Part of the microbiological sampling pillar of the TAAG Learning Center. It completes the environmental monitoring program, which usually focuses on surfaces.
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Air also carries microorganisms
Most environmental monitoring programs concentrate on surfaces, and rightly so: that is where pathogens become established and from where they pass to the product. But between a contaminated surface and an exposed food there is also the air. A review by Masotti and colleagues in Trends in Food Science & Technology describes it as a factor contributing to cross-contamination: although the air entering a plant is usually cooled and filtered, processes, personnel and the facilities themselves release microorganisms indoors. The result is bioaerosols that vary widely between plants, within the same plant and from one day to the next.
In a study at a Brazilian dairy plant published in the Brazilian Journal of Microbiology, the authors point to personnel, floor drains, the ventilation system and pressurized water applied during cleaning and sanitation as the main sources of aerosols.
When it makes sense to sample the air
Air sampling contributes most where the product is exposed after its last treatment and before the package is closed:
- Cooling, slicing and packaging areas for baked goods, a critical point described in the article on microbiology in bakery.
- Filling and packaging rooms for dairy, beverages and long shelf-life products.
- Areas for ready-to-eat foods, where there is no later step that reduces the load.
- Processes sensitive to spoilage by molds and yeasts, which spread easily through the air, as discussed in the article on spoilage microorganisms.
It is also useful for checking the effect of specific changes: construction work, a new filter in the air system, a change in the flow of people or cleaning with high-pressure hoses near exposed product.
Passive method: settle plates
The standard ISO 14698-1:2003, on biocontamination control in cleanrooms and controlled environments, defines the settle plate as a container, for example a Petri dish, with a suitable sterile culture medium, left open for a defined time to collect the viable particles that settle from the air.
It is the simplest and cheapest method, and it measures something with practical meaning: what falls on an open surface, just as it would fall on an exposed product. Its limitation is that it does not tell how many microorganisms there are per volume of air, and small particles, which stay suspended, settle very little.
The Index of Microbial Air Contamination (IMA)
To make settle plates comparable, Pasquarella and colleagues proposed the IMA index, with a standardized "1/1/1" scheme: a plate 9 cm in diameter, left open for 1 hour, 1 meter above the floor and about 1 meter from walls and obstacles. The result is expressed in CFU per plate over that time. A 2020 review in Acta Biomedica gathers the proposed classes (0 to 5, very good; 6 to 25, good; 26 to 50, fair; 51 to 75, poor; more than 75, very poor) and found that food industries were among the indoor environments where it was most applied.
Active methods: air samplers
An active sampler draws in a known volume of air and collects the viable particles, which allows the result to be expressed in CFU/m3. ISO 14698-1 distinguishes two principles:
- Impaction: the air strikes a solid surface, normally an agar plate. It is the most widespread format in plants.
- Impingement: the air strikes a liquid and the particles remain in it, which later allows filtering, plating or analysis by molecular methods.
There are also filtration, centrifugal and cascade samplers. In impact samplers with a perforated lid, several particles can pass through the same hole, which is why manufacturers provide count correction tables; the dairy plant study used that correction for a 400-hole device.
Passive or active: what the data say
The study by Salustiano and colleagues compared both methods in six areas of a dairy plant. The results illustrate their differences well:
- The impact sampler recovered between 2 and 10 times more microorganisms than settling.
- The dominant group depended on the method: yeasts and molds predominated with the sampler; aerobic mesophilic bacteria with the settle plates.
- The settle plates did not detect coliforms or Staphylococcus aureus, which did appear with the sampler, at low levels.
- Air humidity showed a relationship with higher counts; temperature did not.
The authors concluded that, to recover microorganisms from the air, including pathogens, the impact method is preferable. The IMA review adds a nuance: active sampling measures the concentration in the air and passive sampling reflects what is deposited on critical surfaces, and there is no fixed equivalence between the two. Many plants use both for different purposes.
