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4.5.3 Control of air quality andventilationAir can have a number of uses in foodprocessing including drying, agitation,packaging, conveying, operating equipment,cleaning or refrigeration. Some facilities willoperate under positive air pressure on thefinished product side relative to the raw side.In this system, high-risk zones (Figure 1) willbe maintained at a higher pressure (i.e. aboveatmospheric pressure) than an adjacent lowriskzone, thereby preventing contaminatedair entering the high-risk zone (Figure 1).Calibrated pressure measuring and recordingdevices should be used to monitor positive airpressure systems.Compressed air has been identified as asource of L. monocytogenes contamination (26) .Compressed air is sometimes used to cleanequipment, particularly packing equipmentat the end of processing prior to sanitation.However, the use of compressed air asa cleaning aid should be minimised. Ifcompressed air must be used, the air shouldbe dry (i.e. to minimise microbial growth)and filtered at the point of use. The filtersshould be maintained and replaced as permanufacturer’s instructions as these canbecome contaminated with L. monocytogenesalso. Air from compressed air lines implicatedas a source of L. monocytogenes contaminationhas been traced to niche environments nearthe point of use (Section 4.5.4) (26) .Research has not identified atmospheric airas a direct source of L. monocytogenescontamination in foods (90, 96-97) . However, someresearch has indicated that L. monocytogenescan survive in aerosols for extended periods.This suggests practices which produceaerosols should not be used in the processingenvironment (98) . In general, air quality andventilation should be carefully controlled tominimise the formation of visible condensation.Ultraviolet light can be used to treat air atintake points to reduce levels of contamination.It is important to prevent the entrance of nonsterileair during draining or venting. Air ventson equipment, ventilation fans and ducts shouldalso be monitored.4.5.4 Elimination of niche environmentsWhile isolated contamination withL. monocytogenes may occur in the processingenvironment, repeated contamination is morelikely to occur after L. monocytogenes hasbecome established in a niche environment. Aniche environment is a localised site (e.g. hollowcavity in a roller on a poorly designed conveyorbelt) in which food debris and moisture mayaccumulate. Populations of L. monocytogenesmay become established and persist in a nicheand sometimes form biofilms (Appendix 2.9).When L. monocytogenes becomes establishedin niche environment normal sanitationprocedures may become less effective. Poorlydesigned equipment is particularly vulnerableto the establishment of niche environments. IfL. monocytogenes does become established in aniche, the equipment can become a substantialsource of contamination. As equipment isoperated, L. monocytogenes from the niche canbecome deposited on food contact surfaces. Asa product moves over or through this regionit can become contaminated and subsequentlyspread contamination to other areas of thefacility as it moves through production.F O O D S A F E T Y A U T H O R I T Y O F I R E L A N D26