Freudenberg Filtration Technologies
Air Filtration

Air filtration quality in the food industry

Over the past twenty years we have seen the introduction of a range of food related legislation some of which relates to the control of the air supply quality. The aim of air filtration and food related service engineering activity is to guarantee that air quality supplied into the controlled environment is not the vector for food spoilage or source of airborne product contamination.

Food safety legislation and more recently Retailer manufacturing standards and codes of practice for High risk food manufacture has presented an opportunity for specialist engineering maintenance companies to offer a scope of air movement service work that will ensure manufacturing compliance with these requirements.

The air around us contains millions of particles in every cubic meter. The majority of the particulate in the air is generally below one micron in size and a high percentage of dust can remain airborne even in still air. The majority of the dust in the air is the result of incineration and in urban areas the carbonitious content is high. Motor vehicles, town waste incinerators and power station emissions make up the main source of particulates in the atmosphere. There may also be agricultural dusts and of course microorganisms such as yeast, moulds and bacteria (hosted). Urban and rural air quality may vary with the prevailing wind. However, there is no general level of air cleanliness we can expect for urban or rural environments.

Air filtration is designed to remove particulates and microorganisms from the air using a variety of mechanical processes. The degree of air filtration is selected by evaluating the risk of the contamination. Air filters are tested for efficiency using one of two tests which are designed to reflect the operating performance requirements, thus the air filtration system is the first critical control point in the air quality control process for both environmental and process applications in the food industry. Particulate and microbiological control will be linked with one or more services, such as heating, cooling and dehumidification to meet the supply air needs for process control and personnel comfort.

Conditioned supply air can be in contact with food, processing equipment, product input, packaging, personnel, process cleaning, storage and handling, and yet the filtered air does not play a useful role in controlling the spread of contamination, unless a number of factors have been addressed.

The air handling system must react to the following variable conditions: 

  • Ambient climate changes
  • Process condition - temperature, humidity, extraction and containment
  • Air filtration life cycle - system pressure drop, supply and extract air balance

The controls built into a modern air handling system should constantly monitor factors which affect the conditions within the manufacturing space. Some examples would be heating in winter to protect against freezing, cooling to maintain food production space temperature, and the differential pressure of the air filtration system.

The majority, if not all, air processing usually takes place within an air handling and filtration unit (AHU), which delivers air to a ducted system under pressure, for use as environmental (room), direct food contact and other process air requirements. An air handling unit will typically contain many of the following items, generally in the order listed when following the direction of air flow: 

  • Weather louvre or cowl with inlet "trash" control screen 
  • Volume control dampers with optional air mixing plenum – return / outside air
  • Primary filtration
  • Heater battery
  • Cooling coil section (positioned to suit requirements)
  • Fan and motor plenum
  • Air distribution zone and final filter access area (high efficiency filtration)
  • Final filter assembly (high care / high risk)

Some form of humidification control may also be required. The design and location of such equipment upstream of the final filters is critical to ensure successful long-term operation with good access for inspection and maintenance. Hygiene factors must be given careful attention when moisture may be present in the air supply system.

Important points to consider when designing an air handling and filtration unit include: 

  • AHFU casing design and internal finish
  • Primary filtration for long service life and protection of downstream components
  • Fan and motor selection to ensure constant air flow
  • Hygiene requirements
  • Equipment location, especially tertiary filtration

The design specification may also be modified to give a wider use of standard equipment. Whatever design of air handling equipment is chosen, care must be taken to ensure good manufacturing practice with HACCP considerations.

Air leaving the air handling and filtration unit is under pressure and the duct system should be designed and built to avoid air loss (DW144 standard). Duct insulation to prevent condensation forming on cold surfaces may be required.

There are circumstances where a modified design of inlet duct is required, and an example is where an air handling unit is situated some distance from an outside air supply. In such cases the pre-filtration should be located as close to the inlet control screen as possible, thus preventing dust laden air entering the system. Inlet duct systems should be operated in a clean and dry condition offering the minimum of maintenance.

Wherever possible ducts should be installed outside of the food processing space, with adjustable ceiling diffusers fitted for efficient air delivery into the controlled room. Extract air ductwork should receive particular attention with inspection access especially if the food manufacturing process is dry and particulate is released into the atmosphere under normal processing conditions. Extract from a cooking process will require careful attention to ensure that suitable and quick access for inspection and cleaning is available.

