Generation and Behaviorof Airborne Particles (Aerosols)Paul BaronDivision of Applied TechnologyNational Institute for Occupational Safety and HealthCenters for Disease Control and PreventionOverviewI. Particle size rangeII. Inhalation & lung depositionIII. Particle behavior– Settling, impaction, electrostaticeffectsIV. Particle generation– Energy input, size, charge, humidityScenarios• Letter release• Carpet releaseVI. Particle collection and measurementWhat is an AEROSOL?• Simply defined- tiny particlesor droplets suspended in air.• The haze in the picture on theright is caused by lightscattering from numerouswater/oil droplets and mineralparticles released into the airfrom the drilling of rock.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 particleexposure is the respiratory systemToxic 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 thatway. Soluble particles may dissolve and pass through the skin.Read on for more details on aerosol generation and behaviorOverall Scenario:Evaluation of Exposure in WorkplacesAerosol TransportBased on Air FlowAerosol Sampling/MeasurementAerosol InhalationAerosolGeneration from,e.g., GrindingAerosolLossesto SurfacesSecondarySources(Resuspension)Loss MechanismsSettling,Diffusion,Impaction,ElectrostaticDepositionFilterSamplersDirectReadingInstrumentsAerosol Assessment in theWorkplace: Types of Measurements• Sampling, usually with a filter and pump, providesa sample that can be analyzed in the lab forspecific chemicals, quantity of dust, particle shape(fibers), etc.• Direct reading instruments allow continuousobservation of dust concentrations, e.g., mass orconcentration or size distribution, but do notusually provide specifics of the aerosol type.Aerosol Assessment in theWorkplace: Types of Measurements• The most accurate assessment of worker’sexposure is measurement with a personal sampler,i.e, a collection or measurement device placed onthe worker’s chest.• Techniques for control of exposures can use eitherpersonal samplers or (fixed) area measurementdevices. Direct reading devices allow rapidassessment of the effectiveness of dust controldevices or strategies.I. Aerosol Size RangeParticle size is often determined by the process that generated the particle.Combustion particles usually start out in the 0.01-0.05Pm size range, but combinewith each other (agglomerate) to form larger particles. Powder is broken downinto smaller particles and released into the air; it is difficult to break down suchparticles smaller than~0.5Pm. Biological particles usually become airborne fromliquid or powder forms, so these particles are usually larger than ~0.5Pm.VirusesBacteriaSporesMechanical Generation(dust or mist)Combustion Particles(fume)0.001 10 100Particle Diameter (Pm)0.01 0.1 1II. Respiratory System DepositionICRP Model, averagedover males, females,several respiration ratesParticle Diameter (Pm)00.60.81.00.40.2TotalHead AirwaysHead AirwaysAlveolarAlveolar(Gas exchange)TracheobronchialTracheobronchial0.001 0.01 1010.1 100

