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Testing Compressed Air Lines for Microbiological Contamination

An air impaction microbial assessment of your air supply may indicate you have unwanted inhabitants in your compressed air system


Manufacturers of medical devices who label their product as sterile may use compressed air in various applications. Applications include injection molding, operation of conveyor belts, and/or aseptic cleaning processes. The manufacturer may use these systems on a continuous basis or on an as-needed basis. The manufacturer may not realize that the compressed air system could be harboring microorganisms.

When the air supply system is operating, it may unleash contaminants which could adversely affect the product, including operational characteristics, by compromising sterile claims, or product aesthetics.

The condition or the quality of the supplied air, from a microbiological standpoint, may not be obvious unless microbiological testing is performed. A simple air impaction microbial assay of compressed air lines, however, will alert the manufacturer to the various types of viable microorganisms that might be present.

Manufacturers of medical devices, pharmaceutical operations, or those classified as sterile fill, are constantly assessing the environmental impact on the product during the manufacturing process. This assessment usually includes the facility, the equipment, and the personnel involved in the assembly process. Sampling may include, but is not limited to, surface sampling, particulate counts, water analysis, and product testing. However, compressed air can easily be overlooked if not initially inserted into the environmental monitoring protocol or identified by an experienced environmental scientist.

The engineering department may design the facility whereby the air compressor is segregated from the area where the product is assembled for logical hygienic reasons. However, this segregation may lead to a false sense of security because it is possible that the environmental air source that feeds into the compressor may contain biological flora similar to that of the manufacturing environment. It could be possibly even higher in microbiological levels if the location of the air compressor is less controlled than that of the manufacturing environment. If the compressed air supply is not properly engineered with filters, dryers, and appropriate gauges, these microbiological inhabitants could eventually reach the product.

Compressor as the Culprit
The compressor itself can function as the culprit by creating a contaminated environment for the product. For example; the compressors prefilters can become overloaded with dust and lint, causing the filter to cease functioning properly, thereby resulting in migration and potential strike-through. Also, there may be a contaminant in the environment which is smaller than the pore size of the pre-filter. Again, this may lead to the inefficiency of the pre-filter.

A microorganism that is capable of forming a viable colony forming unit (CFU) and which exists within the compressed air line system is called the microbiological particle (MP) as per ISO 8573-4:2001(E). The microbial load on a product is referred to as Bioburden, as per ANSI/AAMI/ISO 11737. The Bioburden can come from various sources, such as human contact, air conditioning systems, the manufacturing process itself, raw materials, and any other vector that the product is exposed to. However, the least likely considered vector for contamination is the compressed air system. Maybe this is because the compressed air network is considered to be a closed system, under high pressure (usually 160 pounds of pressure and greater) and is expected to supply continuous high quality filtered air.

Microorganisms can range from fungal spores, to bacterial spores, and various vegetative microorganisms, depending on the environmental conditions. These organisms can range in size from less than one micron to several microns in size. Therefore, when designing an air supply system, every effort should be taken to ensure entrapment of these contaminants before they get to the product.

Test Compressed Air Lines
Routinely Compressed air lines should be tested on a routine basis in order to determine the microbiological cleanliness of the air supply. Compressed air lines undergo the same changes as other dynamic systems, such as contraction, condensation, sedimentation, and in the case of compressors, oxidation when the air-lines are stagnant over an extended period of time.

Testing should be conducted on a periodic basis, and the frequency of the testing should take into consideration important factors such as:

Increased/Reduced production schedules
Seasonal changes
Equipment changes/modifications
Replacement of hardware and/or filters/dryers
Inactivity of system

A simple test that utilizes air impaction onto growth media is suggested. A general sampling method would be to reduce the pressure of the compressed air line (which usually is 160 pounds per square inch or greater), using a built-in or external regulator; attach a flow meter, and adjust the flow to a suitable rate, for example, 1 cubic foot per minute; and attach the air impaction sampler which has been prepared with a Petri plate. You should select parameters best suited for your application or those noted in ISO 8573-4. See Figure 1.

