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Air Monitoring in Food Plants

FSMA Preventive Controls for Human Food include air as a potential source of contamination. How can you take a risk-based approach and incorporate air monitoring into your Preventive Control plan?

The first consideration is whether there is a need to test only ambient air or also compressed air.

Airborne contamination of foods by bioaerosols (solid or liquid microscopic particles suspended in air that carry microbes) may be a risk factor depending on the product, process area, or processing equipment. A bioaerosol can be created from a number of sources including foot and wheeled vehicle traffic passing through contaminated standing water, the impact of high-pressure hoses on contaminated surfaces, use of compressed air lines without point-of-use filters and poorly designed air handling units. Bioaerosols are suspended in the air for various lengths of time. Bioaerosols of vegetative (nonspore) cells released into an open space will move from high to low pressure on air currents in the plant. This highlights the importance of appropriately filtered, positive-pressure air in rooms requiring elevated hygienic controls such as filling and packaging rooms (3)).

Compressed air is used in a variety of food contact applications such as mixing ingredients, cutting, sparging, drying, transporting ingredients, cleaning/dislodging build-up in lines, and in packaging. While compressors make the processing of foods faster and more efficient, food contamination from compressed air is a food safety concern. The microbiological content of the compressed air can directly impact product quality. When ambient air is drawn in and compressed, the contaminants (Table 1) become further concentrated. Water in compressed air, in particular, is a significant food safety issue. Only properly filtered compressed air should be used and it must be monitored to assure that products are not contaminated with “unlawful indirect food additives” (CFR Title 21 Part 110.40).


Proactive routine air sampling will detect viable airborne particles and establish typical microbial reference data. Counts above that baseline trigger investigations and subsequent corrective actions improving Preventative Controls. Sampling air in a processing environment verifies that preventative controls such as air-handling systems, air filtration and SSOPs are properly designed and working effectively.

How to test: Exposure (or settle) plates (passive monitoring) and centrifugal air sampling devices (active monitoring) are the most common procedures used to measure airborne contamination.

Exposure plates are standard Petri dishes containing culture media (usually non-selective). The agar is simply exposed to the atmosphere over a controlled period of time and whatever settles by gravity is collected on the surface. The plates are then incubated to allow visible colonies to develop and be counted. This approach assumes a fairly steady settling rate and does not take into account air movement. While cutoff values for food processing plant air exist, settling plates will not detect smaller particles or droplets that stay suspended in the air and they cannot sample specific volumes of air, so the results are not quantitative.

However, exposure plates are inexpensive and easy use. They are useful for qualitative analysis of airborne microorganisms and the data they produce may

Using Exposure Plates

Exposure plates are available from the Lab. Place the plates in the key sampling areas as discussed above (or rotate through them as you would in an environmental sponge program), open the lid, and start a timer. A referenced exposure period in common use is 15 minutes (2). If you are sampling dusty areas, you may find it’s best to shorten your exposure time to 10 minutes. After the exposure is complete, bag the plates up and send them into the lab to be incubated and counted.

detect underlying trends in airborne contamination and provide early warning of problems. They are also useful for directly monitoring airborne contamination of specific surfaces. In an environment such as a low risk food factory, settle plates may provide an adequate means of monitoring biological air quality.

Centrifugal air samplers are devices that draw in a known volume of air so that airborne microorganisms impact onto an agar strip or plate. When the correct volume of air has been passed through the sampling head, the agar plate can be removed and incubated directly without further treatment. After incubation, counting the number of visible colonies gives a direct quantitative estimate of the number of colony forming units in the sampled air.

Impaction samplers using standard petri plates offer benefits in terms of convenience. They are also able to handle higher flow rates and larger sample volumes necessary to monitor air quality in rooms where the number of microbes present is likely to be very low. Pre-poured Petri plates are available from the lab.

Sites to sample: Microorganisms can come from the air itself or they can be aerosolized from contaminated surfaces. Areas to consider for sampling include under air vents, in areas of high worker activity or other movement, areas where water is spraying or misting (e.g. sinks, high pressure hoses in use), active floor drains (focus on inadequately trapped drains or drains that back-up), areas in the plant with negative air pressure drawing in air from contaminated outside areas, or areas where a lot of dust is generated (e.g. raw

material ingress or opening of packaging).

