Oxygenslowly killsmostairborne microbesthroughoxidation (Kowalski &
Bahnfleth, 1998). The toxiceffectof oxygen is relatedto moisturecontent;it
usually increases with the degree of desiccation, increasing oxygen con-
centration, timeof exposure and also on whetherthe desiccationis causedby
aerosolizationor drying.It is importantto remember that the combined effects
of oxygenand RH should be consideredwhenexplaining lossesin viability
(Griffiths & DeCosemo,1994).Airborne fungal contaminationalso correlates
withair pollutants,e.g. ozoneconcentration(Lin & Li, 2000).Airborne bacteria
are subjectto dehydration causedby evaporationof waterfromdroplet-carrying
microbes,as wellas evaporation of cellularwater. Dehydrationof microbes
causesosmoticstressand mayresultin decreased survival. Thompsonet al.
(1994)showed that the totalrecoveryof viablePseudomonasfluorescenswas
muchhighersampledfromhighthanfromlow RH levelair. The decreasein
total recovery of microbes with increase in desiccation time was more
pronouncedat low RH levelsof < 50%(Thompsonet al., 1994).For yeast
cellsthe survivalwas fourtimeshigher underhighRH (> 70%)conditions
compared withlow RH (2060%).The mouldsporesofPenicillumsp. werenot
affectedby RH of air (Lin& Li, 1999b).
37.3 Why,howand whatto sample
Concern is growing in the foodindustry to determine the importance of the
airborne routeas a possible source of contamination (Griffiths& DeCosemo,
1994).Manyfoodproducersnowincludebioaerosol monitoringas part of their
HazardAnalysisCriticalControl Point(HACCP) system (Parrett& Crilly,
2000).Anypointat whichthe productis exposedto air is a possible route for
airborne contamination.Bioaerosolmonitoringis carriedout traditionallyfor
three principal reasons (Griffiths & DeCosemo, 1994): (1) to meet legal
requirementsin complyingwithguidelinesthat oftenstatethat air qualitymay
haveto be monitoredbut do not specifythe methodologyor the acceptable
limits to use, (2) to collect epidemiological data,possibly witha viewto set
occupationalexposure limits and (3) for scientificinterestto determine howthe
air affects the products processed.Regularbioaerosol samplingis also important
in controllingthe effectivenessand hygieneof the ventilationsystem. Eventhe
filtersshould havemaintenanceprogrammesand advanced plannedchange
programmes,and sometimes the bioaerosol samplesare the firstsignof the
systemfailure.In additionto mechanical reasons, biological reasons suchas the
growthon filtermaterials can also causeproblems.
The following criteria should be usedto determine a samplingstrategy:the
samplingmethod, specificityand levelof sensitivityrequired as wellas the
speed with which a result is required. Furthermore, information on the
importance of totalcell counts versusviabilityof the cellsin the sampleas
well as the particlesize rangeneededput demandson the methodchosen. When
the reasonsfor carryingout samplingare fullyunderstood, the correct weighting
Improving air sampling 621