Sock or fabric air distribution systems are in common use, especially where a high air change rate is required. The advantages of lightweight construction and good air distribution should be considered carefully with each application, together with the requirement for a high standard of air filtration. The standard of air filtration should be up to "high care" requirement, and the control of the food process room conditions can be critical to the performance of fabric sock systems.

A balance of supply and extract air volume is a critical consideration especially when cooking vessels and dust filter systems are in use. Allow extra air volume at the design stage to ensure air movement from the controlled space. To conserve energy it is advisable to consider a separate ambient air supply to compensate for high air volume extracts such an open cooking and oven exit zones. This dedicated air will not normally be chilled and thus a saving in energy can be realized.

Air movement in a food factory
Outside air


Air handling unitMixing of air
Temperature control
Air filtration
Supply air under pressureSealed ducts with smooth internal surface
Room air distributionSupply and extract location
Differential pressureMinimal air loss through openings
ExtractionHeat, steam and fume
Dust control

Return air

Access for inspection

There are numerous design features which will assist in reducing running costs and add to process efficiency, and we suggest you seek the services of a company with a track record in this type of work.

When the air has been filtered to the required standard (particulate and microbiological control), heated, cooled and distributed through a duct system, control of the air in the manufacturing space or direct product contact is the next step. The application of air for food manufacture is divided into two group’s namely environmental (room) and process air. A breakdown into specific use groups is illustrated as follows:

Environmental air

  • Stock holding
  • Raw food preparation
  • Cooking environments
  • High care / high risk assembly rooms
  • Final packing
  • Storage and office spaces
  • Tank containment rooms

Not all these areas may require a supply of conditioned air and the selection is based on the type of food process and the layout of the factory. The choice of air filtration level can vary from one application to another.

Process air

  • Contained / high risk food manufacture
  • Laminar flow filling machines – aseptic filling
  • Ducted process air – dryers, fluid bed systems
  • Tank venting systems - mobile and fixed
  • Pneumatic conveying
  • Compressed air supplies

Generally process air systems are more critical in the degree of air filtration required, and they are often dedicated to a single application. A combination of environmental and process air requirements may also be serviced with a single air handling and filtration system. Once the conditioned air enters the manufacturing environment there will be a gradual loss of benefit – cleanliness, temperature, directional flow – due to the operating conditions prevalent within the space. The number and effectiveness of the air changes can be critical in making best use of the supply air. Air entering the manufacturing space over the process with extract behind the operators is a useful consideration. Extract points at wash areas, personnel entry, tray and utensil wash rooms and over dust generating processes will assist in maintaining clean conditions in the manufacturing space. The higher the air change rate of a correctly designed system the better the air quality control. Specific design features to suit various manufacturing layouts have evolved and a minimum of five air changes per hour is a good rule to adopt.

An economical way of overcoming an issue of room air quality control is to isolate the "high risk" air requirement, thus limiting the amount of highly cleaned air required. This is now a process air system and is referred to as a controlled or mini-environment. Fan assisted terminal systems which take air from the general pre-cleaned environment are also an effective way of installing an isolated facility. Care must be taken to ensure that the air filtration system within the process space is not subjected to a high level of airborne contamination.

For non "high care" requirements the use of good quality air filtration with efficient ceiling diffusers is usually sufficient to ensure acceptable room cleanliness. A positive room air movement condition is important for most types of food manufacturing facilities, to prevent atmospheric pollution entering the working space from the environment.

Process air systems, where the treated air is delivered directly to the food product are usually of the "high risk" type. Examples are spray and fluidised bed dryers, pneumatic conveying, compressed air supply, tank venting and some tunnel cooling systems and meat slicing operations . Although it is technically correct to operate a process air system under pressure with the final filters installed after the fan, this is not always possible, especially with high pressure blower systems. In this case the risks should be assessed and necessary design changes implemented.

When the correct air movement system is in place, it is time to consider the control of the food manufacture environment as part of the factory layout. Positive air movement from critical food process rooms can only be maintained if air loss restrictions are installed and other access (if any) to the food process rooms is carefully controlled.. Air passing to low care should be kept to a minimum on the basis of at least 0.5 - 1.0m/s at openings.

Temperature and humidity control are two areas where operating problems often occur, many of which cannot be fully appreciated at the factory design stage. Correct thermal barriers and access control to high and low temperature areas are most important. Air volume at the correct temperature and direction across the process area is often the answer to condensation problems. In fact air movement is the key to the majority of complaints about variations in temperature and humidity. At the design stage therefore an additional 10% - 15% capacity in the air supply system can be most useful for balancing out these problems.