A lot of air movement can affect the accuracy of your Exposure (Settle) plates, so it may be best to conduct sampling in those areas at the beginning or end of the shift.

The Baseline Proactive routine air sampling will detect viable airborne particles and establish typical microbial reference data. Counts above that baseline should trigger an investigation and subsequent corrective actions.

What to test for: Air is typically assayed for Yeast and Mold and Aerobic Plate Count (APC). In large volumes of air in open areas, detection of pathogens or specific spoilage agents is unlikely due to the dilution effect. The best approach is to monitor airborne microbial populations for hygienic indicators and take appropriate corrective actions when exceeding acceptable levels.

Interpretation of Results: As with any microbial count testing (APC, Yeast/Mold, etc.), individual results don’t really tell you very much. The goal is to establish a baseline of “normal” counts and then use ongoing results to look for spikes which indicate that something is going on that requires investigation.

That leads to the question of how often to sample. In any new program, it is best to start with more frequent data collection because it allows a baseline to be established. If you have a very consistent system, you can decide at some point to scale back on your sampling, but you have to have the data to show that your system is stable enough first.

There’s also the balance between cost and quality of data. If you sample frequently (i.e. every day), it obviously costs more in testing and employee time, but you have much better oversight of what’s going on in your plant. If you test less often (i.e. weekly), you save money, but you stand a good chance of missing “spike events” or not being able to effectively problem solve the ones you do find. As a start, testing at least three times per week is highly recommended. Once you establish a consistent baseline, you can cut back to weekly.
Finally we come to acceptable results. In the lab an unacceptable air count is >15 cfu per 15 minutes exposure, but that’s in a laboratory. We wouldn’t expect to achieve that in particularly dusty areas in a plant. In that case, a good count would be <20 cfu per 10 minutes exposure. Remember, though, it’s more about baseline and spikes than good numbers and bad numbers.



Where does the contamination come from?

Compressor room filtration and drying is not enough.
Downstream air reservoirs, piping, fittings are ideal harborage sites for microbial and biofilm growth.
Bacteria are compressed along with the air, amplifying

When compressed air contacts food, most GFSI and HACCP risk analyses will call for filtering out microbial contamination and testing compressed air for microbial contamination. Untreated compressed air contains many potentially harmful or dangerous contaminants which must be removed or reduced to acceptable levels in order to protect the consumer and the production facility. Contamination hazards include rust, pipe scale, water, oil, non-microbial particulates, allergens, and microorganisms.

The first line of defense to ward off potential microbial contamination of the food product from compressed air is to use point-of-use sterile air filtration. Even the best compressor room system filtration doesnot eliminate harborage sites and biofilm buildup in the compressed air piping system. With a properly designed compressed air system employing the benchmarked GMPs (outlined below in Table 1) along with well-designed SSOP (Sanitation Standard Operating Procedure) maintenance and monitoring programs – the risk associated with compressed air at points of contact can be reduced significantly. Information available from the Parker Hannifin Corporation details requirements for the production of sterile air for use in food manufacturing (4, 5).

A 5-log (99.999% efficiency) removal of microbial contamination from the air stream is possible with a sterile air filtration system that produces sanitized (commercially sterile) air at point of use. This puts a physical barrier in the air stream guarding against microbial contamination of the food.

Table 1. Point-of-Use Compressed Air Filtration - Best Practices

GMPs For Food Contact Air

1. System Design - Point of use filtration
Wherever the compressed air comes |in contact with the food – either directly or indirectly - the following 3-stages of filtration will significantly reduce the risk of microbial contamination of the food.
• Stage 1: Remove bulk liquid and particulate matter down to 0.01 micron at >= 93% DOP efficiency. Automatic drain in filter.
• Stage 2: Remove oil and water aerosols and smaller particulate matter down to 0.01 micron at >= 99.99+% DOP efficiency. Automatic drain in filter.
• Stage 3: Remove microbial contamination down to 0.01 micron at >= 99.9999% DOP efficiency with a sterile air filter.