Deposition FractionIII. Aerosol Particle Behavior• Settling• Impaction• Charge effects• Release from surfaces• Agglomeration/DeagglomerationParticle Settling in Still AirTime to settle 5 feet by unit density spheres0.5Pm 1Pm 10Pm 100Pm3Pm41 hours12 hours1.5 hours8.2 minutesAerodynamic diameter definition:diameter of a unit density sphere thatsettles at the same velocity as the particlein question5.8 secondsParticle Settling in a Closed RoomStagnant airTurbulent airTimeConc.Conc.TimeParticles of the same size will settleat the same speed in still orstagnant airParticles passing close to ahorizontal surface can settle, butthe rest will continue to be stirred.Concentrationprofiles using adirectmeasurementinstrumentParticle Settling in Turbulent AirHalf-life of particles in 8 foot high room0.5Pm 1Pm 10Pm 100Pm3Pm41 hours12 hours1.5 hours8.2 minutesParticles settling in turbulent air willhave an exponential decay rate asindicated in the previous slide5.8 secondsParticle Transport in Buildings• Most large particle losses by settling• Most small particle losses by exchangewith outdoor air• Complex flow systems• Turbulence productionDoors, people, fans. ventilationVentilation systemIII. Aerosol Particle Behavior• Settling• Impaction• Charge effects• Release from surfaces• Agglomeration/DeagglomerationParticle Impaction• Impaction depends onparticle size, airvelocity, jet diameter• Large particles depositmore easily• Even larger particlescan bounce fromsurface• Impaction surface canbe modified to improvecollection, e.g., add oilCascade ImpactorsPump• Used for size distributionmeasurement• Commercial impactors– Andersen– MOUDI>8>4>2Filter>1Virtual Impactors• Used to reduceparticle bounce• Used to concentratelarger particles– Commercial virtualimpactor up to 100:1– Contains smallerparticles in minorflowQ00.9Q00.1Q0III. Aerosol Particle Behavior• Settling• Impaction• Charge effects• Release from surfaces• Agglomeration/DeagglomerationElectrostatic Effects+++++++++-• Particle-particle interaction small• Particle-surface interaction large• Particle charge depends mostly ongeneration process, surface energy,humidity, time in the air• Airborne particle charge graduallydecreases due to ions in air(particles are nearly neutral afterabout 30 min)Particle Charge Imparted DuringGeneration—Liquid Droplets• In conductive solution,ions equally distributed• In nonconductivesolution, fewer ions• Droplet chargegenerally low• When liquidevaporates, the finalparticle may haverelatively high charge-+--+++-Particle Charge Imparted DuringGeneration—Solid Particles• Difference in surfaceenergy levels• Separation energy• Humidity createsbridge betweenparticle and surface+ + + + + + + +-Space ChargeExpansion of Aerosol– High aerosol concentration– Particles are highly charged– All particles have same polarity– Aerosol will expand because of particle-particlerepulsionIII. Aerosol Particle Behavior• Settling• Impaction• Charge effects• Release from surfaces• Agglomeration/DeagglomerationGeneration from Carpet• Particles deposited incarpet; acts as a sink• Footstep crushesfibers against eachother• Footstep compressescarpet, creating highvelocity air flowParticle Transport from SourcesSmall particlesthroughventilationsystemTransport bylocal turbulenceDirect settling(larger particlesand clumpsResuspensionby activityAsbestos Fiber GenerationEffect of humidity on particle chargeand particle generation efficiencyMeanElectricalMobilityRelativeConcentration0 10 20 30Relative Humidity (%)Particle Removal from Surfacesby Air Flow• Boundary layer nearsurface—producedby motionlesssurface• Factors affectingrelease: Air velocity,particle attraction tosurface versusparticle crosssection• Water (humidity)can increaseadhesion< 0.1Pm virtually impossible> 20Pm relatively easyVacuum Removal• Suctionforces airnear surfaceto removeparticles• VariableremovalefficiencyIII. Aerosol Particle Behavior• Settling• Impaction• Charge effects• Release from surfaces• Agglomeration/DeagglomerationAgglomeration/ Deagglomeration• Particles in a powder are in closecontact, 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 withincreasing energy (air velocity)Particle Size Evolution10510-310-110310

Concentration (#/cm

3 )Particle Diameter (Pm)Grinding aerosolT = 0T = 25 minT = 225 minSettling/ImpactionDiffusionCoagulation (high conc.)SurfacesSurfaces0.001 0.01 0.1 1 10 100IV. Aerosol Generation< 0.1Pm virtually impossible> 20Pm relatively easyEnergy InputAir flowMechanical energyOvercome adhesionbetween particle andsurfaceAirflow to entrainparticlesRelease happensin microsecondsAdhesion dependsmostly on microroughness ofsurface, also onrelative surfaceenergiesV. 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 mediumFiltration• Air filtration differentfrom liquid filtration• Pore size in air filtersgenerally meaningless asindicator of efficiency• Small particles collectedby diffusion, large onesby impaction/interception• Maximum penetration atabout 0.3Pm• Efficiency increases withincreasing air velocityParticle diameter (Pm)0.01 0.1 1 10

Efficiency1Impaction/InterceptionDiffusionDirect ReadingAerosol MeasurementOptical 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 studiesDirect ReadingAerosol MeasurementAerodynamic Particle Sizer– Relatively expensive (~$40K)– Movable, line operated– Higher size resolution, possibly improved sizedistribution signature– Fluorescent detection version– Can be used for tracer studiesOverall ScenarioAerosol Sampling/MeasurementAerosol TransportAerosolSourceCharacteristicsAerosolLossesto SurfacesAerosol InhalationSecondarySources(Resuspension)Resources for AerosolInformation• Hinds, 1999, Aerosol Technology, Wiley• Baron and Willeke, 2001, Aerosol Measurement,Wiley• Hurst, 1997, Manual of EnvironmentalMicrobiology, ASM Press• Spreadsheet: Aerosol Calculator available fromwww.tsi.com or www.bgiusa.com