Atmospheric pollution (particulates in the air) can block air supply systems and result in unsightly contamination within the food process environment, noticeably at ceiling grilles, air extract points and leaks from fabric air distribution systems. Air filters play a critical role in preventing contamination of the air supply system and protection of the high care food product.

Improved clarity for the selection of air filters has simplified what appears to be a rather complex business. Standards relating to clean rooms, Heating, Ventilation and Air Conditioning (HVAC) and HEPA filters were often mixed with limited understanding of particulate performance in regard to microbiological contamination, and the need to satisfy British, US and EC regulations. The CCFRA document twelve, “Guidelines for Air Quality Control in the Food Industry”, of November 1996 is a suitable reference.

All environmental air and many process air systems use disposable HVAC type filters which, in the case of "high care / high risk" applications, should have been designed to ensure they are food safe. Compressed air, some pneumatic conveying and tank ventilation systems make use of circular cartridge filters. Selection is based on operating pressure, process conditions, air volumes, capital and running costs. Filter manufacturing components should not include cardboard, coarse glass mats and untreated metal parts.

Air filters of the type we are discussing are divided into three groups namely primary, secondary and tertiary filters, as illustrated in the following European Normalisation standards:


BS EN779 and BS EN1822 Test Standards
BS EN779 for Primary and Secondary Filters (average performance values)
Arrestance (Primary filters)
Efficiency (Secondary filters)
BS EN1822 for EPA, HEPA Filters (minimum performance value to MPPS)
High efficiency (Tertiary filters)



Note that two tests cover all the above filter types, which are graded by either arrestance (G1-G4), efficiency (M5-6, F7-9) or by specific particulate penetration. (E10 upwards). A new term MPPS (Most Penetrating Particle Size) is now applied to high efficiency filters. For more information read FEL technical note "Air Filter Testing - The Story So Far".

High pressure, 50-200 mbar disposable cartridges are tested for a specific particle efficiency down to 0.2 micron and below. A liquid challenge test is applied to these filters which offers higher performance values than gas testing using high test pressures. Air filters installed in a food processing environment are mostly of the barrier type, where dust particles and microorganisms are captured and retained in the filter media matrix. Air filters are available to collect a wide range of particulate sizes, and the following general guide will prove useful:


Approaching 100% retention
General filter description
Test grading
> 5 micron
Primary filters
G4 - M5
> 2 micron
Secondary filters
> 1 micron
Secondary filters
0.5 micron
0.3 micron
HEPA filters
H13 upwards


It is important to note that secondary filters up to and including F9 grade are classified for average efficiency. This means that when the filters are new their efficiency is some 15% lower than the average value. Filters from E10 offer a guarantee that the stated efficiency will be met from the time the filter is installed. We have discussed filter grading and the size of dust various filters will collect, and we can now consider their application for the food industry. The following chart illustrates the zones where air filtration will be required, and a specific filter quality can be selected to suit these requirements.


Final filtration level
Filter system make-up
Covered product / general office
M5/ M6
and up to F7
Single filter use acceptable if dust loadings are low.
Food Process Hall (General process areas defined as non "high care/high risk")
G4 / M5 pre-filter panels or pocket type followed by F7 rigid cell filter.
"High Care/High Risk" food production
F8 - E11
M5 / M6 pocket filter followed by rigid cell final filter in leakproof framing system located on pressure side of system
High Risk direct product contact
M5 / M6 pocket filter followed by E11 filter in special framework for leakproof confirmation.

Note: A risk assessment in conjunction with the air filtration supplier may prove of benefit to technical personnel.

Air filters of the correct grade must be located within a filter framework designed and installed to ensure total air filtration integrity. Further reading on this subject is available from Filtration Engineering Ltd.

A clean zone or controlled environment is defined as "a room in which the concentration of airborne particles is controlled and the room contains one or more clean zones which can be allocated a particulate cleanliness class". It is important to remember that a high particulate level in the process space air does not necessarily mean a high microbiological contamination. However, to remove potentially contaminating organisms, a degree of air filtration is required. It is correct to say that a low level of particulate in the air will give rise to a low bug count too.

The principle of control is: 

  • Clean the air to the required level for the manufacturing process. 
  • Blanket the product (and process) with clean air – non-laminar flow / laminar flow. 
  • Use sufficient air volume to eliminate ingress of airborne contamination and maintain process integrity. 


