sizesof verysmall particles, up to 40m, at highconcentrationnear to cleaning
operations (Burfootet al., 2003a). These devicesutilisetwo laserbeamsand
lightis scatteredat the intersection of the beams.Analysisof the scatteringcan
be usedto assessthe particlesize distribution, flux and velocity.Image-based
systems use a laser and high-speedcameraand they are usedto measure the size
and velocityof particles beyondthe size rangestudiedwith a phase-Doppler
analyser. Air particlecounters are commonly usedin cleanroomsand theseare
also basedon lasertechnology.Theyextracta sample of air for analysisrather
thananalysing the aerosolin situ. Furtherinformationon particlesize analysers
is givenby Mitchell(1995).
5.2.2 Number of organisms
Settleplatescan be usedto assessthe deposition of airborne organisms.Other
equipment is available fromvariousmanufacturersthat allowsmeasurementsof
the concentrationsof airborne organisms.Mostof thoseusedin the foodsector
rely on the impactionof the organismsonto solidmediain a Petridish as factory
air is drawnacrossthe dish. Calibrationsare thenusedto convert the number of
organismson the dish to an airborne concentration.The Andersen (1958)
sampler is widely usedfor research as, by usingmultiple dishes arrangedin
stages,it can provideinformationon particlesize distribution. Crook(1995)
describes various samplers,though thoseconstructed fromglasswouldgenerally
be consideredunsuitable for use in chilled foodproduction areas.
5.2.3 Air flow
The speedand directionof the air flow in the productionareaare importantas
they can movecontamination arounda factory.Air speedcan be measured using
hot wire anemometerswhilevaneanemometersprovide speeddataand some
indication of flowdirection, althoughcare is needed. Bothof thesedevicescan
be usedin conjunction witha `windicator' suchas a small length of freely
hangingfabric filaments to indicatethe directionof the air flow(Burfootet al.,
2001).Smoketestsmaybe applicable in someareas.Ultrasonicanemometers
are relativelyexpensive but allowthe measurementof air speed,directionand
fluctuation.
5.3 Aerosolgeneration
Burfootet al.(2003a) examined the generationof particles by fourcleaning
operations: low-pressurehosing(100psi (689 103 N/m^2 ), hosetype), boot
scrubbing (mechanical walk through), hand-washing and floor scrubbing
(mechanicalwithbrushes, squeegeeand vacuum). Hosing was foundto produce
a veryhighparticleflux of 144 000 particles per square centimetreper second
below 40 m diameter. Thisflux,which was measured15 cm fromthe impact
Aerosolsas a contamination risk 95