Generation and Behavior 

of Airborne Particles (Aeros

Paul Baron

Division of Applied Technology

National Institute for Occupational Safety and 

Centers for Disease Control and Preventio
Overview

I. Particle size rangeII. Inhalation & lung depositionIII. Particle behavior

– Settling, impaction, electro

effects

IV. Particle generation

– Energy input, size, charge, hu

Scenarios

• Letter release• Carpet release

VI. Particle collection and meas
What is an AEROSOL?

• Simply defined- tiny particles 

or 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 air from the drilling of rock.
Are Aerosols dangerous?

• The air we breathe always contains solid pa

or droplets and is therefore an aerosol.

• These aerosol particles can be from natural 

sources or man-made sources

• Sometimes the particles are of type that, at 

sufficient concentration, are toxic to our bod

• The organ in our body most sensitive to par

exposure is the respiratory system
Toxic Aerosols!?

Our respiratory system is efficient at removing aerosols, but if they fwithin particular size ranges, are highly concentrated, or toxic, they cause adverse health effects. They may also deposit on skin or eyesgenerally only causing irritation, though more toxic effects may occVery small particles may pass through the skin and enter the body thway. Soluble particles may dissolve and pass through the skin.

Read on for more details on aerosol generation and behavior
Overall Scenario:

Evaluation of Exposure in Workp

Aerosol TransportBased on Air Flow

AerosoMeasu

Aeros

Aerosol

Generation from,

e.g., Grinding

Aerosol

Losses

to Surfaces

Secondary

Sources

(Resuspension)

Loss Mechanisms

Settling,

Diffusion,

Impaction,

Electrostatic

Deposition
Aerosol Assessment in the 

Workplace: Types of Measurem

• Sampling, usually with a filter and pump, pr

a sample that can be analyzed in the lab for specific chemicals, quantity of dust, particle (fibers), etc.

• Direct reading instruments allow continuou

observation of dust concentrations, e.g., maconcentration or size distribution, but do nousually provide specifics of the aerosol type
Aerosol Assessment in the 

Workplace: Types of Measurem

• The most accurate assessment of worker’s 

exposure is measurement with a personal sai.e, a collection or measurement device placthe worker’s chest.

• Techniques for control of exposures can use 

personal samplers or (fixed) area measuremdevices. Direct reading devices allow rapid assessment of the effectiveness of dust contrdevices or strategies.
I. Aerosol Size Range

Particle size is often determined by the process that generated the paCombustion particles usually start out in the 0.01-0.05 

Pm size range

with each other (agglomerate) to form larger particles. Powder is brinto smaller particles and released into the air; it is difficult to break particles smaller than 

~0.5

Pm. Biological particles usually become 

liquid or powder forms, so these particles are usually larger than ~0.

Viruses

Bacteria

Spores

Mechanical Gener

(dust or mist

Combustion Particles

(fume)

0.001 10 

Particle Diameter (

Pm)

0.01 0.1 1
II. Respiratory System Deposi

ICRP Model, averaged over males, females, several respiration rates

Particle Diameter (

Pm)

0

0.60.81.00.40.2

Total

Head Airw

H

Alveolar

(G

Trachebronch

0.001 0.01 10 0.1 

1

Deposition Fraction
III. Aerosol Particle Behav

• Settling• Impaction• Charge effects• Release from surfaces• Agglomeration

/

Deagglomeration
Particle Settling in Still Ai

Time to settle 5 feet by unit density sphere

0.5

Pm 1

Pm 10 3

P

m

Pm 

41 hours 12 hours 1.5 hours

8.2 minutes

Aerodynamic diameter definition: diameter of a unit density sphere that settles at the same velocity as the particle in question

5
Particle Settling in a Closed R

Stagnant air Turbulent air

Time

Conc. Conc.

Time

Particles of the same size will settle at the same speed in still or stagnant air

Particles passing close to a horizontal surface can settlethe rest will continue to be s
Particle Settling in Turbulent 

Half-life of particles in 8 foot high room

0.5

Pm 1

Pm 10 3

P

m

Pm 

41 hours 12 hours 1.5 hours

8.2 minutes

Particles settling in turbulent air will have an exponential decay rate as indicated in the previous slide 5
Particle Transport in Buildin

• 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
III. Aerosol Particle Behav

• Settling• Impaction• Charge effects• Release from surfaces• Agglomeration

/

Deagglomeration
Particle Impaction

• Impaction depends on 

particle size, air velocity, jet diameter

• Large particles deposit 

more easily

• Even larger particles 

can bounce from surface

• Impaction surface can 

be modified to improve collection, e.g., add oil
Cascade Impactors

Pump

• Used for size distribution 

measurement

• Commercial impactors

– Andersen– MOUDI
Virtual Impactors

• Used to reduce 

particle bounce

• Used to concentrate 

larger particles

– Commercial virtual 

impactor up to 100:1

– Contains smaller 

particles in minor flow

Q

0

0.1Q

0
III. Aerosol Particle Behav

• Settling• Impaction• Charge effects• Release from surfaces• Agglomeration

/

Deagglomeration
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 after about 30 min)
Particle Charge Imparted Du