Air filtration is only one consideration with regard to the air supply quality once the air has entered the controlled zone. If a food manufacturing process within a controlled environment is to remain uncontaminated it must be protected from all sources of contamination by controlling: 

  • Food ingredients
  • Food containers entering the controlled space
  • The air quality - by air filtration, air movement and air loss
  • Personnel controlling operations (limited access) 
  • Equipment contamination from outside the controlled area - conveyors etc. 


Thus cleaning the air and introducing the air correctly only solves one part of a space contamination control programme. High standards of room air quality and hence low levels of contamination are easier to control using uni-directional or laminar flow type installations. Localised isolation using a mini-environment is a cost-effective method of establishing a high level of contamination control to (say) ISO 5 standard.



Air filtration levels required for controlled environment use are of the high efficiency type and filters tested to BS EN1822 are installed after adequate pre-filtration. The design of an installation will depend on size and location of process and supply air equipment design.

The installation and maintenance of laminar flow air systems is expensive and their application is usually restricted to high risk processes, such as liquid fill machines, some powder drying and meat slicing operations.

For some high risk applications an environment that can be monitored to a recognised standard may be required. The following is a reference guide to the controlled environments (clean zone) standards with BS EN ISO 14644 being the replacement for all others listed below. 

Comparison of BS EN ISO 14644, UK & USA Standards for Controlled Environments


Classification Number (N) 

BS EN ISO 14644

Maximum Particle Concentration per m3 @ 0.5 micron
BS 5295 (1989)
BS 5295 (Pre-1989 – for reference only)
3 520
E / F
Class 1
35 200
G / H
1 000
352 000
Class 2
10 000
3 520 000
Class 3
100 000
35 200 000

The Food Safety Regulations which were adopted from EC Directive 93/43/EEC on 15th September 1995 are deregulatory in nature, which means that food manufacturers are responsible for controlling their manufacturing conditions, to ensure wholesome foodstuffs are handled, prepared, manufactured, processed, packed, stored, transported and distributed hygienically. Allied to this legislation is the responsibility on the part of food producers to identify and control potential hazards to food safety by utilising a risk assessment system such as HACCP.

With many items of process plant in use it is important that maintenance be carried out by personnel qualified in all aspects of the air quality control system. The term "facilities management" can imply that the contractor has an in-depth knowledge of the air movement equipment and of customer requirements. The contractor is selling his know-how to assist the customer in the production of hygienic food at all times, and at the most competitive price for the service. The customer must determine the ability of his prospective contractor and apply technical and commercial judgement in his evaluation. Certain factors such as track record in the industry, written technical information, presentation, quality assurance, commitment to training of personnel and investment in test and analytical equipment, all help to reassure the customer that the contractor has the necessary skills and training to perform the tasks required.

Unlike some equipment installed in food factory air handling systems, the air filtration requires a periodic service check to ensure the air filters are operating within design limits. Automatic monitoring is available for air filter systems and a separate indication is required for each installation such as for primary and secondary filter systems. Reliable dry type analogue gauges can be installed which should be monitored and readings recorded at intervals. Minimum filter use must be the aim with any air filtration maintenance programme. The air handling and air movement process is the "lungs" of the manufacturing environment, and air from this system can permeate most factory areas. It is essential therefore that a series of checks and controls are in place to ensure air quality and air distribution meet the design and operating requirements. 

A contractor must be able to perform a range of action points based on the following list of skills: 

  • Particulate analysis and interpretation for process requirements. 
  • Airflow pattern and distribution control in the process rooms. 
  • Heating, cooling and humidity evaluation. 
  • Air filtration performance control and integrity checks. 
  • Mechanical and electrical equipment - basic evaluation. 
  • Process controls – basic understanding. 
  • Familiarity with the manufacturing process to identify problems and assess new and associated work quickly and accurately. 


The choice of work to be considered for contract will depend on the size and type of the business, and the need for a specialised contractor. Whatever the need, it is vital that the customer understands clearly what the contractor proposes after the basic work description has been presented.

Once a contract has been established for selective or general air systems management, regular meetings with customer representatives is recommended to ensure all parties are aware of progress. A basic programme for discussion would include work completed, proposals for the following month, new work and allied issues. The management of the contract is based on assessment, action and control - to reduce the cost of such work to the food industry. It is important to ensure that time spent at the customer’s premises be kept to a minimum, and every effort should be made to standardise the work schedule, which will lead to a more cost effective service in the future. An Air Quality Manual (AQM) is suggested as a document for reference between the customer and the contractor.

For further information on selecting a suitable contractor please read “What is an air quality service contract and why do I need one” (FEL Technical information note - 092).