Test Equipment

The Slit Sampler (also known as the slit-to-agar sampler) is a device which utilizes a rotating stage which holds the Petri plate. The air impacts the surface of the agar with whatever organisms are present, the organisms become impinged onto the agar, the plate is incubated, and the organisms are allowed to grow. ISO 8573-7 offers further details and should be reviewed before testing is conducted. Two companies that manufacture Slit Samplers are New Brunswick Scientific and the Barramundi Corporation. See Figures 2 and 3.

The general concept is that compressed air, under reduced pressure, called Partial Flow is forced over the surface of a Petri plate. The Petri plate is 100 millimeters in diameter or greater, based on the sampling device used. Bacteria are impinged onto the surface of the agar. Subsequent incubation of the Petri plate will allow the bacteria to grow. Care should be taken to ensure that equipment used has been properly disinfected in order to minimize the introduction of bacteria not associated with the compressed air supply. A negative control should be used as a qualitative measure, and the end point to be tested should be purged and aseptically cleaned to minimize false results.

Important factors to consider when developing a sampling plan include:

1. Compressed air lines being tested versus sampling time
A compressed air system may require various sampling times, for example:

If the air system is static, a longer sampling time may be appropriate;
If the air system is in use, a shorter sampling time may be appropriate Caution should be taken to minimize the possibility of confluent growth.

2. Selection of media
A broad spectrum, non-selective media will allow the growth of all microorganisms and can overwhelm the process and result in confluent growth.
A selective media can potentially limit overall growth and result in counts that are quantitative. However these results may not ref lect the total flora in your compressed air system.

The agar used can be a broad spectrum agar such as Soybean Casein Digest Agar (SCDA), commonly referred to as Tryptic Soy Agar (TSA) which will allow the growth of most non-fastidious organisms. A selective agar may be used if you are trying to isolate a certain type of microorganisms, such as Sabouraud Dextrose Agar, which is used for the cultivation of fungi.

Following incubation, the agar plate(s) are read on a colony counter and recorded. A detailed description of sampling techniques and incubation periods can be found in ISO 8573-7. It is recommended that the viable organisms be identified in order to assess the impact on the environment and product.

It may be beneficial to conduct particle counts of the compressed air lines before conducting microbiological counts to more accurately determine the microbiological sampling time. This can be performed under the general rule that the higher the particle count the higher the microbiological count. Correlation between these two variables should be determined so that particulate testing can be used to predict future microbiological results.

Alternative Methods
Industry offers various other instruments which also serve as microbial detection systems. These instruments may or may not comply with the ISO 8573 standards and are offered here for informational purposes only. See Table 1.

The process of conducting a routine environmental monitoring program of your compressed air lines will assist you in identifying the microorganisms, if any, that may be present in your system. Once you know the types and quantities of these potential environmental contaminants the sooner you can start to identify their impact on your finished product, and you can begin to develop systems to protect your product.

References 1 International Standard ISO 8573-1:2001, Technical Corrigendum 1, Compressed air-Part 1: contaminants and purity classes, 2002-04-01.
2 International Standard ISO 8573-4:2001, Compressed air-Part 4: Test methods for solid particle content, First Edition 2001- 06-15.
3 International Standard ISO 8573-7:2003, Compressed air-Part 7: Test method for viable microbiological contaminant content, First Edition 2003-05-01.
4 ANSI/AAMI/ISO 11137:1994 & ANSI/AAMI/ISO 11137:1994/A1:2002, Sterilization of healthcare products- Requirements for validation and routine control-Radiation sterilization, 3ed.
5 ANSI/AAMI/ISO 11737-3:2004, Sterilization of Medical Devices-Microbiological Methods-Part 3: Guidance on evaluation and interpretation of bioburden data.



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