2. System Design - Drying Compressed Air
Indirect food contact air: dry air to <= +37° F
Direct food contact air: dry air to <= -40° F


1. Maintenance of Filters

Stage 1: Change filter element every 6-12 months.
Stage 2: Change filter element every 6-12 months.
Stage 3: Change filter element every 3-6 months – or sooner – as necessary based on point-of-use air quality test for microbial content. Optional: Steam sterilize stage 3 (provided the filter is designed for CIP sterilization). Follow manufacturer’s instructions.

Note: Sterile air filters are designed to capture microbial matter larger than the filter rating. Microbial matter will not create a differential in pressure across the filter. Therefore, measuring differential pressure across the element will not give an accurate reading of contamination. Air testing and/or regularly scheduled element changes are the best practice for verification.

The established Good Manufacturing Practices that relate to compressed air used in a food processing facility are listed in the Table 2 below. The standards come from a variety of regulatory and GFSI schemes. Collectively they point to the use of the most discriminating filtration standard which is at least a 5 log reduction of microbiological particles and additional impurities like dirt, water and oil that could have a food safety impact.

Table 2. Global compressed air GMP requirements (reprinted with permission of the Parker Hannifin Corporation).

Monitoring the microbial purity of compressed air verifies that filters are effective and that maintenance programs are operating as intended is

How to test using a Compressed Air Sampling Device:

  1. Use an aseptic compressed air sampling device.
  2. Expose agar plate to compressed air.
  3. Incubate agar plate.

Count total colony forming units.

How to test if you don’t have a Sampling Device (1):

  1. You will need:
    • A quick connect air nozzle sanitized with 200-400 ppm quat-based solution or 50-200ppm chlorine, one for each air hose checked. 
    • Premoistened sterile pathogen sponge swabs in whirl-pak bags and sterile gloves.
  2. Blow air through the nozzle to ensure the insides are dry.
  3. Remove a sterile pathogen sponge swab from the whirl-pak bag with a sterile gloved hand and place the nozzle in contact with the sterile sponge swab while blowing air onto the sponge for 20 seconds. 
  4. Place the sponge back into the labeled whirl-pak bag and seal.
  5. Check the air nuzzle gun for signs of any residual moisture by triggering the nozzle into a clean white cloth for one minute and examine the cloth for any signs of moisture or particulates. Photograph the cloth before and after use. 
  6. Send the sponge samples out for microbiological analyses.

Sites to sample: Develop a baseline by testing compressed air at each food contact point frequently. Once the absence of microbial contamination has been assured, a less frequent sampling and testing scheme can be used.

What to test for: Compressed air is typically assayed for Aerobic Plate Count (APC). The
objective is to determine the presence or absence of microbial contamination (beyond ISO
8573-1 air standards).

Interpretation of Results: RTE foods are at high risk of contamination from sources such as
compressed air. Any microbial contamination introduced in the later stages of RTE food
processing can stay with the food all the way to the consumer, as few hurdles or barriers are
generally in place to eliminate the hazards. If air is used as an ingredient in food production or on product contact surfaces, microbial counts should be below the limit of detection. Any counts on the agar plate require corrective action.


  1. Deibel, Charles. 2015. Personal communication
  2. Evancho, G.M., et al. 2001. Microbiological monitoring of the food processing environment, Chapter 3. In Downes, F.P. and K. Ito (eds.), Compendium of methods for the microbiological examination of foods, 4th ed. Washington, DC: American Public Health Association.
  3. Kornacki, J. Food Safety Magazine. June/July 2014. (Airborne Contamination a Microbiologist’s Perspective - Food Safety Magazine).
  4. Parker Hannifin Corporation. Balston Filters. “Sterile Air for Food Plants”. Brochure
  5. Scott. L. White Paper: Reducing Contamination Risks of Compressed Air in Food Plants: Benchmarking Good Manufacturing Practices. May 2013. Parker Hannifin Corporation. http://www.Balstonfilters.com/. White Paper:

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