size of aerosolparticles is generally in the range0.5±50m. Particle size is the
majorfactorinfluencingaerodynamic behaviour (Kang& Frank,1989a)that
affectssignificantlythe samplingeffectivenessand yield.
Generally, the performanceof an active bioaerosolsampling device is
characterized by the device'sability to aspirate particles intothe inlet, to
transmit them through the sampler's interior and to collect them on the
collection surface. In the case of viable bioaerosol sampling,the performanceof
samplers must maintain the stability of microbialviability as an additional
component duringsampling(Thompsonet al., 1994).The samplers must also
collect a representative sampleof the requiredfractionof bioaerosolwitha
minimumof stress, so that the biological activity of the aerosol is not too
impaired (Griffiths& DeCosemo, 1994). The efficiency of a sampler in
collecting a particle of a given size is relatedto the air velocityin the impaction
nozzle.Too low a velocity in the inletwill resultin failureto collectthe particles
of interest. Toohigha velocityresultsin a highshearforceand maycause
seriousdamage to the microbes,thusdecreasingtheirviable recovery. The
largerthe aerosolparticles, the greaterthe over-or underestimationof the
aerosolconcentrationis likelyto be. If the sampler is not mounted on the axis of
the ambientairflow, the measured aerosolconcentrationmaybe significantly
differentfromthat in the ambientair environment(Thompsonet al., 1994).
Samplers differ fromeachother in flowrate,and optimal sample densities, as
well as optimal samplingtimes,differ considerably fromone sampler to another
(Nevalainenet al., 1992).Furthermore, the ambient bioaerosol is a mixture of
manyspecies of microbes. Sporeformersand hardyspeciesmay maskthe
sampleand impedethe recoveryof more sensitive organisms.Thus,sample
analysis is severely limited sincelittleinformationis availableon the causative
variablesthatleadto differing colony recoveries(Thompsonet al., 1994).
Airbornecontaminant counts are alsodependent on the person operatingthe
measurementdevice.The morestringentthe air purityrequirements,the greater
the influence of air-sampling techniques(Meier& Zingre, 2000).Contamination
fromthe sampler,especially in clean environments,will decreaseif thereis a
waitingperiodset for 1±2 minafterinstallationand beforestartingthe air
sampler. If measurements are taken in a conventionallyventilated room,
airborne contaminantcountsmayalso varygreatlyif peopleare movingaround
or if workis beingperformed(Meier& Zingre, 2000).
37.4.1 Efficiencyof the bioaerosolsamplers
Sampler efficiency is a measure of howwellthe inletof a sampler drawsin
particles without being affectedby the particles' sizes, shapes, velocitiesand
direction(Nevalainenet al., 1992).The totalefficiency(TE)of a sampler is
determined by several factorssuchas the designof the inlet, collection stageand
choiceof collection medium,whichaffectthe viability of the collected microbes
(Henningson& Ahlberg, 1994).TE can be dividedinto collection efficiency
(CE)and preservationefficiency(PE).Nevalainenet al.(1992)determinedPE
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