, primarily agglomerates Shaken powder releases clumps
(agglomerates) and single particles
Shear forces, caused by differencein air velocity across the particle,can break apart clumps
Shear forces increase with
increasing energy (air velocity)
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Particle Size Evolution
105
10-3
10
-1
103
10
C
oncentration(#/cm
3)
Particle Diameter (m)
Grinding aerosolT = 0
T = 25 min
T = 225 minSettling/
Impaction
Diffusion
Coagulation (high conc.)
Surfaces
Surfaces
0.001 0.01 0.1 1 10 100
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IV. Aerosol Generation< 0.1 m virtually impossible
> 20 m relatively easyEnergy Input
Air flowMechanical energy
Overcome adhesion
between particle andsurface
Airflow to entrain
particles
Release happens
in microseconds
Adhesion depends
mostly on micro-
roughness ofsurface, also on
relative surface
energies
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V. Particle Collection and Measurement Filter sampling
Filter efficiency, pore size, filter type Sufficient volume for analysis
Dries particles because of continuous air flow
Removal from filter can be an issue Impactor sampling
Cascade impactor: 3 to 8 stages, size resolution
Sufficient volume for analysis Dries particles, though less than filter
Inert (oiled?) surface or direct to growth medium
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Filtration Air filtration different
from liquid filtration
Pore size in air filtersgenerally meaningless asindicator of efficiency
Small particles collected
by diffusion, large onesby impaction/interception
Maximum penetration at
about 0.3 m Efficiency increases with
increasing air velocity
Particle diameter (m)
0.01 0.1 1 10
Efficiency
1
Impaction/
InterceptionDiffusion
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Direct Reading
Aerosol Measurement
Optical particle counter
Relatively inexpensive (~2K $10K)
Portable, battery operated
Rapid detection
Nonspecific for bacteria
Toxic concentrations near or belowambient particle concentrations
Can be used for tracer studies
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Direct ReadingAerosol Measurement
Aerodynamic Particle Sizer Relatively expensive (~$40K)
Movable, line operated
Higher size resolution, possibly improved sizedistribution signature
Fluorescent detection version
Can be used for tracer studies
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Overall ScenarioAerosol Sampling/
MeasurementAerosol Transport
Aerosol
SourceCharacteristics
Aerosol
Lossesto Surfaces
Aerosol Inhalation
Secondary
Sources(Resuspension)
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Resources for AerosolInformation
Hinds, 1999, Aerosol Technology, Wiley Baron and Willeke, 2001, Aerosol Measurement,
Wiley
Hurst, 1997, Manual of Environmental
Microbiology, ASM Press
Spreadsheet: Aerosol Calculator available fromwww.tsi.com or www.bgiusa.com