How to design the sampling
Points, times and states
ISO 14698-1 asks that monitoring be designed by risk zones and that the sampling itself not contaminate the product or the area. It also distinguishes three states of a facility: as-built, at-rest and operational. For a food plant, the most informative approach is usually to sample during operation, near exposed product and at approximately line height, and to complement it with at-rest samples to separate the contribution of the process from that of the air system.
Media and incubation
In the dairy plant study, plate count agar was used for aerobic mesophiles, potato dextrose agar for molds and yeasts, VRB agar for coliforms and Baird-Parker for S. aureus, with volumes of 100 to 1000 liters of air depending on the expected load. The sampler lid was sterilized and disinfected with 70% alcohol before and after each sampling. As with any plating, the result depends on the media being controlled, a topic we cover in the article on quality control of culture media according to ISO 11133. The transfer of the plates to the laboratory follows the guidelines of the article on sample transport and preservation.
How to read the results: there is no universal limit
It is common to see reference values cited. The Brazilian study mentions those recommended by the American Public Health Association (APHA) for aerobic mesophiles in process areas: 90 CFU/m3 with a sampler and 30 CFU/cm2/week with settle plates; and cites other proposals, from 180 to 360 CFU/m3 for mesophiles and from 70 to 430 CFU/m3 for molds and yeasts depending on the area.
A systematic review by Loveniers and colleagues, published in 2024 in the International Journal of Food Microbiology, puts those numbers in context. After analyzing twenty years of literature, it concludes that there are no established standards or guidelines with quantitative limits for airborne microorganisms in the food industry, that solid evidence of a direct correlation between the air load and product contamination is lacking, and that the lack of standardized sampling methods prevents studies from being compared. Its proposal is to set criteria adapted to each product and production context, supported by risk assessment.
In practice, that matches what ISO 14698-1 asks: that each user establish their own alert and action levels, appropriate to their activity and to each risk zone, based on a baseline obtained at start-up and reviewed periodically. What has value is the trend over time at the same point, with the same method, rather than an isolated number compared with a figure from another plant.
The regulatory framework
ISO 14698-1 and 14698-2 were written for cleanrooms, but their principles are used as a reference in the food industry. In Europe, CEN published the standard EN 17141 in 2020, which replaced the European versions of ISO 14698 and includes an informative annex for food processing. The ISO 14698 standards remain in force outside that scope.
When counts rise, the Masotti review goes over air treatments such as chemical fogging, ozone, UV radiation or cold plasma, always as a complement to cleaning and sanitation, not a substitute. And a sustained increase in the air usually has an origin in the plant, such as a drain, a piece of equipment with biofilm or a cleaning practice that generates aerosols, which should be looked for with surface sampling.
Common mistakes
- Comparing settle plate results with limits expressed in CFU/m3, or the other way around.
- Changing the method, the medium or the exposure time between campaigns and losing the comparability of the trend.
- Sampling only at rest, when the plant is not producing and the air is cleaner.
- Adopting a limit published for another industry without building your own baseline.
- Treating the air as the cause and not investigating the source that contaminates it.
Conclusion
Air sampling completes what surface sampling does not see: how much and what is deposited on an exposed product. Settle plates are simple and reflect that fallout; active samplers quantify by volume and recover more microorganisms. Since there are no universal limits for the food industry, what gives the program its value is keeping the same method, building your own baseline, setting alert and action levels by zone and looking for the source when the trend rises.
About TAAG
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Frequently asked questions
What is the difference between a settle plate and an air sampler?
A settle plate collects what deposits on it over a period of time and is expressed in CFU per plate and time. An active sampler draws in a known volume of air and allows the result to be expressed in CFU/m3. In a study at a dairy plant, the active method recovered between 2 and 10 times more microorganisms.
What is the IMA index?
It is a standardized index for settle plates: a 9 cm plate left open for 1 hour, 1 meter above the floor and about 1 meter from walls. The result in CFU per plate is classified into categories ranging from very good to very poor.
Is there an official limit for airborne microorganisms for food plants?
There are no standards or guidelines with established quantitative limits for the food industry. There are published reference values, such as those from APHA, but a 2024 systematic review recommends setting criteria adapted to each product and process, based on risk assessment and on your own baseline.