Generation—Liquid Drople

• In conductive solution, 

ions equally distributed

• In nonconductive 

solution, fewer ions

• Droplet charge 

generally low

• When liquid 

evaporates, the final particle may have relatively high charge

-

+

-

-+

+

-
Particle Charge Imparted Du

Generation—Solid Particle

• Difference in surface 

energy levels

• Separation energy• Humidity creates 

bridge between particle and surface

+ + + + + + + + 

-
Space Charge 

Expansion of Aerosol

– High aerosol concentration– Particles are highly charged– All particles have same polarity– Aerosol will expand because of particle-p

repulsion
III. Aerosol Particle Behav

• Settling• Impaction• Charge effects• Release from surfaces• Agglomeration

/

Deagglomeration
Generation from Carpet

• Particles deposited in 

carpet; acts as a sink

• Footstep crushes 

fibers against each other

• Footstep compresses 

carpet, creating high velocity air flow
Particle Transport from Sour

Small particles throughventilationsystem Transport by 

local turbulence

Direct settling (larger particles and clumps

Resuspeby activ
Asbestos Fiber Generation

Effect of humidity on particle charge 

and particle generation efficiency

Mean

Electrical

Mobility

RC

0 10 20 30

Relative Humidity (%)
Particle Removal from Surfac

by Air Flow

• Boundary layer near 

surface—producedby motionless surface

• Factors affecting 

release: Air velocity, particle attraction to surface versus particle cross section

• Water (humidity) 

can increase adhesion

< 0.1 

Pm virtually impos

> 20 

Pm relatively easy
Vacuum Removal

• Suction

forces air near surface to removeparticles

• Variable

removalefficiency
III. Aerosol Particle Behav

• Settling• Impaction• Charge effects• Release from surfaces• Agglomeration

/

Deagglomeration
Agglomeration/ Deagglomera

• Particles in a powder are in close 

contact, primarily agglomerates

• Shaken powder releases clumps 

(agglomerates) and single particles

• Shear forces, caused by difference 

in air velocity across the particle, can break apart clumps

• Shear forces increase with 

increasing energy (air velocity)
Particle Size Evolution

10

5

10-310-110

3

10

Concentration (#/cm3)

Particle Diameter (

Pm)

Grinding aeroso

T = 0

T = 25 min

T = 225 min 

I

Diffusion

Coagulation (high conc.)

Surfaces

0.001 0.01 0.1 1 10 
IV. Aerosol Generation

< 0.1 

Pm virtually 

> 20 

Pm relativel

Energy Input Air flow

Mechanical energy

Overcome adhesion 

between particle and 

surface

Airflow 

part

Relein m

Adhesion depends mostly on microroughness of surface, also on relative surface energies
V. Particle Collection and Measu

• Filter sampling

– Filter efficiency, pore size, filter type– Sufficient volume for analysis– Dries particles because of continuous air – Removal from filter can be an issue

• Impactor sampling

– Cascade impactor: 3 to 8 stages, size reso– Sufficient volume for analysis– Dries particles, though less than filter– Inert (oiled?) surface or direct to growth 
Filtration

• Air filtration different 

from liquid filtration

• Pore size in air filters 

generally meaningless as indicator of efficiency

• Small particles collected 

by diffusion, large ones by impaction/interception

• Maximum penetration at 

about 0.3 

P

m

• Efficiency increases with 

increasing air velocity

Particle diame

0.01 0.1 

Efficiency

1

Diffusion
Direct Reading

Aerosol Measurement

Optical particle counter

– Relatively inexpensive (~2K – $10– Portable, battery operated– Rapid detection– Nonspecific for bacteria– Toxic concentrations near or below 

ambient particle concentrations

– Can be used for tracer studies
Direct Reading

Aerosol Measurement

Aerodynamic Particle Sizer

– Relatively expensive (~$40K)– Movable, line operated– Higher size resolution, possibly improve

distribution signature

– Fluorescent detection version– Can be used for tracer studies
Overall Scenario

Aeroso

Aerosol Transport Measu

Aerosol

Source

Characteristics

Aerosol

Losses

to Surfaces

Aeroso

Secondary

Sources

(Resuspension)
Resources for Aerosol 

Information

• Hinds, 1999, Aerosol Technology, Wiley• Baron and Willeke, 2001, Aerosol Measu

Wiley

• Hurst, 1997, Manual of Environmental 

Microbiology, ASM Press

• Spreadsheet: Aerosol Calculator availabl

www.tsi.com or www.bgiusa.com